[CPS Science]Skylights

[Jim Kaler]

 Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, November 2, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon fades this week from near full to its third quarter,
reaching the phase about the time of moonset the night of Wednesday
the 7th (or the morning of Thursday, the 8th).  As it goes, it will
be seen to the west of Saturn the night of Friday, the 2nd, to the
east of it and closer the night of Saturday, the 3rd.  The night of
Monday, the 5th, be sure to watch the Moon play closely with
Jupiter, our satellite less than two degrees north of the giant
planet around midnight.  

The two brightest planets in the sky, Venus and Jupiter, are
highlighted this week.  (Yes, Mars can get brighter than Jupiter,
but for only a very short amount of time near its favorable
oppositions.)  The morning of Friday, the 2nd, Venus stands four
degrees north of Spica, which the Sun has just cleared, meaning
that Venus is now rising rather late, about 5 AM Standard Time,
just after the birth of morning twilight.  On the same day, Jupiter
becomes momentarily "stationary," that is, it ceases its normal
forward motion easterly through the stars, and reverses into
retrograde, as the Earth prepares to come between it and the Sun. 
Two days later, on the morning of Sunday, the 4th, Mars passes 2
degrees south of Neptune, an event only visible if you have a
telescope, Neptune rather far below naked-eye brightness.  Since
Venus and Mercury maintain their close connection this week, the
two less than a degree apart for most of this period, both actually
are seen north of Spica.  Go look, and use Venus to find the
smallest inner planet (Mercury), which will be the brightest body
close to bright Venus, both notably bright in morning twilight.  

With the Moon now gone from the evening sky, we can look again at
the stars.  Even in early evening, the Great Square of Pegasus can
be seen moving high in the southeastern sky.  The Square's
northeastern star is part of both Pegasus and Andromeda, which
climbs in streams of stars to the northeast.  In the middle of
Andromeda, if you have a dark sky, you might spot the fuzzy patch
of the Andromeda Nebula, which in the early twentieth century was
discovered to be a large nearby galaxy comparable to our own (our
200-billion-star assembly that makes the Milky Way).  "Nearby" here
takes on a relative meaning, as this great spiral galaxy, also
called M 31, is two million light years away, the farthest thing
visible to the naked eye.  Comparable in distance is another, M 33,
the great spiral galaxy in Triangulum, which under ideal
circumstances (which includes being young!) is also visible to the
naked eye, though just barely.  The southern hemisphere contains
two more naked-eye galaxies, the Large and Small Magellanic Clouds
in the constellations Dorado (the Swordfish) and Tucana (the
Toucan), these two requiring the observer's latitude to be well
south of 20 degrees north latitude.

STAR OF THE WEEK.  MATAR (Eta Pegasi).  Pegasus (the Flying Horse)
is so well known for its Great Square that we sometimes give the
other stars little thought.  Coming off the northwestern star of
the Square, Scheat, is a pair of stars that with Scheat make a
rather prominent triangle, the northern one Matar, which Bayer
called "Eta," and which (at mid third-magnitude, 2.95) actually
ranks fifth in brightness rank (ignoring Alpheratz, Delta Pegasi,
which is actually Alpha Andromedae).  "Matar," from Arabic, has to
do not with a horse, but with "rain," though just what is unclear,
one source suggesting "lucky rain."  At a distance of 215 light
years, Matar is double and may well be quadruple, consisting of a
very unequal pair of pairs, an unbalanced double-double.  The
bright naked-eye star is actually a close pair separated on the
average by only three astronomical units (a bit over half the size
of Jupiter's orbit).  The brighter, 262 times the luminosity of the
Sun, is an evolving class G (G2) 5100-Kelvin giant with a quiet,
contracting helium core, the fainter a hotter (7800 Kelvin) class
A (A5) hydrogen-fusing solar type dwarf.  The measured orbit (its
period 2.24 years) reveals the stars to contain respective masses
3.2 and 2.0 times the mass of the Sun.  Ninety seconds of arc away
is a much fainter (ninth magnitude) class G (G5, a bit cooler than
the Sun) star that separates into another pair only 0.2 seconds of
arc (at least 13 astronomical units) apart that take at least 34
years to orbit.  That the two doubles are actually related is not
fully known, some say yes, others no, that they are a line-of-sight
coincidence.  The luminosity of the dim pair, however, is close to
being right for G stars if assumed to be at Matar's measured
distance, so they

[CPS Science]Skylights (late)

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 26, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

As a result of error and travel, Skylights is late this week.
My apologies.

The Moon passes through its full and brightest phase this week the
night of Wednesday, October 31.  This one, more than the last full
Moon of October 2, deserves to be called the "Hunter's Moon," as
the early evening around the time of the full Moon is still
dominated by moonlight.  As a result of various misinterpretations,
the second full Moon in a month is sometimes called a "blue Moon." 
This full Moon is curious in that the technical full phase occurs
on the morning of Thursday, November 1, at 5 hours 41 minutes
Greenwich Time (better known as Universal Time), and therefore does
not qualify, nor does it in Eastern Time, which is 5 hours behind
Greenwich.  However, for the remainder of the Americas (Central
Standard Time, 6 hours behind Greenwich, and west), this one is
indeed a "blue Moon," as it takes place at 11:41 PM CST.  Such
timing differences can also confound the much more important date
of Easter, which falls on the first Sunday following the first full
Moon after the Sun passes the vernal equinox in Pisces.

The big news of the week, however, involve a remarkable interplay
between Mercury and Venus.  If you have never seen Mercury, now is
the time, as brilliant Venus, which now rises in the east just as
dawn starts to light the sky, shows the way.  On Monday the 29th,
Mercury reaches its greatest western elongation from the Sun.  From
Saturday, October 27th until Wednesday November 7, Venus and
Mercury will be within a degree of each other, an extraordinary
event that is rarely repeated.  Just find Venus, which is hard to
miss as it will be the brightest body in eastern morning twilight,
and the next brightest thing close to it will be Mercury!  What
makes the close pass even rarer is that the two are never in formal
conjunction, wherein one planet lies due north of the other.  Of
much lesser interest, since it cannot be seen (not in Moonlight
anyway), is an event that involves Uranus, which begins its
retrograde motion within the confines of dim Capricornus on the
night of Monday, October 29.

On the morning of Sunday October 28, Daylight Savings Time ends in
the UK, Canada, and the US, that is, we drop back an hour to our
own Standard Time rather than using the one to the east of us.  The
sky darkens an hour earlier (a strictly artificial event, as it is
the clock changing, not the sky), but we gain daylight back in the
morning.  Look to the south now about 8 PM STANDARD Time, and you
will see the bright star Fomalhaut crossing the meridian.  If you
are far enough south, from just above 40 degrees north on down, you
can see -- once the Moon is out of the way -- the modern
constellation Grus, the Crane, which, unlike most of the
constellations in the sky, rather looks like what it is supposed to
be, a great bird walking along the southern horizon -- providing
your horizon is clear of trees, corn, or even soybeans.

STAR OF THE WEEK.  ANWAR AL FARKADAIN (Eta Ursae Minoris).  The Big
Dipper (the Plough in England), the major figure of Ursa Major (the
Great Bear), is so well known that few except the dedicated pay
much attention to its counterpart, the Little Dipper, which in
parallel is the most (in fact the only) recognizable portion of
Ursa Minor, the Small Bear.  And no wonder, since the Little Dipper
is so faint that it cannot be seen in any town with bright lights. 
The figure is recognizable mostly by the North Star (Polaris, Alpha
Ursae Minoris) and by the two front bowl stars, Kochab and Pherkad
(Beta and Gamma).  Kochab was at one time called "Nair (or Anwar)
al Farkadain," meaning the "bright one" (or "the lights") of the
"two calves," and Pherkad was called "Alifa al Farkadain," meaning
the "dim one of the two calves" (hence the name "Pherkad").  As so
often happenes, the names have been transferred to other stars, in
this case to Zeta and Eta Ursae Minoris.  And here, at the bottom
of the Little Dipper, we find dim fifth magnitude (4.95) "Anwar,"
the faintest star of the Dipper's seven.  Physically, the star is
nearly (but not quite) sunlike, a class F (in the middle of the
range, F5) dwarf with an estimated temperature of 6400 Kelvin,
right at the point at which we do not have to correct for infrared
or ultraviolet radiation.  The star's luminosity of only 7.4 times
that of the Sun leads to a radius twice solar and a mass 1.4 solar. 
Rather well along in its hydrogen-fusing lifetime, Anwar is a bit
brighter than normal for its temperature, and seems close
to becoming a "subdwarf," a star that has shut down hydrogen
fusion, if it has not alr

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 19, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Skylights is a day early this week.

The Moon passes through its first quarter this week the night of
Tuesday, October 23.  On its way there it makes a close pass to
Mars in the middle of the American afternoon of Tuesday the 23rd,
and by evening will make a fine configuration just to the east of
the red planet, both bodies to the east of the Little Milk Dipper
in Sagittarius.  

Though the sky changes only slowly from week to week, it changes
surely.  At the same time each night, from one week to the next the
stars slip another seven degrees to the west, and to see the same
sight you have to look another (roughly) half an hour earlier.  We
therefore lose the western stars to the Sun, the loss compensated
by the ever-earlier risings of the eastern stars.  Two bright
constellations, Taurus and Gemini, representing late autumn and
true winter, and tagged with the giant planets Saturn and Jupiter,
now rise around 8:30 and 10:30 Daylight Time.  

The planets of course have their own motions within these
constellations.  Saturn, to the east of Jupiter, is now in
retrograde (as a result of the Earth beginning to pass between it
and the Sun) and is moving westward against the background stars. 
Jupiter, on the other hand, is still in direct (easterly) motion. 
It will not enter retrograde ("retro" in the trade) until November
2.  As a result, the two planets are (as seen in the sky) moving
farther apart.  After Jupiter enters retrograde, they will slightly
approach each other.  But that is temporary.  Jupiter will quickly
thereafter pull away from the ringed planet, and the two will not
be back together again for nearly 20 years, as Jupiter takes 12
years to orbit the Sun, Saturn nearly 30.  As a result, Jupiter on
the average spends about a year in a given constellation of the
Zodiac, while Saturn visits each for just over two years.

Watch Cassiopeia now climb the northeastern sky opposite the Big
Dipper, her "W" beginning to go over the pole like a splayed "M." 
Unlike the Dipper, all of whose are named, Cassiopeia, bright as it
is, has few that are.  Following behind is bright Perseus, whose
central concentration of stars is actually a wide cluster.  Between
the two, those under a dark sky can make out the "Double Cluster in
Perseus," the only example of two clusters, undoubtedly born at the
same time from the same interstellar cloud, moving through space
together.  Eventually, as a result of gravitational forces from the
Galaxy, they will separate.  Through such forces, and as a result
of simple "evaporation" (stars just leaving), each will
individually mostly dissolve, as will the bright stars of the
Pleiades in Taurus. 

STAR OF THE WEEK.  RHO CAS (Rho Cassiopiae).  Cassiopeia is full of
bright stars, yet precious few have proper names.  Even very bright
Gamma Cassiopeiae has none, at least in western lore.  Pity then
the seemingly lesser stars, which have no chance at all.  At least
in one spectacular case, however, the "lesser" tag is totally
wrong.  Look to fifth magnitude (4.54, just over the line from
fourth) Rho Cas, way down on the Bayer Greek Letter list. 
Estimated to be an amazing 8000 light years away, Rho Cas, greater
even than a supergiant, is a class G (G2, some say F8)
"hypergiant."  Dimmed by two magnitudes by interstellar dust, still
it shines at near-fourth magnitude, radiating 550,000 times more
light than the Sun from a surface measured at 7300 Kelvin, the
star's energy mostly pouring out in the visible part of the
spectrum.  The temperature and luminosity tell of a distended
surface 450 times larger than the Sun, one 4.3 Astronomical Units
across, 40 percent larger than the Martian orbit.  Rotating at
least at 29 kilometers per second, Rho Cas could take up to two
years to make a full spin.  Though it has no companion from which
to gauge its mass, the immense luminosity suggests roughly 40 times
solar.  Theory shows that hydrogen-fusing dwarf stars from 10 to
about 60 solar masses evolve from blue class O first to become blue
supergiants and then into red class M supergiants.  From around 40
to 60, however, they loop back, turning from red supergiants back
into much hotter and smaller blue supergiants.  Higher than 60,
they bump into a wall and stay as blue supergiants.  Rho Cas now
seems to be on its way back from being a red supergiant, when it
may have been some five times larger.  If so, it is bouncing
against the "yellow evolutionary void," in which stars become
unstable and do not like to linger.  And Rho Cas certainly is
unstable.  It is an irregular variable, or at best a semi-regular,
and seems to have multipl

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 5, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon descends in phase and brightness toward its third quarter,
the phase reached the night of Tuesday, October 9, shortly before
Moonrise in the Americas.  The Moon will then begin its waning
crescent phase as it moves through Cancer and then into Leo.  In
the early part of the week the Moon will make a fine passage
between the giant planets Saturn and Jupiter.  The night of
Saturday the 6th, the Moon will approach Saturn.  The next two
nights, Sunday the 7th and Monday the 8th, it will be between the
pair, and then before moonrise the night of Tuesday the 9th will
pass north of Jupiter, appearing to the east of the planet upon its
rising. 

Mars hangs low in the southwestern sky, where it will be for the
rest of the year as (while trying to keep up with faster Earth) it
travels easterly along the ecliptic from its current low position
within the constellation Sagittarius.  As it moves, it will
noticeably dim as the Earth pulls away from it.  Our evening
attention now is slowly being displaced from Mars to Saturn, which
is now rising around 9:30 PM Daylight Time, and then to Jupiter,
which this week rises as Mars sets, around 11:30 PM.  These two
great planets are then high in the sky near dawn when brilliant
Venus hovers over the eastern horizon.

Early October evenings, especially those with no Moonlight, provide
a fine time to view the remaining summer stars, the Summer Triangle
of Vega, Deneb, and Altair high in the sky, the northern two of the
stars nearly overhead in mid-latitudes.  As the Big Dipper falls in
the northwest, The "W" of Cassiopeia rises in the northeast,
followed by Perseus and bright Capella in Auriga.  From out of
Perseus flows the Milky Way through Cassiopeia and then through dim
Cepheus and into Cygnus, from which it falls through Aquila and
down to Sagittarius past Mars.  

As the evening progresses, watch for the passage of the lonely
first magnitude star Fomalhaut, which in mid-northern latitudes
appears to glide slowly across the far southern sky, the star the
luminary of Piscis Austrinus, the "Southern Fish."  Both Fomalhaut
and high brilliant Vega are surrounded by dusty disks of matter
that may hold some kind of planetary system, though no planets have
ever actually been detected.  To the northwest of Fomalhaut is
rather dim Capricornus, the Zodiac's "water goat," while to the
northeast lies Aquarius, the "water bearer" and then farther to the
northeast Pisces, the classic "fishes."  The whole area represents
a "wet quarter" of the sky that once signalled a rainy season in
some ancient land.
  
STAR OF THE WEEK.  ETA AQL (Eta Aquilae).  Lying almost exactly one
degree north of the celestial equator, this quite-wonderful mid-
fourth magnitude (nominally 3.90) star in Aquila (the Eagle) glides
across the sky just 8 degrees to the south of much brighter first
magnitude Altair.  Unfortunately given no proper name by the
ancients, it is now known principally as Eta Aquilae, Eta the
seventh letter of the Greek alphabet.  Near one of the Eagle's
talons, Eta also represents the head of the now-defunct no-longer-
recognized constellation Antinous, who was honored in the sky by
the Roman emperor Hadrian, and depicted as being carried by Aquila. 
The star seems relatively dim only because it is so far away, an
uncertain 1200 light years.  It remains quite visible to us only
because it is a luminous yellow-white class F (nominally F6)
supergiant that shines 3100 times more brightly than the Sun. 
While such brilliance may pale behind that of the blue hydrogen-
fusing dwarfs or the great red supergiants, the star's moderate
temperature of 5600 Kelvin assures that nearly all of its radiation
pours out in the visual where we can see it rather than hidden in
the invisible ultraviolet (as it is for hot stars) or the infrared
(cool stars).  The luminosity and temperature conspire to give a
radius 65 times that of the Sun, while direct measures of angular
diameter give a close 59 times.  But these characteristics are only
an aside compared with the star's status as one of the sky's most
prominent Cepheid variables (the variability discovered long ago,
in 1784), Eta Aquilae comparable to the prototype Delta Cephei (the
name "Cepheid" taken from the constellation "Cepheus"), Mekbuda
(Zeta Geminorum), and the southern hemisphere's Beta Doradus and W
Sagittarii.  Polaris is actually the brightest of all Cepheids, but
its small variations are not sensible to the eye.  The variations
of Eta Aquilae, however (as they are for the others listed here),
are obvious, the star changing its brightness from magnitude 3.6 to
4.4 and back agai

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, September 28, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Welcome to the first full week of autumn (if you are in the north)
or spring (if in say Chile, Argentina, or Australia), the seasons
reversed between hemispheres.  The lunar phases of course are not,
as everyone in the world will see the Moon pass through its full
phase on Tuesday, October 2.  In the Americas, full phase actually
occurs shortly after moonset, so the night of Monday October 1 the
Moon will be just before full and will rise before sunset, the
night of Tuesday the 2nd it will be just after full phase and will
rise just after sunset within the Earth's rising shadow.  Full
moonrise is often spectacular, the Moon looking larger than normal,
the effect purely an optical illusion.  Because of the low tilt of
the eastern portion of the ecliptic against the horizon, the
nightly delay in moonrise is short and the evening seems flooded
for several nights with near-full-moonlight.  Though the October
full Moon is traditionally called the "Hunter's Moon," this year it
is closer to the fall equinox than was the September full Moon, and
more fits the concept of the "Harvest Moon." 

Because the Sun has just passed the autumnal equinox in Virgo, this
full Moon will be just past the vernal equinox in Pisces, though
the dimness of the constellation will render it quite invisible. 
In between fall and spring lies winter, so between the autumnal and
vernal equinoxes lies the winter solstice in Sagittarius, which is
right on the meridian to the south at sunset.  Embracing bright
reddish Mars, Sagittarius will thus be just to the west of south as
twilight ends.  Look just below Mars to see Sagittarius's five-star
Little Milk Dipper, the red planet passing just north of bright
Nunki (Sigma Sagittarii) the night of Sunday September 30.  

In the evening sky, among the bright planet Mars is still quite
alone.  Watch, however, for the rising of Saturn around 10 PM
Daylight Time (the ringed planet to the east of Aldebaran in
Taurus).  Two hours later, bright Jupiter, in Gemini, crosses the
evening divide, rising now about midnight Daylight Time.  In the
morning hours, Venus, though getting closer to the horizon, still
gleams brilliantly in growing dawn.  

At 9 PM, Deneb, in Cygnus, flies is high in northern-hemisphere
skies near the zenith.  Look to the south to find one of the sky's
more exquisite constellations, Delphinus, the Dolphin, appearing
something like a parallelogram that makes a hand with a finger
pointing southward.  Just west and a bit north lies Sagitta, the
Arrow, and north of that very dim Vulpecula, the Fox, a challenge
even on a moonless night (which these are not).

STAR OF THE WEEK.  ARKAB PRIOR (Beta-1 Sagittarii).  "In 1603
Johannes Bayer gave Greek letters to the stars according to the
apparent brightnesses," so goes the story.  Sagittarius, however,
shows how fickle the rule.  The constellation's brightest star,
Kaus Australis, is Epsilon, while number two, Nunki, is of all
things Sigma.  The Alpha and Beta stars (Rukbat and Arkab), far to
the south, are hardly consequential at all except that they make a
significant part of the Archer ("Arkab" from Arabic meaning "the
archer's achilles tendon").  In his "Uranometria," however, Bayer,
seems to have been fooled into thinking that the stars are really
brighter than they are, perhaps because of their low position and
an overestimation of dimming by the Earth's thick atmosphere. 
Arkab is really two stars, the western and brighter Arkab Prior
(the leading one as the pair goes across the sky), or Beta-1, and
Arkab Posterior (Beta-2), both only fourth magnitude (respectively
3.95 and 4.28).  In spite of their proximity, they are not a real
couple, Arkab Prior 380 light years away, Arkab Posterior, at 140
light years, only about a third as far.  Both Arkabs are
understudied.  Arkab Prior's listed temperature of 13,630 Kelvin is
way out of line with its B9 dwarf class, which implies more like
11,000 Kelvin.  Adopting the lower value (from which we derive the
amount of invisible ultraviolet light), its magnitude and distance
then giving a luminosity 440 times that of the Sun, a radius nearly
6 times solar, and a mass 3.5 times solar.  Near the end of its
hydrogen-fusing lifetime (if indeed the star has not already given
it up), Arkab Prior has a dimmer, hydrogen-fusing, seventh
magnitude (7.11), class A (A5), 1.8 solar mass dwarf companion,
which lies at least 3300 Astronomical Units away (83 times Pluto's
distance from the Sun), the pair taking at least 82,000 years to
orbit each other.  From Arkab-1 A (the brighter), the companion
would shine with the brightness of our full Moon, whe

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, September 21, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Our Moon passes its first quarter early in the week, on Monday the
24th, brightening as it heads toward its full phase next week. 
That same night it will make an especially good pass at Mars (now
in Sagittarius), when in the early evening it will be only 2
degrees directly to the north of the red planet.  The configuration
will provide a fine chance to see the lunar motion, as the Moon
moves through its own diameter in about an hour.  The night of
Wednesday the 26th, the Moon will visit Neptune, the night of
Thursday the 27th Uranus, both planets within the confines of
Capricornus.

The morning sky remains glorious, with Saturn, brighter Jupiter,
and yet-brighter Venus all strung out on a line from high in the
sky toward the eastern horizon, the three planets roughly defining
the ecliptic -- the plane of the Solar System -- rather like a
dotted line.  Saturn (still in Taurus), now rising around 10:30
Daylight Time, passes a watershed during this round of evening
visibility when it begins retrograde -- backward -- motion the
night of Wednesday the 26th, as the Earth prepares to pass in
between the Sun and the ringed planet.  Jupiter (beautifully set
amidst the stars of Gemini), rising around 12:30 AM Daylight Time,
prepares to move into evening as well.

It is planet Earth, however, that takes center stage, when it
crosses into the sky's southern hemisphere at the autumnal equinox
in Virgo at 6:04 PM Central Daylight Time (7:04 Eastern, 4:04
Pacific) on Saturday the 22nd, and astronomical autumn begins in
the northern hemisphere (and spring in the southern).  On that day
the Sun will rise very close to due east and set close to due west
all over the Earth except at the poles.  At the north pole, the Sun
will officially set (though because of its finite diameter and
atmospheric refraction it will still be visible); at the south pole
scientific stations, the Sun will officially rise and be up for the
next six months.  At the Earth's equator, the Sun will pass through
the zenith, the point overhead.  

As we move into fall, the summer constellations begin to make their
exit to the west, Scorpius appearing low in the southwest, giant
Ophiuchus, the Serpent Bearer, standing above the scorpion more to
the west.  To the east look for the Great Square of Pegasus, which
has already risen by nightfall.  To the left of the Great Square,
runs the graceful string of stars that makes most of Andromeda, and
coming up in the northeast is the "W" of Cassiopeia.  Still, the
memory of summer lingers through the Summer Triangle of Vega,
Deneb, and Altair, which in early evening is nearly overhead in
mid-northern latitudes.

STAR OF THE WEEK.  ZETA OPH (Zeta Ophiuchi).  Just over the line
into third magnitude (2.56), and third brightest star within the
constellation Ophiuchus (the Serpent Bearer), Zeta Ophiuchi is
oddly not graced with a proper name, which is odder still since it
is in the middle of the line of stars that make the bottom border
of the constellation, the others, from west to east being Yed Prior
(Delta Ophiuchi), Yed Posterior (Epsilon), and Sabik (Eta) (though
Zeta has been known to share that name with just-barely-brighter
Eta).  Too bad too, as Zeta Oph is truly the magnificent star, a
blue-white class O (though at 09.5 just barely) hydrogen-fusing
"dwarf" (a strange term for a star with a diameter 8 times that of
the Sun).  The star (which is very slightly variable), however,
does not LOOK so blue-white.  Zeta Oph is one of the brighter stars
in the sky to be significantly affected by absorption and reddening
of its light by passage through interstellar dust (which lies
everywhere within the Milky Way).  At a distance of 460 light
years, the star is deeply involved with dust gas clouds (and even
illuminates one of them), and is used as a background light source
with which to examine the stuff of interstellar space.  If the dust
were not in the way, Zeta would shine at almost first magnitude. 
>From its distance and great temperature of 32,500 Kelvin (from
which we can account for the star's fierce ultraviolet light), we
calculate a magnificent luminosity of 68,000 times that of the Sun,
and from that a mass of 20 times solar, the star about in the
middle of its short (8 million years) hydrogen-fusing lifetime. 
Like most luminous stars, Zeta Oph is losing mass through a strong
wind that in this case blows at about 1600 kilometers per second at
a rate of about a hundredth of a millionth of a solar mass per
year.  The star's only fate seems to blow up as a supernova.  Among
Zeta Ophiuchi's most interesting properties is that it is one of
the sk

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, September 14, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

To those who have suffered losses, my heartfelt sympathies.

The Moon passes through its new phase this week on Monday the 17th,
rendering the stars wonderfully visible throughout the night
(weather and stray lights permitting).  In the morning sky, the
waning crescent puts on a show, when on Saturday the 15th it passes
through the Sickle of Leo, and will be found down and to the left
of brilliant Venus, and up and to the left of the star Regulus. 
The waxing crescent will then begin to make its evening appearance
in the western sky the night of Tuesday the 18th.  The day before
the new phase, the Moon passes perigee, when it is closest to the
Earth at a distance of only 358,130 kilometers (222,530 miles).  

Mercury makes an appearance this week, as it passes greatest
eastern elongation relative to the Sun also on Tuesday the 18th
(when it is down and to the left of the lunar crescent). 
Unfortunately, the evening ecliptic lies near its flattest against
the horizon this time of year, so the little planet will appear low
and difficult to see.  On Thursday the 20th, Mercury will pass only
a degree to the south of Spica in Virgo.  The morning sky again
puts on a better show when Venus passes only half a degree (the
angular diameter of the Moon) north of Regulus, also on the 20th. 
We thus have the odd and rare situation where the two stars closest
to the ecliptic -- which hold the autumnal equinox between them --
are being visited by the two inner planets, and not just on the
same date, but at nearly the same moment, about 3 PM Central
Daylight time.

The outer planets are not to be ignored, however.  In the evening,
Mars glows in the southwest.  Moving easterly against the
background, the reddish planet has shifted into the confines of
Sagittarius.  In the morning, Jupiter, second only to Venus in
brilliance, shines high above that planet, with fainter but still-
prominent Saturn higher yet, the two giant planets still in fairly
close proximity.

In the evening, the Summer Triangle, made of Vega, Deneb, and
Altair, are at their best, Vega nearly overhead for those at mid-
northern latitudes as twilight fades.  To the north of Vega, find
the two stars that make most of the head of Draco the Dragon, the
pair appearing as the Dragon's baleful eyes.  Winding toward the
north, Draco's tail eventually passes between the Big Dipper in
Ursa Major (now going into its autumn hibernation beneath the pole)
and the Little Dipper in Ursa Minor, which in early evening stands
high upon its curved, though faint, handle.  The handle ends at
Polaris, which closely marks the North Celestial Pole, about which
the northern sky seems to turn.

STAR OF THE WEEK.  NODUS SECUNDUS (Delta Draconis).  Not a terribly
romantic name, Nodus Secundus, Draco the Dragon's Delta star, is
less-well-known from Arabic as Al Tais, the Goat.  Better to use
the Latin "Nodus," as it more refers to its host constellation,
where it marks the second (Secundus) of four loops in the Dragon's
long winding mythological body.  Nicely visible at mid-third
magnitude (3.07), and one of the brighter stars of the far north,
it lies only a bit more than 20 degrees from the sky's north pole
(marked by Polaris), and is circumpolar (always visible) from
everywhere north of the Tropic of Cancer.  Within its daily circle
about the North Celestial Pole, it is exceeded in brightness only
by Polaris, Kochab, and Pherkad, the Alpha, Beta, and Gamma stars
of Ursa Minor.  Physically, Nodus number two is a class G (G9,
almost class K) giant star, though since it is a bit warm (4830
Kelvin) for such a giant, not quite so large as many.  At a
distance of almost exactly a light-century (100 light years), the
star radiates at a rate of 63 solar luminosities, giving it a
radius 11 times that of the Sun.  Its luminosity and temperature
combine to yield a mass almost exactly 2.5 times solar and an age
of 700 million years.  Nodus Secundus is a fine example of a
helium-fusing giant, one that is now converting its core helium
into carbon and oxygen.  Sometime in the astronomically near
future, the helium will run out and the star will brighten as it
prepares to slough its outer envelope and become a mid-mass white
dwarf.  It is not clear whether Nodus-2 is a single or double star. 
Lying 88 seconds of arc away is a faint twelfth magnitude
"companion" about which nearly nothing is known.  The seeming
proximity may be just a line of sight coincidence.  Yet the two
seem to be regarded as a real double.  If so, "Delta Draconis B" is
at least 2700 Astronomical Units from the giant.  Nearly 70 times
Pluto's distance from the 

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, September 7, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon, waning early in the week through its gibbous phase,
passes its third quarter shortly after it sets during the day on
Monday the 10th.  It will thereafter wane as a crescent as it loops
through the most northerly constellation of the Zodiac, Gemini.  

Just before the Moon reaches the quarter, it will occult, or pass
over, Saturn, now in Taurus.  Unfortunately, the event takes place
in daylight for a good fraction of North America.  On the west
coast, however, the sky will still be dark enough however, the
occultation beginning in California around 5 AM Daylight Time
(specific times need a city-by-city timetable).  Hawaiians will get
a grand view around 1 AM.  Two days later, on Wednesday the 12th,
the Moon will do the same to Jupiter.  The Americas lose out even
more, however, this event visible best in Europe.  At least on the
night of Sunday the 9th we can see the Moon approach Saturn, then
be on the other side the night of the 10th, and then watch the same
act the nights of the 11th and 12th as the Moon approaches, then
recedes from, Jupiter.

The brightness of the Moon (which shines only by reflected
sunlight) at different phases depends on the degree to which
mountains (really crater walls) throw shadows, and on the local
reflectivity of the rock.  The full Moon is 8 or so times brighter
than the first quarter.  Although the same area appears illuminated
at both first and third quarters, the first is twice as bright as
the third, principally because of the extent of a huge dark lava
plain called Oceanus Procellarum (the "Ocean of Storms"), the
largest dark area visible to the naked eye.  (The allusion to
oceans and seas on the Moon comes from a time when people thought
they might really exist -- the dark areas are lava-filled impact
basins, not ocean basins.  The Moon has no significant water).

The two planets that flank the Earth remain firmly set as opposing
brilliant jewels that grace the early evening and morning.  Look
for bright reddish Mars to the just to the west of south just after
dusk, the planet setting around midnight, and then for even-
brighter white Venus, which rises around 4 AM, well before dawn. 
If you are in a dark location, Venus may lead your eye to one of
the more subtle sights of the Solar System, the "zodiacal light,"
which is caused by the scattering of sunlight from dust particles
that lie in the plane of the solar system (and thus through the
constellations of the Zodiac).  The best time to see the phenomenon
in the northern hemisphere is in autumn mornings and spring
evenings.  Before the beginning or morning twilight, the zodiacal
light (sometimes called the "false dawn") is now nicely visible as
a faint cone of light standing upward from the eastern horizon.

STAR OF THE WEEK.  ALPHA SCT (Alpha Scuti).  Eighty-eight formal
constellations grace the sky (along with many more informal ones
plus figures that have long-since fallen into disuse).  Over half
come down from ancient times.  The rest are of "modern" origin,
that is, created between about 1600 and 1800 as astronomers
furiously competed with "filling in the blanks" between the ancient
constellations.  Lying in the Milky Way between ancient Aquila and
Sagittarius is the modern constellation Scutum, the Shield, which
honors the Polish king John Sobieski for his defense of Vienna in
1683.  Though the stars are faint, the northern part of the Shield
stands out mightily as a bright patch of the Milky Way.  None of
its stars carry proper names, even the brightest of them, which is
known simply as Alpha Scuti, abbreviated as an unpronounceable
"Alpha Sct."  Alpha Scuti, which shines at only fourth magnitude
(3.85), is yet one more orange class K giant, though one with a bit
of a difference, at class K3 slightly cooler than most, its
temperature measured at 4280 Kelvin.  From its distance of 175
light years it radiates 134 solar luminosities from a surface
swollen to 21 times that of the Sun, about a quarter the size of
the orbit of Mercury.  Decidedly single, Alpha Scuti is reported to
be slightly variable, its brightness varying by about 10 percent. 
No one seems to know the period of oscillation, however (if indeed
it has one), or what class of variable it might be.  The most
interesting aspect of the star is its ambiguous evolutionary
status.  As solar-type stars age after they cease hydrogen fusion
in their cores, they (1) brighten as they become giants with dead
helium cores; (2) shrink a bit as they begin to fuse helium to
carbon; (3) fuse carbon; (4) brighten even more with dead carbon
cores; (5) lose their outer envelopes and become dim wh

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, August 31 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Precious little actually "happens" this week.  The Moon brightens
and heads to full, the phase reached early, the night of Sunday,
September 2.  Almost exactly one day before technical "fullness,"
the Moon also passes its apogee point at a distance of 406,330
kilometers (252,480 miles) from the Earth, the maximum distance
minimizing high ocean tides.  As autumn comes on, the full Moons
fall farther to the north, this one near the Aquarius-Pisces
border.  Traditionally, the September full Moon is called the
"Harvest Moon."  The ecliptic, the apparent path of the Sun, is in
the evening near its flattest to the horizon, and as a result the
delay in moonrise from night to night is at a minimum, the effect
flooding the fields with bright moonlight after sundown.  The
October full Moon, the "Hunter's Moon," actually fills the bill
better, as this year it occurs quite a bit closer to the beginning
of fall on September 22, when the Sun will cross the autumnal
equinox.

In spite of the week's quietness, the sky still keeps on turning,
stars and planets rising and setting daily and shifting to the west
an extra degree every day as the Earth goes around the Sun.  In the
evening, bright Mars hangs to the south-southwest after dark, while
Saturn (much fainter, but still brighter than any star of the
northern hemisphere) begins to make its move into the evening sky,
now rising just before midnight daylight time.  About an hour and
a half later, bright Jupiter (which has pulled well the west of
Venus) lofts itself over the northeastern horizon; Venus saves
itself for around 4 AM, rising just over an hour before morning
twilight commences.

Though bright moonlight floods the nightly scene early in the week,
you can still admire the glorious Summer Triangle, which consists
of Vega (in Lyra), now high in the sky for northerners as night
falls, Deneb (in Cygnus) to the east of Vega, and Altair (in
Aquila) to the south of the pair.  When the Moon finally disappears
from early evening, if you live in a dark site you can see the
Milky Way flooding southward through the Triangle.  As the Milky
Way leaves Aquila, it brightens through the modern constellation
Scutum (the Shield), and then even more so through Sagittarius, the
Archer, whose arrow points toward the center of the Galaxy.  In the
other direction, the Milky Way flows from Cygnus into the coming
autumn constellations, through Cassiopeia and then into Perseus. 
Already the flying horse of the Perseus myth, Pegasus and its Great
Square, are climbing the eastern sky in late evening.  There are
only three weeks to go until the beginning of autumn in the
northern hemisphere, spring in the southern.
STAR OF THE WEEK.  RUKBAT (Alpha Sagittarii).  Alpha stars are,
according to logic, tradition, and expectation, supposed to be the
brightest in their constellations.  Sure there are exceptions --
Alpha Orionis (Betelgeuse) is slightly fainter than Beta (Rigel) --
but they are minor and understandable.  Rukbat, of Sagittarius, is
among the most dramatic of counter-examples, an Alpha star that
lies at mid-fourth magnitude (3.97) and is hard to see from any
lighted town.  (Sagittarius's brightest star, second magnitude Kaus
Australis, Epsilon Sagittarii, is actually the brightest.)  The
name, which from Arabic refers to "the Archer's Knee," clearly
indicates that the star's residence is Sagittarius, and that it is
not some interloper.  (It even has a second name, "Alrami," that
means "the Archer.")  Not only did Bayer assign it "Alpha," but in
his great star atlas of 1602 (the "Uranometria"), he draws it
vastly brighter than it really is (as he does also-dim Arkab, the
Beta star).  No one knows why.  Rukbat is very far south, indeed
not even visible north of 50 degrees north latitude, so Bayer may
have had difficulty in knowing its brightness.  An alternative
speculation might be that Rukbat has simply faded over the past 500
years, but rather ordinary class B (B8) hydrogen-fusing dwarfs do
not do that.  Rukbat, 170 light years away, radiates 112 solar
luminosities from its 12,370 Kelvin blue-white surface, the star
2.3 solar diameters across.  Its temperature and luminosity give an
ambiguous status.  Rukbat may indeed be a dwarf, one of 3.2 solar
masses; but it may also be near the end of its hydrogen-fusing
lifetime, and at 3 solar masses may be becoming a growing
"subgiant."  Practically ignored in the scientific literature
(mentioned in but one publication per year), the star still has a
few things to recommend it.  Rukbat's spectrum indicates that it
may have a companion -- a careful search for one, however, turned
up

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Sunday, August 24 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon passes through its first quarter early in the week, on
Saturday, the 25th, thereafter waxing toward its full phase,
brightening as it goes through Scorpius and Sagittarius, bottoming
out at its most southerly position of the month the night of
Tuesday, the 28th.  The night of Saturday, the 25th, the Moon will
pass 12 degrees north of Antares in Scorpius, while the following
night it will be approaching its passage to the north of Mars.  The
night of Thursday the 30th, it is Neptune's turn to be visited.  

Mars is now transiting the meridian to the south in bright
twilight; by darkness it has moved into the southwest, where, as
seen in the early evening, it will remain throughout the year.  For
observers in the mid-northern hemisphere, setting time has moved to
just after midnight daylight time.  But as Mars prepares to set,
Saturn rises in the northeast, followed around 2 AM by Jupiter,
which has now pulled rather far to the west of Venus, the brilliant
"morning star" rising before the onset of twilight until the end of
October (by which time Saturn and Jupiter will be rising in early-
to-mid evening).  

Not that anyone will notice, but Pluto ceases its westerly
retrograde movement against the stars of southern Ophiuchus this
week, on Saturday the 25th.  Three days later, Ceres, the largest
asteroid (570 miles -- 910 kilometers -- wide and also invisible
without a telescope), does the same thing.  The orbits of asteroids
are commonly more highly tilted than are those of the planets
(Pluto excepted).  Ceres, now beneath the Little Milk Dipper in
Sagittarius, is about as far south as it gets, some 8 degrees below
the ecliptic (the apparent path of the Sun).  

As August heads towards September, the sky's fifth brightest star,
Vega in Lyra, passes nearly overhead in early evening for those in
mid-northern latitudes.  A bit farther north (and a bit west) is
the much fainter head of Draco, the Dragon.  A line drawn south
from Vega passes through the line of stars that makes the tail of
Serpens the Serpent (the only constellation that comes in two
parts, the head and tail divided by Ophiuchus), then much farther
down back to Sagittarius, which sits atop Corona Australis, the
Southern Crown.  Once the Moon gets out of the way, you can admire
the great star clouds of the Milky Way that seem to blanket
Sagittarius, the celestial archer, one of two mythological centaurs
in the sky, the other Centaurus, a much larger constellation now
escaping to the west, its southern portions far below the horizon
for most people in the northern hemisphere.

STAR OF THE WEEK.  MENKENT (Theta Centauri).  Centaurus is
dominated by its two brightest stars, Rigil Kentaurus (Alpha
Centauri, third brightest star in the sky and the nearest star to
the Earth) and first magnitude Hadar (Beta Centauri).  Though these
two shine brilliantly to the lucky residents of the southern
hemisphere, neither is visible from mid-northern latitudes, so if
those that live there wish to know Centaurus, they must begin with
the third brightest star in the constellation, Menkent, to which is
assigned the rather lowly letter Greek letter Theta (Gamma Centauri
rather oddly coming in second).  The name comes from an Arabic word
for "shoulder" (of the Centaur), to which is attached the Latin
abbreviation for "Kentaurus" for Centaur, tying Menkent back to the
constellation's luminary, Rigil Kentaurus.  Menkent, at mid-second
magnitude (2.06) is but four percent fainter to the eye than
Polaris.  Much closer than Polaris, however, only 61 light years
away, it is intrinsically much less luminous.  At the warm end of
class K (K0), the star is a near-clone of the northern hemisphere's
Pollux, just fainter to the eye than Pollux because it is 80
percent more distant.  Menkent, with no known or even suspected
companions, is about as pure a sample of its class you can come by. 
>From its 4780 Kelvin surface, this yellow-orange star radiates at
a luminosity 60 times that of the Sun, the star's radius 11 times
solar.  Well along in its evolution, Menkent is now fusing helium
into carbon and oxygen in its deep core.  The star's only offbeat
property is its rather high "proper motion," its speed across the
line of sight.  Approaching us at only one kilometer per second, it
is speeding past us at 65 kilometers per second, about twice
"normal," suggesting that the star really belongs to the outer part
of the Galaxy's disk and is only visiting the solar neighborhood. 



Jim Kaler
Professor of Astronomy   Phone: (217) 333-9382
Universit

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the 5-day period starting Sunday, August 19
2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

During this shortened period, the Moon waxes through its crescent
phase, first quarter reached next Saturday, August 25th.  The night
of Wednesday, the 22nd, the Moon will make a nice pass to the north
of the star Spica in Virgo, which is now seen well to the southwest
as twilight draws to a close.  

In spite of the growing Moon, the evening still belongs to Mars,
which shines brightly just to the west of south as the sky darkens. 
Follow its progress as it ever-so-slowly (but with increasing
speed) pulls to the east of reddish Antares in Scorpius, the star
not very far to the right of the red planet.  Three planets ride
the morning sky, the leader Venus.  Moving to the east against the
stars of Gemini, the brilliant planet passes seven degrees to the
south of Pollux, Gemini's brightest star, on Wednesday, the 22nd. 
To the west of Venus, find bright Jupiter, and rather well to the
west of Jupiter is Saturn, the two giant planets of course far
beyond Venus, Jupiter almost 5 times farther away, Saturn nearly
eight.  

The night of Tuesday, the 21st, the Winter Solstice in Sagittarius
will be directly south at 9 PM Daylight Time.  The Solstice, the
most southerly point of the ecliptic, marks the position of the Sun
on the first day of northern winter.  Just rising exactly in the
east at that time will be the Vernal Equinox, the point at which
the solar path crosses the equator.  

Immediately to the left of the equinox, which lies 23.4 degrees
south of the celestial equator, is a small fuzzy spot visible to
the naked eye, a large cloud of interstellar gas and dust, the
Lagoon Nebula, or Messier 8.  Binoculars will make the cloud
immediately jump from the background.  Almost directly north, and
nearly overhead at mid-northern latitudes, lies one of the most
ancient constellations of the sky, Hercules, the celestial memorial
to the great hero, the figure originally known as the "Kneeler." 
Toward the northwestern corner lies another fuzzy spot, the
greatest of the northern globular clusters, Messier 13, the "Great
Cluster in Hercules."  The telescopic view of the cluster, which
contains somewhere around a million stars, is stunning.  To the
west of Hercules is the semi-circle that makes Corona Borealis, the
Northern Crown; its southern hemisphere counterpart, Corona
Australis, the Southern Crown, lies south of Sagittarius.  To the
east of Hercules find Lyra, the Harp, brilliantly marked by Vega,
the fifth brightest star in the sky.

STAR OF THE WEEK.  ZETA HER (Zeta Herculis).  With the exception of
the brightest stars, star names were handed out more by position
than brightness, as attested to by Zeta Herculis.  At bright third
magnitude (2.81), just barely the second brightest star in the
constellation Hercules (right behind Kornephoros, Beta Herculis),
Zeta Herculis was ignored by the ancients.  Even Bayer rather
ignored it by giving it the sixth letter in the Greek alphabet, the
Alpha designation going to faint-third-magnitude Rasalgethi clearly
because of its position in the Hero's head.  In spite of the star's
lack of public prominence, it has a lot to recommend it.  Zeta Her
is actually double, a modestly bright third magnitude star orbited
by a sixth (5.53) companion only a second of arc or so away.  The
brighter star, Zeta Her A, is a class G (G2) subgiant with the same
temperature (5780 Kelvin) as the Sun (which is also a G2 star). 
With a mass some 50 percent greater than the Sun, however, and
beginning its evolution toward gianthood (its core hydrogen fusion
likely shut down), Zeta Her A is 6 times more luminous than the Sun
with a radius 2.5 times as large.  Nevertheless, the star gives a
good idea of what the Sun would look like from a great distance, in
Zeta Her's case 35 light years.  The companion, a cooler class G
(G7) hydrogen-fusing dwarf with a luminosity only 65 percent that
of the Sun and a mass about 85 percent solar, orbits with a period
of 34.5 years at a mean distance of 15 Astronomical Units (over 50
percent farther than Saturn is from the Sun).  A rather high
eccentricity takes the two as far apart as 21 AU and as close as 8
AU.  Under such conditions, planets would very likely be
impossible.  Astronomers have identified a number of extended
"moving groups" of stars that seem to have some common origin (the
most famed the stars that are related to the Ursa Major cluster). 
The "Zeta Herculis moving group," of which the star is the leader,
contains stars as far removed as Perseus, Lupus, and Octans, the
dim constellation that surrounds the South Celestial Pole.


  


*

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, August 10 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The next Skylights will appear Sunday, August 19.  As the week
begins, the Moon is nearly in its third quarter, the phase passed
the night of Saturday, the 11th.  During the remainder of the week
it will wane in the crescent phase until Saturday the 18th, when it
will pass new.  Only three hours after new, the Moon passes
perigee, the coincidence causing near-maximum ocean tides.  The
last glimpse of the crescent will be in twilight the morning of
Friday, the 17th.  

As the crescent wanes, it will appear to the west of Saturn (still
in Taurus to the east of the Hyades) the morning of Monday the
13th, and to the east of the ringed planet the following morning. 
The morning of Wednesday the 15th, the Moon will appear up and to
the right of Jupiter (the planet in western Gemini).  During the
day, around 3 PM Central Time, the Moon will actually occult (pass
over) Jupiter, the event visible through a telescope.  (Saturn's
occultation on the 13th is visible only in Africa and India).  The
most striking pairing will occur the morning of Thursday the 16th,
when Moon will appear nearly to brush brilliant Venus, the event
well worth getting up at dawn to see.  While less dramatic, the
evening still holds bright Mars, the planet still just a bit to the
east of Antares in Scorpius.  Finally, distant Uranus, now in
retrograde in eastern Capricornus, passes opposition to the Sun on
Wednesday, the 15th.  

The week, however, as always this time of year belongs to the famed
Perseid meteor shower, which will be at maximum the morning of
Sunday the 12th.  Meteors are caused by small rocks from space that
heat and streak through the Earth's upper atmosphere.  The Perseid
shower typically sends 50 to 100 meteors per hour (the show best
seen well after midnight).  Unfortunately, the third quarter Moon
will light the sky this year, taking out the fainter meteors. 
Nevertheless, the brighter ones will shine through.  The meteors
are the stony debris from Comet Swift-Tuttle, which on its 130 year
orbit last passed by the Earth in 1992.  The direction of the
meteoroid stream coupled with the direction of the Earth makes the
meteors seem to come from the constellation Perseus.  The best
place to look, however, is straight up.

When observed at the same time of night, the stars slip slowly past
us by a degree each successive day as the Earth orbits the Sun.  To
the south at 9 PM Daylight Time find the tail of Scorpius, to the
east the figure that makes Sagittarius.  Above Sagittarius is a
bright patch of Milky Way coupled to modern constellation Scutum,
the Shield.  Below Sagittarius are a few stars that make the modern
constellation Telescopium, the Telescope, whose stars appear
overhead to those near 50 degrees south latitude.  To the far
north, the head of Draco, the Dragon, is high in the sky, and near-
overhead for those near latitude 50 degrees north.  

STAR OF THE WEEK.  ALPHA TEL (Alpha Telescopii).  The modern
constellations, those that survive, honor two cognate scientific
instruments, Microscopium, the Microscope and Telescopium, the
Telescope, the first south of Capricornus, the latter south of
neighboring Sagittarius.  Neither constellation is very bright, the
luminary of Telescopium barely fourth magnitude (3.51) Alpha
Telescopii, which like most of the stars in modern constellations
has no formal proper name.  Though the brightest star of its
constellation, it is nearly as obscure as the constellation itself,
mentioned in only about one scientific paper a year.  A class B
(B3) blue subgiant (more about that below) almost exactly 250 light
years away, this quite-luminous star shines with the light of
almost 900 Suns from a surface heated to 18,400 Kelvin.  The
luminosity and temperature tell that the star has a mass six times
that of the Sun.  In opposition to the subgiant spectral class,
which suggests that the star is beginning to evolve from being a
core-hydrogen-fusing dwarf, the luminosity and temperature strongly
suggest that the Alpha Tel is fairly young and has a long way to go
before any kind of evolution sets in.  The star seems decidedly
single.  Its most significant characteristic, other than being a
rather hot class B star, is that of chemical peculiarity.  There
are several classes of such stars in which various chemical
elements are enriched or depleted as a result of diffusion of atoms
(some settling below the star's gaseous surface, others lofted
upward by radiation, the effect sometimes coupled with magnetic
fields).  The most prominent are the "Am" metallic line stars
(Sirius, for example), the "Ap" (for "class A peculiar," like Cor
Car

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, July 27, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

We begin the week with the Moon passing its first quarter.  For the
next seven days it will wax through gibbous, getting brighter each
night as it moves at its 13 degree-per-day pace against the
background stars.  Since the Sun is now moving through the
northerly constellations of the Zodiac (this week in Cancer), the
Moon -- as it approaches full -- passes through the southerly
constellations, bottoming out in Sagittarius the night of
Wednesday, August 1.  

Aside from the Moon, the evening sky is dominated by reddish Mars,
the next planet out from the Earth.  Shining from southern
Ophiuchus (between the classic zodiacal constellations of Scorpius
and Sagittarius), the planet is slowly picking up easterly speed
against the background stars.  The tilt of its orbit has sent it
several degrees below the ecliptic (the apparent solar path), while
the tilt of the lunar orbit has sent the Moon just north of it.  As
a result, as the Moon approaches Mars the night of Sunday the 29th,
the Moon will be some 6 degrees to the north, just greater than the
spread between the front bowl stars (the "Pointers") of the Big
Dipper (which in the evening is now descending the sky to the
northwest).  The morning, on the other hand, is dominated by the
mythological opposite of the "god of war," Venus, the "goddess of
love and beauty," which is strikingly lovely in the morning hours
to the east before dawn.

It is dim Neptune's week, however, as the farthest of the large
planets passes through opposition to the Sun on Monday, July 30, as
it moves retrograde in Capricornus.  Discovered in 1846 as a result
of its gravitational influence on Uranus, Neptune has yet to make
a full orbit since found, as it takes 165 years to make a complete
circuit of the Sun.  (It will come full circle in 2011.)  Of the
traditional planets of the Solar System (including tiny Pluto,
which now hangs out in Ophiuchus 15 degrees almost exactly to the
north of Mars), only Neptune and Pluto require a telescope to see,
Uranus (somewhat to the east of Neptune) faintly visible to the
naked eye. 

As July moves into August, we find Scorpius on the meridian to the
south as evening descends.  Part of a "double constellation," the
scorpion's claws stretch out to the west as the bright stars of
Libra (the celestial scales), Zubenelgenubi the southern claw,
Zubeneschamali the northern.  Two and a half thousand years ago the
"Balance" held the autumnal equinox (the point where the Sun
crosses the celestial equator on its way south).  Precession, the
26,000 year wobble of the Earth's axis, has long since moved it
westerly into Virgo, just as it has moved the vernal equinox (where
the Sun moves north of the equator) from Aries into Pisces.

STAR OF THE WEEK.  ZUBENHAKRABI (Sigma Librae = Gamma Scorpii). 
There are a small number of "linking stars" that traditionally
belong to two constellations.  Alpheratz, Alpha Andromedae, is also
Delta Pegasi, while Elnath, Beta Tauri, is also Gamma Aurigae (the
two alternatives no longer used since the stars are within the
modern boundaries of Andromeda and Taurus).  Zubenhakrabi is more
extreme.  The star, whose name means "the scorpion's  claw," was
originally part of Scorpius along with the more-traditional "claws"
Zubeneschamali and Zubenelgenubi (which refer to the "northern" and
"southern" claw).  As such, Bayer called it Gamma Scorpii.  But it
was so far west of Scorpius proper and so much a part of Libra that
in the nineteenth century B. A. Gould (who in 1851 founded the
"Astronomical Journal" (one of the world's premier research
publications) gave it to Libra as Sigma Librae, which is how it is
referred to today.  To add to the confusion, early in that century
Elijah Burritt coupled the name "Zubenhakrabi" to Eta Librae!  The
star is as interesting as the story of its name.  Notably fainter
than Zubenelgenubi or Zubeneschamali, third magnitude (3.29)
Zubenhakrabi is a cool class M (M3) rather-luminous red giant. 
>From a distance of 290 light years, it radiates 1900 solar
luminosities from a reddish 3600 Kelvin surface that is swollen to
a radius 110 times that of the Sun (0.52 astronomical units, which
would take the star about halfway between the orbits of Mercury and
Venus).  It is a subtle "semi-regular variable" (in the astro-
trade, an "SRb") that changes its brightness by only 0.16
magnitudes over a 20-day period.  This dying 2 (?) solar mass star,
with its dead carbon-oxygen core, is expanding and brightening as
a giant for the second time (the first brightening was with a dead
helium core) fueled by internal nuclear-burning shells of helium
and hydrogen.

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, July 20, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The week begins with the Moon in its new phase, from which it will
begin to wax through crescent toward its first quarter, that phase
to be reached on Friday, the 27th.  Enjoy the sight as the
partially-lit lunar disk climbs nightly from above the evening
horizon, the crescent outlined to the east with earthlight.  You
might make the first sighting the night of Saturday, the 21st.  By
the following night, the crescent will become easily visible. 
Almost exactly a day after new Moon, the Moon passes its apogee
point, where it is closest the to Earth.  

Mars, moving ever-so-slowly eastward against the stellar background
stands almost exactly on the formal border between Scorpius (to the
west) and Ophiuchus (to the east).  Almost directly east of first
magnitude Antares in Scorpius and almost due south of second
magnitude Sabik in Ophiuchus, and quickly picking up angular speed,
the red planet will cross into Sagittarius on September 1.  Very
near its southerly limit of 27 degrees south of the equator, Mars
is currently invisible from the Arctic, indeed north of 63 degrees
north latitude.  Venus, on the other hand, shining in brilliant
splendor in morning skies to the east of the Hyades star cluster in
Taurus, is quite far north, and is not now visible over most of
Antarctica.  Saturn, having just passed conjunction with Venus, now
stands slightly to the west of her and even closer to the Hyades. 
Look down and to the left of Venus to find Jupiter, which has just
crossed over into Gemini and rises about as morning twilight
begins.  Jupiter, nearly opposite Mars, is about as far north as it
will become, 23 degrees north of the equator.  

Mars in the evening; Venus, Saturn, and Jupiter in the morning; at
midnight a full view of the constellations of northern summer. 
Vega, in Lyra, shines nearly overhead from mid-northern latitudes. 
With Deneb, a bit to the east, and Altair to the south, it forms
the Summer Triangle.  Low to the south, below the celestial
equator, is the "Teapot" of Sagittarius.  Having crossed the
meridian to the south about 10 PM daylight time, at midnight Mars
stands in the southwest rather than in the southeast.  

Overflowing constellation boundaries, the Milky Way cascades
through Cygnus southward into Sagittarius and then into the
glorious deep southern hemisphere not visible from northern climes. 
Consisting of the combined light of billions of faint stars, the
Milky Way is the disk of our flattened Galaxy.  Some 26,000 light
years away, in the heart of the black blotches seen against the
Milky Way's background, lies the center of the Galaxy, thought to
be made of a two and a half million solar mass black hole with
gravity so fierce even light cannot escape.

STAR OF THE WEEK.  SABIK (Eta Ophiuchi).   At the dim end of second
magnitude (2.43), and (after Rasalhague, Alpha Ophiuchi) the second
brightest star in Ophiuchus, Sabik still received the lowly Eta
appellation from Bayer.  The anomaly is the result of Bayer's
distributing the Greek letters not so much in order of brightness
in the constellation but in order of position: Alpha, Beta, and
Gamma lie at the northern end of the sprawling figure, while Delta
through Eta form the Serpent Bearer's lower "skirt," Sabik the
southern-most.  The star's Arabic name, something of a mystery,
refers to one that "precedes" or "comes in first," and may perhaps
have to do with Sabik's position at the end of the stream of stars
at the bottom of the constellation (though if it were a race, it
would come in last as Ophiuchus moves toward the west).  The star
is a close and rather unusual double that is very difficult for the
amateur to resolve.  Two third magnitude class A stars (A2 at
magnitude 3.0 and A3 at magnitude 3.5) swing around each other in
mutual orbit every 88 years.  The angular orbital size is only 1.3
seconds of arc, and the two are usually much closer than that.  At
the star's distance of 84 light years, 1.3 seconds corresponds to
33.5 Astronomical Units, a bit farther than Neptune is from the
Sun.  The most unusual aspect of the system is the very high
orbital eccentricity of 0.94, which means that the stars come as
close as 2 AU (0.5 AU farther than Mars is from the Sun) and then
only 44 years later are 65 AU (over half again farther than Pluto
is from the Sun) apart, the separation varying by a factor of 32. 
The gravitational disturbances caused by such an orbit would make
planets impossible (and indeed there is no evidence for
circumstellar dust).  Otherwise, the stars are rather ordinary. 
The brighter has a temperature of around 8900 Kelvin, a luminosity
35 times th

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, July 13, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Skylights is presented a day early this week.
We begin the week, Friday the 13th, with the Moon in its third
quarter, from which it will wane toward new, that phase reached
next Friday, the 20th.  

The week is highlighted by numerous passages.  As the Moon wanes,
it will in succession occult, or cover (seen only in specific parts
of the world), Saturn, Venus, Jupiter, and Mercury, the first two
on Tuesday the 17th, Jupiter a day later, and Mercury a day after
that.  (Be sure to look in the morning sky on Wednesday, the 18th,
to see the Moon approaching Jupiter.)  The occultations of Saturn
and Jupiter will respectively be seen in South America and in the
west Pacific, that of Mercury in northern Europe and the Arctic. 
The occultation of Venus, however, will be beautifully visible in
North America.  Unfortunately, the event takes place during
daylight, shortly after noon.  Fortunately, that matters little to
those with a telescope -- even binoculars -- as both the Moon and
Venus are visible in the daytime.  Just scan the binoculars well to
the west of the Sun (avoiding the Sun itself!) until you pick up
the Moon, and there will be Venus.  Exact times depend on latitude
and longitude (and time zone).  For Chicago, Venus disappears
behind the Moon at 1:16 PM CDT and reappears at 2:28.  For New York
the times are 2:32 and 3:33, while for San Francisco they are 10:08
and 11:42.

The planets pass through other conjunctions as well.  On the
morning of Saturday the 15th, brilliant Venus makes an extremely
close pass to Saturn, the two (Saturn much the farther away and
fainter) only 0.3 degrees apart!  At the same time, both are only
3 degrees north of the star Aldebaran in Taurus.  (Be sure to
admire the Pleiades just above the group.)  

Mars, the lone planet in the evening sky, calls for some attention
as well.  Since last May 11, the red planet has been moving
retrograde, or to the west against the background stars.  Now well
past its opposition with the Sun, and beautifully visible in the
early evening to the southeast, Mars stops retrograding on Thursday
the 19th.  Seemingly stationary for a few days (of course ignoring
its daily passage across the sky), it will soon begin an obvious
and rapid motion eastward as the Earth slowly pulls away from it.

Directly to the west of Mars is the ever-engaging Scorpius with its
bright reddish star Antares.  North of Antares lies the sprawling
paired constellations of Ophiuchus and Serpens, north of these
Hercules.  Directly south of Scorpius's curved tail is Ara the
Altar, which requires you to be south of roughly 30 degrees north
latitude to see much of it at all.   South of Scorpius's head and
claws (represented by Libra, the Scales), however, is the bright
constellation of Lupus the Wolf, the northern part of which is
easily visible up to 45 degrees north latitude or even a bit
higher.  

STAR OF THE WEEK.  KAKKAB (Alpha Lupi).  Don't take the proper name
too seriously: it's only partial and no one ever uses it anyway. 
According to R. H. Allen, the full name, apparently from the
ancient Euphrates Valley, is "Kakkab Su-gub Gud-Elim," meaning "the
Star Left of the Horned Bull" (Centaurus).  Far better to know it
by its Greek letter name Alpha Lupi (the luminary of Lupus, the
Wolf).  This blue-white second magnitude (2.30) star lies in one of
the most southerly of all the ancient constellations.   Shining
south of Scorpius, the figure is hardly known in the north, but
from southern latitudes it is glorious, Alpha Lupi bright even
though 550 light years away.  A hot, class B (B1.5) giant, Alpha
Lupi pours 18,000 solar luminosities (most of it in the invisible
ultraviolet) into space from a 21,600 Kelvin surface with a radius
nearly 10 times that of the Sun.  Like a great many hot O and B
stars, "Kakkab" is a member of a loosely organized grouping, an "OB
association."  Huge numbers of them flock the Galaxy.  Alpha Lupi
is a member of the subassociation "Upper Centaurus-Lupus," or UCL,
which in turn is a part of a huge super-collection called the
Scorpius-Centaurus Association.  From analysis of all its members,
UCL lies at an average distance of 450 light years, which fits in
very nicely with our star's individually measured distance of 550. 
As are many hot class B giants, Alpha Lupi is a "Beta Cephei star,"
one exemplified by Mirzam, Beta Canis Majoris.  These are all
subtle variables that pulsate with multiple periods.  With a major
oscillation cycle of 0.259847 days (the periods really known to
such accuracy, to the fraction of a second), in which it varies by
only 0.03 magnitudes (about 3 percent), Alpha

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, July 6, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon falls "between quarters" this week, spending the entire
seven day period in its waning gibbous phase, full moon having
taken place last Thursday, July 5th, the third quarter not being
passed until next Friday, July 13.  As it moves through the
constellations of the Zodiac, it passes south of Neptune on
Saturday the 7th, south of Uranus on Sunday the 8th (both planets
in Capricornus), and its apogee (where it is farthest from the
Earth) on Monday the 9th.  

The planets put on quite a show for us this week.  In the evening,
Mars, four degrees south of the ecliptic and about as southerly as
it can get, glows a brilliant red just to the east of the bright
reddish star Antares in Scorpius.  The planet, brighter than the
brightest star and comparable to Jupiter, lies above the great
curve of stars that make the Scorpion's tail.  Since Mars moves in
orbit only a bit slower than Earth, we will have it with us in the
evening for the rest of the year.  

The big event, however, takes place the morning of Friday, the 13th
(a lucky day!) when Venus, Saturn, and the bright star Aldebaran of
Taurus all gather together into a tight equilateral triangle below
the Pleiades star cluster (Saturn down and to the left of Venus,
Aldebaran below, to the south of Saturn).  Venus is now at its
earliest rising of the year, coming up just before 3 AM daylight
time, about an hour ahead of twilight.  As the sky grows light, you
might then glimpse Jupiter and Mercury just above the east-
northeast horizon, Mercury (the fainter) about two degrees down and
to the right of Jupiter.  The pair comes into conjunction the day
before, on Thursday the 12th.  Three days before, on Monday the
9th, Mercury passes its greatest western elongation, the little
planet 21 degrees to the west of the Sun.  

Even the asteroids get into the act, as Ceres, the largest of them
(though only 900 kilometers across and invisible to the naked eye)
passes opposition with the Sun on Saturday, the 7th.  Only number
4 in discovery order, 500-kilometer wide Vesta (the third largest
of them), can be seen without optical aid, and then just barely. 
Nearly 100,000 of the critters have been discovered.  Vast numbers
of tiny ones small enough to hold in your hand hit the Earth every
day as meteorites.

Two crowns ride the nightly sky, one in the north, the other in the
south.  As the sky darkens, look 30 or so degrees north of the
equator (nearly overhead for mid-northern latitudes) to find the
gentle curve that makes Corona Borealis, the Northern Crown.  If
you are far enough south, around 1 AM you can watch the passage of
Corona Australis, another curve of stars that makes the Southern
Crown, which lies directly below the Little Milk Dipper of
Sagittarius.

STAR OF THE WEEK.  BETA COM (Beta Comae Berenices).  The naked-eye
sky is dominated by luminous stars, stars that are far brighter
than the Sun.  Only a few stars like the Sun and fainter sneak
through.  They simply do not have enough radiative power to be
visible unless they are quite close to us.  ("Selection effects"
like this one, in which Nature shows us "what she wishes," pervade
science: the population of microbes far exceeds that of elephants,
yet only the elephants are visible without some kind of aid.)  It
is then quite surprising to find the luminary of a constellation to
be a near-solar clone.  Fourth magnitude (4.26) Beta Com, with no
proper name at all, just barely beats out Diadem (Alpha Comae
Berenices) for the honor (such as it is) of being the brightest
star within the faint but glorious constellation Coma Berenices
(Berenices Hair).  Though within the formal constellation
boundaries, Beta Com is not a part of the star cluster that makes
the constellation's heart, its distance of only 30 light years
placing it 1/9 as far as the cluster and half as far as Diadem.  At
6000 Kelvin, this class G (G0, alternatively classed as F9.5) star
is only slightly warmer than our class G2 (5780 Kelvin) Sun.  A
hydrogen-fusing dwarf like the Sun, Beta Com is only 37 percent
more luminous than is the Sun and but 10 percent larger, the result
of 10 percent greater mass.  There is some  suggestion that the
star might have a close companion (detectable only via
spectrograph), though such a neighbor is unconfirmed and probably
unlikely.  How sunlike is Beta Com?  It is a bit metal-rich,
containing perhaps 7 percent more iron (relative to dominant
hydrogen) than the Sun.  No planets have yet been spectroscopically
detected as they have for several similar stars.  A search for a
residual dusty disk (one left over from planet formation) around
the star has

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, June 29, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon waxes through its gibbous phase early in the week, and
passes through full on Thursday, July 5.  The night of Monday, the
2nd, it will pass six degrees north of Mars, the red planet so
brilliant that we will have little trouble seeing it even in bright
moonlight.  The tilt of the Martian orbit has taken Mars a bit
below the ecliptic (the apparent solar path), whereas that of the
lunar orbit has taken it a bit above, hence the rather large
separation of the two bodies upon their conjunction.  

If conditions are right for a solar eclipse, as they were on June
21st, then they are usually right for a preceding or succeeding
lunar eclipse (that is, the Moon must be crossing the ecliptic
while new in the case of a solar eclipse, full in the case of a
lunar).  As a result, the Moon will undergo a partial eclipse at
this full phase, on Thursday the 5th.  Unfortunately, the event --
in which the Moon only partly immerses itself in the Earth's shadow
-- will not be visible in North America.  Those in the opposite
hemisphere (Australia, New Zealand, Pacific Ocean), however, will
get a nice view, including Hawaiians.

The Earth also takes the stage, as it passes through its aphelion
point, where it is farthest from the Sun, on the Fourth of July. 
At a distance of 94,502,836 miles (152.088 million kilometers), our
planet will be 3.5 percent farther from the Sun than when it passed
perihelion, its closest point, last January 4, giving us 7 percent
less solar warmth.  Obviously, given the usual northern hemisphere
July heat, the distance between the Earth and the Sun has little to
do with the seasons, which are caused by the 23.4 degree tilt of
the Earth's axis of rotation relative to its orbital perpendicular. 
All things being equal, the distance variation would cause southern
hemisphere seasons to be more extreme than those in the northern
hemisphere, but the effect is lost in the asymmetric distribution
of the oceans, the southern hemisphere far more watery than the
northern.

Venus remains a stunning morning sight, while Mars maintains its
rule over night, the red planet retrograding through the southern
zodiac between Scorpius and Sagittarius.  Look to the right of
bright Mars to find Antares, the scorpion's alpha star.  If you
look down and to the right of Antares, and if you are far enough
south, you can see the bright stars of Lupus the Wolf, one of the
most southerly of the ancient constellations, the wolf held in the
grip of Centaurus, the Centaur, even farther south and to the east. 
If Lupus and Centaurus are out of sight, instead admire the Big
Dipper high overhead in early evening.  Look then to the south to
see the pair of stars that makes most of Canes Venatici (the
Hunting Dogs) and then to the sprawl of fainter stars that make
Coma Berenices, Berenices Hair, a cluster that makes a lovely sight
in binoculars. 

Correction: Last week's Skylights claimed Alpha Comae Berenices
to be the faintest "alpha star."  The magnitudes of the individual
stars of the double were not combined.  The faintest alpha star is
Alpha Octantis, of Octans, the Octant, the constellation that
contains the South Celestial Pole.

STAR OF THE WEEK.  DIADEM (Alpha Comae Berenices).  Not quite the
brightest star in Coma Berenices (Berenices Hair), the Alpha star
just loses out to Beta.  The prominent portion of Coma Berenices is
the beautiful Coma Berenices cluster.  Diadem (Alpha) and Beta are
both off to the eastern side of the constellation, and neither is
a part of the cluster itself (which is 4.4 times farther away). 
The name "Diadem," a jewelled crown in the hair, is of modern and
unknown origin, and is never really used for the Alpha star, which
is commonly known just as Alpha Comae.  Alpha Comae is a close
double star, its two class F (F5) dwarfs (ordinary hydrogen fusing
stars) almost identical to each other, very much as are the twin
class F0 (and just  slightly warmer) stars of Porrima (Gamma
Virginis).  Their apparent magnitudes of 5.07 together make Alpha
Comae a fourth magnitude (4.3) star.  The orbit of the pair is
almost exactly edge-on, causing the two to appear to move back and
forth in a straight line over a period of 25.85 years.  At maximum
separation they are not quite a second of arc apart, while at close
passage (which takes place during the year 2001) they are
effectively inseparable.  The orbital tilt, however, a mere tenth
of a degree against the line of sight, is enough to keep the stars
from eclipsing each other.  Averaging 12 astronomical units apart
(a bit farther than Saturn is from the Sun), they come as close as
6 AU and go as fa

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, June 22, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

As busy as the sky was last week, it is quiet this week.  The
biggest event seems to be the Moon's passing its first quarter on
the evening of Wednesday the 27th just about the time the sky
darkens in North America.  Four days before that, on Saturday the
23rd, during its ascent of the evening sky in the waxing crescent
phase, the Moon passes through perigee, when it is closest to the
Earth.  

Two planets, mythological opposites, now rule opposite portions of
the sky.  In the morning, Venus, the ancient epitome of love and
beauty shines gloriously, its light a creamy white.  This by-far-
brightest of all planets now rises south of the classical figure of
Aries around 3 AM well before twilight begins to brighten the
eastern sky.  In the evening, Mars has already risen by sunset.  By
late evening, its brilliant orange-red glow dominates the
southeastern sky between the classical zodiacal constellations
Sagittarius and Scorpius.  The other ancient planets, those known
from ancient times, Jupiter, Saturn, and Mercury, are out of sight,
though Saturn can be glimpsed in the early dawn.  

Venus and Mars are in reality the opposites of their mythological
natures.  Venus, for all its planetary beauty, is an inhospitable
place, the bright reflecting clouds made of sulfuric acid that
float in a thick, dense carbon dioxide atmosphere that drives the
surface temperature to 470 degrees C (nearly 900 degrees F), about
the temperature of a self-cleaning oven.  Mars, on the other hand,
is for all its cold near-airlessness, a place we could actually
visit, and almost certainly will sometime in the future.  The
temperature can reach the freezing mark, and powerful evidence
shows that water once flowed on the planet, though the air (again
carbon dioxide) pressure -- about 1 percent that of Earth's -- no
longer allows it.  

As the sky darkens, orange Arcturus shines high to the south,
reddish Antares down and to the left and to the right of Mars. 
Compare their colors.  Which appears the reddest?  We probably all
differ in our assessments.  Down and to the right of Arcturus,
Spica shines blue-white, the color contrast between it and the
other three bodies quite noticeable.  To the right of Spica is the
box of stars that makes Corvus the Crow, a springtime constellation
now making way for the stars of summer.  Farther yet below Spica
are the sprawling stars of northern Centaurus.  If you are
reasonably south of about 40 degrees north latitude, you might
glimpse the fuzzy ball of the grandest globular cluster of stars in
the Galaxy, magnificent Omega Centauri.

STAR OF THE WEEK.  ALCHIBA (Alpha Corvi).  It is standard
"knowledge" in astronomy that "Alpha" represents the brightest star
in a constellation, "Beta" the second brightest, and so on.  While
such is often true, the rule is as much broken as held to,
sometimes dramatically.  Alchiba, the Alpha star of Corvus, the
Crow, is a fine example.  One wonders what Johannes Bayer, who
lettered the stars in his great "Uranographia" of 1603, had in
mind.  The name, which from Arabic refers to a "tent" and is meant
to describe the four fairly bright stars that make the distorted
box of Corvus, is now erroneously applied to dim Alpha, which drops
down from the right-hand side of the box and is outstripped by
Beta, Gamma (Gienah), Delta (Algorab), and even Epsilon.  Of the
Alpha stars of the classical constellations, Alchiba, at mid-fourth
magnitude (4.02), ranks number 3 from the bottom, beaten out (if
that is the word) only by Alpha Crateris (Alkes) and Alpha Coronae
Australis (Alfecca Merdiana).  (Of all Alpha stars, including the
modern constellations, the dimness record goes to faint-fifth
magnitude and un-named Alpha Comae Berenices.)  Classification of
Alchiba has been a bit confused.  Once considered a giant and now
given as a class F (F0) dwarf or subdwarf, its luminosity (only
four times that of the Sun) and temperature (7000 Kelvin) strongly
suggest an ordinary hydrogen-fusing dwarf, in fact a near-subdwarf
that shines less brightly than other stars of its temperature
class.  Quite close to us, at a distance of only 48 light years, if 
the star were only 3 times farther away, it would be invisible to
the naked eye.  It is more similar to the Sun than it actually
appears, its mass only about 1.2 times solar, just younger and
hotter.  Subdwarfs are not really too faint for their temperatures,
but too hot for their luminosities, the result of low metal
contents in their atmospheres.  While clearly not one of the
classical subdwarfs (which have quite low metal contents), the
temperature-luminosity status of Alchiba 

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, June 15, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon disappears from the sky toward the end of the week, as it
passes its new phase on Thursday, the 21st.  As the lunar crescent
wanes during the early morning hours, it will make a nice
configuration with brilliant Venus the morning of Monday, the 18th. 
The following morning, Tuesday the 19th, the slimming crescent will
be found beneath the Pleiades star cluster in Taurus and up and to
the right of Saturn, which is now just clearing the glare of the
Sun.  Little Mercury, however, is not so lucky (or perhaps it is we
who are not), as this smallest of the inner planets passes inferior
conjunction with the Sun on Saturday, June 16.

The big date takes place 5 days later.  First, the Sun will cross
the Summer Solstice in Gemini at 2:38 AM Central Daylight Time
(1:38 AM EST, 12:38 PST), marking the first day of astronomical
summer in the northern hemisphere, astronomical winter in the
southern.  At that moment, the northern end of the Earth's axis
will be tipped in the direction of the Sun, the Sun will shine
overhead at the Tropic of Cancer, will be circumpolar (not setting)
at the Arctic Circle, and will not rise at the Antarctic Circle
(technically anyway: atmospheric refraction and the extended
diameter of the Sun will still make it visible).  The Sun will then
be as far north as it can get, 23.4 degrees from the equator.  For
the next 6 months, solar movement will be southerly.

On the same date, Thursday the 21st, the new Moon will exactly
cover the Sun to produce a total solar eclipse that will be visible
along a path through the South Atlantic Ocean and across southern
Africa.  None of it will be visible in North America, though
eastern South America will see a bit of a partial eclipse.  The
geometry of eclipses requires at least two solar eclipses a year. 
The June event is the only total eclipse.  One other, on December
14, is annular (that is, the Moon will be too far away to
completely cover the Sun), and will be visible principally through
the Pacific Ocean.

None of the news of the Sun can eclipse the current glory of Mars,
however.  Moving retrograde between Sagittarius and Scorpius, the
planet is nicely up in the southeast at the end of twilight.  Just
look for the brightest thing you can see!  Though Mars passed
through opposition with the Sun on June 13, its eccentric orbit
causes it to get slightly closer to us until -- again -- Thursday,
the 21st, when the red planet will be 67,344,000 kilometers
(41,846,000 million miles) from us, and at its best for viewing. 
Even a small telescope can show polar caps and dark markings.  To
the right of Mars is bright Antares in Scorpius.  If you are not
too far north, the figure of the celestial scorpion, curved tail,
stinger, and all, is quite obvious.

STAR OF THE WEEK.  GIRTAB (Theta Scorpii).  Girtab, Bayer's Theta
star within Scorpius, the celestial scorpion, stands out in almost
any way you can look at it.  The name alone is unusual, in that it
comes to us from Sumerian rather than Arabic or Greek, and means
simply "the scorpion."  Moreover it carries an additional name,
"Sargas," also Sumerian, whose meaning is unknown.  We'll stick
here with Girtab, though Sargas is commonly used as well.  Though
the most southerly extent of the Sun is in neighboring Sagittarius,
Scorpius is the most southerly of the zodiacal constellations.  And
bright second magnitude (1.87) Girtab is the most southerly BRIGHT
star in the Scorpion, closely anchoring the southern curve of the
scorpion's tail.  At almost exactly 40 degrees below the celestial
equator (and beat out only by much dimmer third magnitude Eta
Scorpii, and then by only 1/6 degree), Girtab is invisible north of
50 degrees north latitude.  The star's southerly position has
allowed northern observers to use its visibility as a test of the
night-sky brightness near the horizon.  Scorpius is filled with
bright blue-white stars of class B.  As a yellow-white class F (F1)
bright giant, Girtab is again an exception.  From its distance of
272 light years, the star radiates 960 times more energy than the
Sun from a surface with a temperature of 7200 Kelvin, its radius 20
times solar, making it a true giant indeed.  Though its equatorial
rotation speed is high (over 50 times that of the Sun), the large
size still gives it a fairly long rotation period of 10 days (or
less).  Girtab is unusual too in our knowledge of its evolutionary
status.  There is no question that the star, which weighs in with
a mass 3.7 times that of the Sun, is rapidly evolving with a dead
helium core toward lower temperatures.  One hundred million years
ago, it WAS a blue cla

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the short week starting Sunday, June 10, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon, beginning the week in its waning gibbous phase,
passes third quarter the evening of Wednesday, the 13th, and
thereafter wanes through crescent, every day rising later after
midnight.  It passes south of Neptune, in Capricornus, on Sunday
the 10th, and south of Uranus, in the far eastern end of the same
constellation, the following day.  

The two planets outward from the Earth provide a wonderful
contrast.  Jupiter, the Solar System's giant, and ordinarily the
second brightest of planets, passes conjunction with the Sun on
Thursday the 14th, and will be quite invisible.  Mars, on the other
hand, is now at its best, passing opposition to the Sun the day
before, Wednesday, the 13th.  On opposition night, Mars will rise
at sunset, set at sunrise, cross the celestial meridian to the
south at midnight (1 AM daylight time), and have its greatest
angular retrograde (westerly) speed.  It will also about as close
to the Earth as possible during this orbital round, 68.6 million
kilometers, or 42.6 million miles.  Unfortunately, the red planet,
which now shines brighter than any star in the sky (and is now
exceeded only by the morning's Venus, the Moon, and the Sun), will
also be at about its most southerly position (just to the west of
Sagittarius), and for northern observers about as low as it can
get.  Turbulence in the Earth's atmosphere then makes detail more
difficult to view by telescope.  After opposition, Mars will begin
to rise before sunset, will move more and more into the early
evening sky, and will be with nicely us for the remainder of the
year.  Mars passes opposition to the Sun every 780 days.  The orbit
is so eccentric that opposition distance vary considerably.  The
next one will be better.  The minimum distance is 56 million
kilometers (35 million miles), the maximum almost twice as great. 
Only Venus comes closer.  

Mars actually lies in far southern Ophiuchus between Sagittarius
and Scorpius, providing a good chance to compare the color of the
planet with its namesake Antares (Ares the Greek version of the war
god), which is nicely visible to the far right of Mars.
As spring slowly blends into summer, these beautiful
summertime constellations, which contain the Milky Way, will begin
to overtake the more drab spring skies, which now still dominate in
the early evening, Virgo and Spica nearly due south as night falls. 
Watch as they slip away a degree per day to the west as the sky
reflects the degree-per-day motion of the Earth around the Sun.  If
your skies are dark, you might make out the dim box of stars that
forms Libra, the scales, the zodiacal constellation that lies
between Virgo and Scorpius.

STAR OF THE WEEK.  ZANIAH (Eta Virginis).  An old term  -- before
we understood that stars do move -- referred to the "fixed stars,"
the phrase really meant to distinguish real stars from the
"wandering stars," the planets.  The names applied to stars
sometimes appear about as fixed as the stars themselves, that is
they, or at least some of them, move around too.  "Zaniah," the Eta
star of Virgo, the Virgin, refers an angel, the "angel of a-awwa"
(the meaning unknown, and with apologies to Arabic readers for
leaving out the necessary accents), and was originally applied to
Porrima (Gamma Virginis), and later fell to dimmer (mind-fourth
magnitude, 3.89) Eta.  In a rather special place, Zaniah, to the
west of Porrima, is squeezed between the celestial equator and the
ecliptic.  Of the brighter stars that make the constellation
figures, Zaniah is one of the closest to the equator, only 2/3 of
a degree to the south of it, and only 10 degrees to the east of the
autumnal equinox, the point where the Sun crosses the equator on
its way south in September.  Zaniah, 250 light years away, is
classed as an A (A3) subgiant, the latter meaning that the star
seems to be about to give up its central hydrogen fusion, if it has
not already.  From its surface temperature of 8800 Kelvin (and of
course its distance) we can calculate a luminosity 130 times that
of the Sun.  The star's status and properties, however, are
seriously compromised by its seeming triple nature.  None of the
components can be resolved by eye at the telescope.  Ultrashort
imaging (to avoid smearing of the image by twinkling) in addition
to occultations by the Moon reveal a pair of stars (one fourth
magnitude, the other fifth) separated by but 0.12 seconds of arc,
or around 10 Astronomical Units.  One of them, probably the
brighter, is revealed by the spectrograph to be a much closer
double with a period of 72 days and an average separation of only
half an astronomical u

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, June 1, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The next Skylights will appear Sunday, June 10.  The Moon passes
through its full phase this week, when it is opposite the Sun, on
Tuesday, June 5.  It will on that day rise near sunset and set near
sunrise.  Thereafter, it begins to thin through its waning gibbous
phase.  The night of June 5 the Moon will appear up and to the
right of the planet Mars, while the following night it will have
moved to appear up and to the left of the red planet.  As we
approach the beginning of summer in the northern hemisphere, when
the Sun will be as far north as it can get (and as high as it can
get for northerners), this full Moon will be the year's second-most
southerly, the "Rose Moon" rising in the southeast, setting in the
southwest.

The brightest and dimmest planets (as seen from Earth) make the
rest of the planetary news.  Venus, very slightly dimming, reaches
its greatest elongation west, when it is 46 degrees to the west of
the Sun.  This lovely planet, third only to the Sun and Moon in
apparent brightness, now rises in the east just ahead of morning
twilight.  Even though the angle between it and the Sun now
decreases, however, Venus will continue to rise earlier, and until
mid-August into ever darker skies.  Earliest Venus-rise will occur
around mid-July.  At the same time, dim Pluto, not visible without
a good-sized telescope, is in opposition to the Sun the night of
Monday, June 4.  When the Moon reaches its full phase, it will lie
roughly 10 degrees below the frigid outer planet, which some take
not to be a planet at all.  In truth, Pluto appears to be some kind
of hybrid object that bridges the gap between the outer planets and
the building blocks (the comets) that created them.  Apparently
there was just not enough raw material in these distant reaches of
the Solar System to make a respectable planet like Neptune or
Uranus.  
 
The early evening presents us with the tail of the longest
constellation in the sky, Hydra , the Water Serpent, which wraps
itself a third of the way around the celestial sphere.  Find
Corvus, the Crow, a small irregular box of stars that for
northerners appears rather low in the south around 9 PM.  The top
two stars point leftward to Spica in Virgo, while the bottom two
point to otherwise un-named Gamma Hydrae, the next-to-the-last
bright star (such as it is) that lies in the celestial snake. 
Snakes of some sort are quite popular, others being summer's
Serpens (the Serpent), which comes in two parts, the southern
hemisphere's Hydrus (another water snake), and, if you wish to
stretch the definition a bit, the northern hemisphere's Draco, the
Dragon, whose tail winds between the Dippers.

STAR OF THE WEEK.  GIAUSAR (Lambda Draconis).  The front bowl stars
of the Big Dipper are famed for pointing at Polaris in Ursa Major's
Little Dipper.  What, however, of the stars along the way?  The
path to Polaris is so familiar that we rarely stop to see the other
sights that lie along it.  About a third of the way from Dubhe (the
Big Dipper's front bowl star) to Polaris (and a just a bit to the
east) lies Giausar, the tail star of Draco the Dragon, to which
Bayer assigned the Greek letter Lambda.  The Arabic name of this
mid-fourth magnitude (3.84) star is confusing at best.  At times
thought to refer to a "central one" much as does the Arabic name of
Orion, the word actually refers to the "nodes" of the lunar orbit,
the points at which the Moon crosses the ecliptic plane twice a
lunar month -- which makes little sense, since Draco contains the
ecliptic POLE, and is therefore quite distant from the ecliptic
itself.  Rather clearly, the name was applied in error.  The star
is about as neglected by research astronomers as it is by even
dedicated skywatchers, rather too bad as it has -- as a class M
(M0) red giant -- one of the rarer of naked-eye types.  Over the
past 20 years it has been mentioned less than 40 times.  It is
neglected by other stars too, as it has no known companions. 
Giausar is one of the sky's cooler and larger stars.  With a
temperature of 3525 Kelvin, it shines to us (if the estimate of
invisible infrared radiation is correct) from a distance of 335
light years with a luminosity 1870 times that of the Sun, which
leads to a radius of 0.55 astronomical units, half the size of
Earth's orbit.  Large enough to have had its angular diameter
measured (at 0.0073 seconds of arc), direct measure of radius makes
it somewhat smaller, a "mere" 0.37 astronomical units, about the
size of Mercury's orbit.  Even the star's general behavior is
obscure.  Classified as a "semi-regular variable," there is some
indication that it changes

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, May 25, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Skylights appears a day early this week.

The Moon passes through its first quarter this week on Tuesday, May
29.  The night of Friday, the 25th, the slim crescent will (for
those in the Americas) stand smack in the middle of Gemini with
Castor and Pollux right above it as it sets.  Two days before the
quarter, the Moon passes its perigee point, where it is closest to
the Earth.  

The dark markings on the lunar surface, the "maria," or "lunar
seas," are huge lava-filled impact basins -- just very large
craters.  Three of these circular features are readily visible at
first quarter.  From upper left to lower right, see if you can make
out Mare Serenitatis (the "Sea of Serenity"), Mare Tranquillitatis
(the "Sea of Tranquillity"), and Mare Foecunditatis (the "Sea of
Fertility").  Toward the right edge of the Moon perhaps you can
also make out smaller Mare Crisium (the "Sea of Crises").  Apollo
11, carrying the first lunar voyagers, set down on Mare
Tranquillitatis on July 20, 1969.  

Saturn is truly gone from sight as it passes conjunction with the
Sun this week, on Friday, the 25th.  You can forget Jupiter too, at
least for now.  Both planets, however, will make nice display
pieces in the morning sky by mid-summer.  By odd coincidence (such
abound in astronomy, as there are so many events going on), the
planet Uranus enters retrograde motion in eastern Capricornus at
almost exactly the moment that the Moon passes first quarter.  At
bright sixth magnitude, Uranus is just visible to the naked eye in
a dark site with no moonlight present.  Neptune (a telescopic
planet), to the west of Uranus, began its retrograde movement
earlier in the month.  Mars is moving backwards too though, since
it has just reversed its direction, at a not-very-fast pace.  That
will pick up as the red planet approaches its very bright
opposition to the Sun on June 13.  By then it will have moved from
its current residence in Sagittarius into Ophiuchus, the only
constellation not of the classical zodiac through which runs the
ecliptic (the apparent path of the Sun).  

If you are far enough south, this is the moment for the Southern
Cross, which crosses the meridian in early evening around the time
that northern observers see Corvus the Crow to the exact south.  In
between is the tail of Hydra, the Water Serpent, and the bright
stars of western Centaurus.  The top two stars of Corvus point
leftward to Spica, in Virgo.  If decently south of 40 degrees north
latitude, look about 35 degrees due south of Spica to see the fuzzy
ball made by the greatest star cluster in the Galaxy, the
magnificent globular cluster Omega Centauri.

Corrections: In last week's Star of the Week, Alkes, note
that the star is converting helium into carbon and oxygen
(not helium into hydrogen as stated!) and that the proper
motion is given 1000 times too high.  Thanks to those who
caught the errors. 

STAR OF THE WEEK.  MIMOSA (Beta Crucis).  Few are the first
magnitude stars, as their apparent brightness requires either great
luminosity, closeness, or both.  Mimosa satisfies the first
requirement.  Tied for apparent brightness at number 19 in the sky
with Deneb (apparent magnitude 1.25), among the stars of first
magnitude it ranks number 10 in luminosity.  The second brightest
star of Crux, the Southern Cross, Mimosa is too far south to have
a traditional proper name, one assigned by the ancients.  The
origin of its name, which comes from Latin and means "actor" (the
word also used in botany), is unknown.  Mimosa is a magnificent
blue-white, very hot class B (B0.5) giant star with a temperature
that soars to 27,600 Kelvin.  Such heat causes the star to radiate
most of its light in the invisible ultraviolet.  To the eye, Mimosa
would appear 3000 times brighter than the Sun; if all the radiation
is taken into account, the luminosity climbs over 11 times higher
to 34,000 solar.  From these figures we calculate a radius of 8.1
times that of the Sun, in good agreement with the value of 8.4
solar found from the small angular radius.  Mimosa is clearly a
close double, but one whose components are too near to each other
to resolve separately and that take almost exactly 5 years to
orbit.  The nature of the second star is unknown, but the agreement
between the two radii above suggests that it does not add much to
the total luminosity.  If it does not, Mimosa has a mass of 14
times that of the Sun, and the pair is separated by around 8
astronomical units.  Mimosa is also a multiply-periodic "Beta
Cephei" star (named after Alfirk) that varies between magnitudes
1.23 and 1.31 with periods of 5.68, 3.87, and 2.91 hou

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, May 18, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Our Moon passes through its new phase this week on Tuesday, the
22nd.  Watch as the waning morning crescent, seen in eastern dawn,
thins early in the week and then appears on the other side of the
Sun to the west the evening of Thursday, the 24th.  The morning of
Saturday the 19th, the Moon will appear a few degrees to the south
of brilliant Venus.  

Saturn is now gone, lost to evening twilight, and Jupiter, just a
little behind, is quite difficult to find, as it now sets just a
bit over an hour after the Sun.  The evening sky does present us
with a fine apparition of Mercury, however.  Only a day before the
Moon passes new (on Monday the 21st), the little planet, the one
closest to the Sun, passes its greatest eastern elongation, when it
is 22 degrees to the east of the Sun and maximally visible.  For
those in North America, the Moon will provide a fine guide, as the
lunar crescent will be just a few degrees to the left of the
planet the night of Thursday, the 24th.  Look in bright twilight
and follow the Moon as it sets and the sky darkens to see Mercury
emerge from the fading glow.  The planet remains mysterious.  Of
the nine planets, only Pluto is smaller.  Not quite 40 percent the
size of Earth, Mercury has the largest iron core relative to its
size of any of them.  Dangerously close to the Sun, it has been
visited by but one spacecraft (Mariner 10 passing it 3 times in
1974 and 1975), and only about half has been imaged.  So close in
angle to the Sun that it is visible only in the daytime or in
twilight, surface features are nearly impossible to see from Earth.

Once Mercury sets, the evening sky awaits the rising of Mars, which
climbs above the southwestern horizon around 10:30 PM Daylight
Time.  Now moving retrograde in far western Sagittarius, the red
planet, fourth from the Sun, and the last of the "terrestrial
planets" (those constructed like Earth), will be rising in evening
twilight by the end of the month.

In the early evening, for those in mid-northern latitudes, look out
perpendicular to the plane of our Galaxy.  The Milky Way is about
as absent as it can get, and lies around the horizon where it is
invisible.  Our view is unobstructed by the dust in the Galactic
plane, allowing us to see outward as far as our instruments will
carry us, to distant galaxies billions of light years away.  From
the southern hemisphere, however, the Milky Circle is high and
spectacular as it passes through Centaurus and Crux, the Southern
Cross.

STAR OF THE WEEK.  ALKES (Alpha Crateris).  Among the dimmest of
all classical constellations is Crater, the Cup, which with Corvus,
the Crow, rides the back of Hydra, the Water Serpent.  The only
proper name in the constellation, "Alkes" comes from Arabic and
means "the wine cup," the star standing in for the whole figure
(the name also clearly related to the English "alcohol.")  Though
mid-fourth magnitude (4.07), Alkes received the Alpha designation
from Bayer.  It takes second place to the un-named Delta star (that
is, to Delta Crateris), and is in a virtual dead heat with Gamma
Crateris.  Alkes is yet one more orange class K (K0) giant star,
though one with an interesting difference.  At a distance of 175
light years, Alkes shines 80 solar luminosities into space from a
4725 Kelvin surface, giving the star a calculated radius 13 times
that of the Sun.  Alkes, with a mass estimated at around 2.5 times
solar, is clearly "in the clump," a set of stars that all have
about the same characteristics of luminosity and temperature and
that are all fusing helium to hydrogen in their cores, Arcturus and
Aldebaran bright examples.  Unlike most helium-burning "clump
stars," unlike most stars around us, Alkes is also a modest "high
velocity" star.  Most of our neighbors are going around the Galaxy
at a speed somewhat in excess of 200 kilometers per second. 
However, all the orbits are a bit different, so they drift relative
to each other at speeds of 20 to 40 or so kilometers per second. 
>From its rate of angular motion across the sky (480 seconds of arc
per year relative to the distant background) and its speed away
from us of 47 km/s, Alkes is moving relative to the Sun at 130
km/s, showing it to be a visitor from a different part of the
Galaxy.  The star has on occasion been placed into the group of
"super-metal-rich" stars.  Though the metal content is probably
more solar, it is clear that the star has come to us from the inner
metal-rich part of the Galaxy, the so-called "bulge." 
Consistently, Alkes has also been dropped into an odd category of
"4150" stars, which seem to have a high abundance of cyanogen, the

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[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, May 4, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon approaches full the early part of the week, passing that
brilliant phase on Monday May 7th among the dim stars of Libra,
thereafter waning through gibbous, rising after sunset.  Sending so
much sunlight back to Earth, the full (or near-full) Moon is so
bright that the surrounding stars are all but invisible.

In eastern dawn, Venus also passes through its greatest possible
brightness, on Friday the 4th, the planet shining brightly amidst
the equally dim stars of Pisces and very close to the vernal
equinox, the point on the celestial equator passed by the Sun on
the first day of spring.  A view through the telescope shows Venus
as a large crescent.  Though we see only a small section of Venus's
daylight side, its proximity to us still throws a maximum amount of
sunlight in our direction.  The observations of Venus's phases was
one of the key proofs to Galileo that Copernicus was right, that
the Earth really does go about the Sun.  

Western twilight holds the disappearing treasure of bright Jupiter,
Saturn, an hour to the west and in a bright sky, difficult to view. 
If you can find it, perhaps you can also find Mercury down and to
the right early in the week.  Jupiter will soon follow into near-
invisibility.  Brightening Mars, however, makes up the difference,
as it now rises in Sagittarius around 11:30 PM Daylight Time. 
Mimicking Venus as a celestial marker, Mars now lies only a couple
degrees above the center of the Galaxy, the center of the Milky
Way.  

The range of celestial brightness is astonishing.  As bright as
Venus may appear to us, the full Moon is 1000 times brighter, and
the Sun a million times brighter yet (so bright as to be able to
damage the eyes).  Venus, however, is at maximum 15 times brighter
than the brightest star, Sirius, and a remarkable 25,000 times
brighter than the faintest star we can see without optical aid.  
With telescopes the range is even more astonishing: the Hubble
Space Telescope can go another 2.5 billion times fainter!

Two other items of note include Neptune, which begins retrograde
motion in Capricornus on Thursday the 10th.  The most distant large
planet, Neptune is now 90,000 times fainter than Venus, and
requires a telescope to see.  The bright Moon will unfortunately
wash out one of the year's better meteor showers, the "Eta
Aquarids," which will peak the morning of Sunday, May 5.  The
debris of Halley's Comet, the shower typically produces some 30
meteors per hour in a dark sky.  

In spite of the Moon's brightness, look to the south in late
twilight to find the great figure of northern spring, Leo the Lion,
the bright star Regulus at the end of the "Sickle" that makes the
great beast's head.

STAR OF THE WEEK.  PRAECIPUA (46 Leonis Minoris).  While Leo is
easy to admire, its modern-constellation counterpart, faint Leo
Minor, which rides the back of the Zodiac's King of the Beasts, is
not.  Leo Minor, the "Lesser Lion," is so dim that few bother with
him, and is so unimportant a constellation that no one would think
of making Leo into "Leo Major," thus raising Leo Minor's rank. 
Because the modern constellations were invented long after proper
names were assigned, most well after Bayer assigned Greek letters
(and to some degree were carved from other constellations), the
names of their stars can be a bit of a mess.  Only one star in Leo
Minor carries a Greek letter name, and that is "Beta;" there is no
Alpha, and Beta Leonis Minoris is not even brightest, coming in
second.  The little constellation does have its charm however,
mostly in a flat quadrilateral with the constellation's luminary,
bright fourth magnitude (3.83) 46 Leonis at the eastern end (the
"46" a Flamsteed number, the only Flamsteed number whose star ranks
"number 1" in a constellation).  Leo Minor is also one of the few
modern constellation whose brightest star carries a proper name, 46
Leo Minoris also called "Praecipua," or "Chief," a "modern" term
from Latin telling that "46" is the brightest star.  Praecipua is
otherwise ordinary, an orange class K (K0) giant-subgiant with a
temperature of 4690 Kelvin.  At a distance of 98 light years, it is
not quite up to average giant brightness, radiating 32 solar
luminosities into space, from which we derive a modest diameter
(for a giant) 8.5 times that of the Sun.  A star of around 1.5
solar masses, once a hydrogen-fusing cool class A star, it is now
evolved, and is quietly fusing helium to carbon in its core.  The
star is known to be somewhat metal poor compared with the Sun, its
iron content down by about a third.  Of most interest perhaps is
how well we know it.  Recent accura

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 27, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon, in its perpetual journey around the Earth, passes through
its first quarter this week, when it is 90 degrees to the east of
the Sun, on Monday, April 30, thereafter waxing toward full.  Two
days after the quarter, it passes perigee, its closest point to the
Earth (typically 363,000 kilometers, or 226,000 miles, 5.5 percent
closer than average).  The night of Friday, the 27th, the crescent
will occult, or cover, two modestly bright stars of Gemini.  Eta
Geminorum will be occulted for east and central North America
around 9:30 Central Time (add an hour for the east coast), while Mu
Geminorum (Tejat) will be occulted for westerners around 10:45
Pacific Time.  The exact time depends on location.   

As May begins the giant planets slip away to the west.  On Sunday
April 29th, Saturn sets just as twilight ends around 9:30 PM
daylight time, though Jupiter lingers another hour.  Mars, however,
is quickly rising ever-earlier, and is now up in the southwest in
Sagittarius by midnight.  In the morning, Venus rises in the east
just minutes after twilight commences.  On Friday May 4th, this
closest of all planets reaches greatest brilliancy for its current
morning appearance.  Since Venus has just passed between us and the
Sun, we are still mostly seeing its nighttime side, and the planet
appears in the telescope as a small crescent.

Twilight is caused by sunlight that illuminates the Earth's
atmosphere after sunset or before sunrise.  There are three
definitions.  As the Sun sets, "civil twilight" ends when the Sun
reaches 6 degrees below the horizon and it becomes too dark for
ordinary outdoor activities.  At 12 degrees we reach the end of
"nautical twilight," when the seagoing navigator can no longer see
the ocean horizon (against which to measure the altitudes of stars
to determine latitude and longitude).  At 18 degrees, the end of
"astronomical twilight," the sky becomes as dark as it can get. 
That is the time for the astronomer to get to work.  The sequence
is repeated backwards in the morning.  Professional observatories
run on "-18 degrees."

As twilight ends, the Big Dipper of Ursa Major is now high overhead
for those in the mid-northern hemisphere, the Little Dipper of Ursa
Minor pointing up to meet it.  If far enough north, above 35
degrees north latitude, you can see Cassiopeia swing beneath the
pole opposite the Dipper.  To the west of the Dippers and around
toward Cassiopeia is an immense area of sky so devoid of bright
stars that the ancients invented no constellations within it.  Not
until after 1600 did astronomers need to organize the sky better
and fill in the blanks, giving us (among many others) the obscure
and large northern figures of Camelopardalis ("the Giraffe," which
lies between Auriga and the Pole) and Lynx (the obvious Lynx),
which sprawls in a long line from east of Auriga to nearly under
the Big Dipper.

STAR OF THE WEEK.  BETA CAM.  Camelopardalis, the Giraffe, a huge
"modern" northern constellation invented only about 400 years ago,
is so obscure that no star within it carries a proper name, and
only three carry Greek letter names.  Only four of its stars are
even as bright as fourth magnitude, the brightest, Beta
Camelopardalis, in the middle of the range (4.03).  Nobody points
out the dim figure at astronomy open houses.  Yet the ghostly
Giraffe is not without its highlights, one of which is its
luminary, Beta.  This great star looks faint only because of its
large distance of 1000 light years.  In truth it is a class G (G0)
yellow-white supergiant (though of a somewhat fainter variety) that
shines with a luminosity 3300 times that of the Sun from a surface
just a bit cooler than solar (about 5500 Kelvin) that would stretch
three-tenths of the way from the Sun to the Earth.  The star is far
enough away, and close enough to the Milky Way, that it is dimmed
some 15 percent by interstellar dust.  With a mass around 7 times
that of the Sun, Beta Cam is only around 40 million years old. 
Lying just over a minute of arc away is a companion that is itself
double, about which nothing is known except that the brighter is
class A, the dimmer probably F.  Separated by at least 25,000
Astronomical Units, the small double takes at least a million years
to orbit the supergiant (which from the little double would shine
with the brightness of four full Moons).  Beta Cam is also a double
mystery.  It is most likely making the transition from being a
hydrogen fusing dwarf (of hot class B) to a larger helium-fusing
red giant.  Whatever its status, it falls into a zone of
temperature and luminosity in which stars become unstable

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 20, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon passes through its new phase this week on Monday the 23rd. 
As the waning crescent thins, it passes ten degrees south of Venus
around mid-day on Friday the 20th, and on the morning of Saturday
the 21st will be well below the bright planet, which now lights up
the eastern dawn sky.  After passing the Sun, the waxing lunar
crescent will be barely visible in western twilight the evening of
Tuesday the 24th.  The following night, Wednesday the 25th, the
crescent, glowing softly with earthlight, will make a fine four-
part configuration with Saturn, Jupiter, and Taurus's bright star
Aldebaran, the Moon positioned about halfway between the two
planets.  By the evening of Thursday, the 26th, the Moon will
appear well up and to the left of Jupiter, having passed south of
it during the day (for those in the Americas).

The two bright evening planets will not be with us too much longer,
as Saturn now sets around 9 PM (daylight time), Jupiter an hour
later.  Mars, however, takes over, the red planet now rising around
midnight between the classical figures of Scorpius and Sagittarius. 
Mercury is completely out of sight, as it passes in back of the Sun
in superior conjunction on Monday, the 23rd.

The minor bodies of the Solar System weigh in this week in the form
of the Lyrid meteor shower, which peaks the morning of Sunday the
22nd.  The Lyrids, which appear to emanate from the constellation
Lyra, are the rocky debris of Comet Thatcher (1861 I), whose orbit
we are passing.  It is usually a modest shower, producing perhaps
10 meteors per hour, but is capable of up to 100.  

The summer stars, represented here by Lyra, are yet to come
however.  Now it is time to admire those of spring.  Just after the
end of twilight look high to the south to find the great figure of
Leo, the Lion, this zodiacal constellation looking for all the
world like a great beast stalking the sky, his head outlined by the
"Sickle," which ends in Regulus.  His hindquarters are marked by a
prominent triangle whose eastern star is Denebola.  Then follow the
curve of the handle of the Big Dipper (nearly overhead for most in
North America) to find Arcturus in Bootes and then Spica in the
Zodiac's Virgo, this lonely star seen to the left of the box that
makes Corvus the Crow.  Almost exactly between Regulus and Spica
lies the autumnal equinox, the point where the Sun crosses the
celestial equator to mark the beginning of fall (which this year
will happen on September 22).  Since the Sun has recently passed
the Vernal Equinox in Pisces, its opposite is nicely "visible" (in
our imaginations) at night.  Down and to the right of Leo is
Alphard in Hydra, the water serpent, and just up and to the left of
Alphard is the modern constellation Sextans, which honors the
venerable sextant, the prime tool of the celestial navigator.

STAR OF THE WEEK.  ADHAFERA (Zeta Leonis).  The Sickle of Leo is
known for the bright star Regulus and the famous double Algieba. 
Just above Algieba, you can admire a fainter star, third magnitude
(but just barely so, 3.44) Adhafera.  The name, which comes
directly from Arabic and means "the lock of hair," makes sense, as
the star lies in Leo's great mane.  Unfortunately, however,
"Adhafera" actually refers to nearby Coma Berenices (Berenices
Hair), and was given to Leo's star in error, a common failing of
those who applied star names.  Adhafera, a yellow-white class F
(F0) giant star 260 light years away, actually shines 207 times
more brightly than does the Sun.  With a temperature of 7030
Kelvin, almost of its radiation pours out in the visible, where we
can see it (that is, there is almost no correction for invisible
ultraviolet or infrared, quite like our Sun), from which we find a
radius 10 times solar.  Spinning with an equatorial velocity of 84
kilometers per second (48 times solar), this three solar mass star
takes less than six days to make a full rotation.  Adhafera's real
interest lies in its very clean state of evolution.  Class F giants
are rare, as they are making a remarkably fast transition from the
main sequence (where they once fused hydrogen to helium in their
cores) to the giant state (where they will eventually fuse helium
to carbon).  Only a million years ago, Adhafera was a white class
A (almost class B) dwarf.  Now with a shrinking, dead helium core,
it will become an orange class K giant star in only another million
years, and will then make its "run to the top," taking a leisurely
100 million years to expand from 12 solar radii to a to a red class
M giant with a radius near that of the Earth's orbit!  At that
point, it will fire up its 

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 13, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Our Moon passes through third quarter this week on Sunday the 15th,
about the time of moonset.  It then wanes through its crescent
towards new, that phase reached next week.  While the crescent
begins to narrow in the early morning sky, the Moon passes north of
Neptune on Monday the 16th, then north of Uranus the following day. 
Two days after the quarter, our companion passes apogee, where it
is farthest from the Earth.

The sky presents an interesting set of planetary pairings.  The
outer four planets are often lumped together as "Jovian planets,"
as if they were of one kind.  Jupiter and Saturn, however, the two
giants of the Solar System (which are quite similar to each other),
are very different from much smaller and more distant Uranus and
Neptune, which themselves are near-twins.  The couples are also
currently paired in space.  As the Sun sets and twilight ends, the
two evening planets, Jupiter and Saturn, find themselves ever
closer to the horizon.  Both still in Taurus, Saturn now sets just
after 10 PM daylight time, Jupiter about an hour later.  At the
same time, both are running ever faster toward the east against the
stellar background.  Jupiter, the closer of the pair, is moving the
faster, causing it to pull away from Saturn.  While the ringed
planet will linger in Taurus, Jupiter (which will pass north of
Aldebaran on Monday the 16th) is heading toward Gemini.  Uranus and
Neptune are in much the same configuration.  Neptune rises around
3 AM (Daylight time), Uranus an hour later.  Both now in
Capricornus and also moving easterly, closer Uranus is pulling away
from Neptune, and will soon enter Aquarius on its 84-year journey
around the Sun, leaving distant Neptune behind.  Uranus, faint but
visible to the naked eye, is now passing roughly north of Deneb
Algedi (Delta Capricorni), making it rather easy to find.

Not paired, but shining in solitary splendor, Venus rules the
eastern morning twilight sky.  Our nearest neighbor has been in
retrograde motion, moving to the west against the stars.  On
Tuesday the 17th it ceases retrograde and begins its direct
westerly motion once again, though at a slow pace, the planet
continuing to separate from the Sun and rising ever earlier.

Directly to the south at 8 PM, between Gemini and Leo, lies the dim
constellation Cancer, the Crab.  Its greatest distinction is a
small box of 4 stars within which lies a fuzzy patch, an open
cluster called the "Beehive."  Not all that much farther than the
famed Pleiades in Taurus, the Beehive (a lovely sight in a small
telescope) is older and contains intrinsically less-luminous stars
and is therefore not so prominent. 
A 
STAR OF THE WEEK.  ACUBENS (Alpha Cancri).  Though Bayer's Alpha
star, Acubens, at faint fourth magnitude (4.25), ranks only fourth
in the constellation Cancer, after Beta (Al Tarf), Delta (Asellus
Australis), even Iota, probably because of its position as a
southern claw of the celestial crab.  The star's name, which it
actually shares with a northern claw (Iota), is derived from an
Arabic word that means just that, "the claw," and is the same root
from which is derived the name Zubenelgenubi (Alpha Librae), the
star that represents the southern claw of Scorpius.  Acubens, 175
light years away, is something of a mystery.  Seemingly a simple
white class A (A5) star, its spectrum displays ultra-strong
absorptions of particular metals that make it a "metallic line" or
"Am" star.  Metallic line stars are typically strongly enhanced in
elements like zinc, strontium, zirconium, barium and others.  The
phenomenon is a surface effect in which some elements sink to lower
layers under the action of gravity, while others rise, pushed
upward by radiation.  To be such chemically peculiar stars, they
must rotate slowly, such that the surface gases are not stirred up. 
Class A stars tend to rotate quickly however, so there should be
slowing mechanism, which for the Am stars is duplicity (each acting
tidally to slow the other).  Acubens, though, rotates more quickly
than usual (at least 68 kilometers per second at the equator).  It
is, however, a double star.  It is close enough to the ecliptic
plane that the Moon occasionally passes over and occults it.  Stars
normally wink out very quickly when the Moon covers them; Acubens
winked out twice, showing that it consists of two identical stars
only 0.1 seconds of arc (5.3 astronomical units) apart.  Each is
presumably a dwarf Am star with double the mass of the Sun, each
shining with 23 solar luminosities.  Given the masses and
separation, they should orbit each other every 6.1 years.  Neither
can be discr

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 6, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon passes through its full phase this week the night of
Saturday, April 7, somewhat after Moonrise in North America.  The
Moon will therefore rise a bit before sunset, but since the full
phase is passed while the Moon is visible, it will also set a bit
after sunrise.  The night of Thursday the 12th, the waning gibbous
Moon will pass to the north of Mars and make a handy reference with
which to find the red planet.  

Mars now officially moves into the evening sky, rising just before
midnight (1 AM daylight time).  It is also very close to bottoming
out in its current journey along the ecliptic path, having recently
entered Sagittarius.  Nearly 23 degrees south of the celestial
equator, the planet rises well into the southwest.  During most of
this month, as Mars crosses -- transits -- the celestial meridian
to the south, Venus rises in the northwest, but after the beginning
of twilight, so the planet will appear low in the sky.  On Friday
the 6th, Venus passes 10 degrees north of Mercury, the little
planet so close to the Sun that it is not possible to see from
North America.

The evening sky still provides the best planetary view, with
Jupiter and Saturn glorious to the west among the stars of Taurus. 
Saturn now sets around 10:30 (Daylight Time), Jupiter an hour
later, the two slowly drawing apart.  Slowly being overtaken by the
Sun, the pair will, one at a time, disappear into twilight during
the month of May.  

Daylight time has just been launched over most of the US and
Canada.  The time of day refers to the "hour angle" of the Sun
(with the effects of orbital eccentricity and axial tilt smoothed
out), the angle the Sun makes with the celestial meridian, each
hour past noon corresponding to another 15 degrees.  Time is really
longitude dependent.  Standard time is local time at specified
meridians.  Technically, everyone in a band 15 degrees wide
centered on that meridian keeps the same time (though the bands are
wildly shifted for political and social reasons).  If you are west
of your meridian, the Sun seems to set late, while if east it will
seem to set early.  In daylight time, we shift to the time zone to
the east of us, and as a result, the Sun seems to set an hour later
than "normal."  

Dim Cancer, flanked by Gemini to the west and Leo to the east,
crosses the meridian around 9 PM.  Above it is even dimmer Lynx,
below it the circlet that makes the head of Hydra, the Water
Serpent.  Much farther down is Pyxis, the Compass, all a challenge
and nearly impossible to see under the glare of full Moonlight.

STAR OF THE WEEK.  TANIA AUSTRALIS (Mu Ursae Majoris).  Our Ursa
Major, the Greater Bear, contains remnants of ancient Arabic
constellations, the best-known example the star Alkaid, which
refers to the leader of the daughters of a funeral bier.  Southwest
of the Dipper's bowl lie three obvious pairs of stars that
represent the bear's paws, but to the Arabs were the tracks of
leaping gazelles.  The middle pair is the "second leap," from which
comes the name "Tania"(for "second").  In the multi-cultural mix of
constellation lore, the northern one received the Arab-Latin name
Tania Borealis, the southern Tania Australis.  Bayer assigned the
three "leaps" ordered Greek letters, Tania Borealis receiving
Lambda, Tania Australis Mu.  The two make a lovely contrast, Tania
Borealis a white class A subgiant, Australis a fairly rare (for
naked eye stars) red class M (M0) giant.  Tania Australis shines at
mid third magnitude (3.05) from a distance of 250 light years
(double the distance of Borealis, the two only a line-of-sight
coincidence).  When we account for infrared radiation from a 3950
Kelvin surface, Tania Australis is found to have a luminosity 850
times that of the Sun, which leads to a radius 62 times solar (0.28
Astronomical Units, three-fourths the size of Mercury's orbit). 
Having used its core hydrogen, Tania Australis seems to be
brightening along the "red giant branch" with a contracting helium
core.  Before long, the helium will fire up to fuse to carbon, and
the star will dim some and stabilize as a class K giant.  Tania
Australis is an unresolvable binary, the companion (known only from
spectroscopic observations) circling the M giant every 230 days at
a distance of at least 1.5 Astronomical Units, suggesting a
combined mass over 9 times solar, double that expected on the basis
of luminosity and temperature.  Tania Australis proper (ignoring
the companion) is also a rare "hybrid star."  Magnetically active
stars like the Sun blow a relatively fast but thin wind from their
surfaces.  Larger giants blow slower, but much thi

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 30, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

We begin the week with the Moon two days day shy of first quarter,
the phase reached on Sunday, April 1st (no fooling!), the lunar
disk to the east of Jupiter.  Four days later, in its waxing
gibbous phase, the Moon passes perigee, when it is closest to the
Earth and angularly largest.  Many are the lunar cycles.  The
period of the phases (the synodic period, that relative to the Sun)
is 29.531 days, from which comes our calendrical month.  The period
relative to the stars (the sidereal period), the true orbital
period, however, is 27.322 days: the time the Moon takes to go from
one constellation, around the sky, and back.  The elliptical lunar
orbit rotates, which moves the point of perigee forward (it goes
all the way around in 8.85 years), so the period from one perigee
to the next (the "anomalistic month") is a bit longer, 27.555 days. 
The lunar orbit is also tilted relative to the Earth's orbit. 
Twice a month the Moon crosses the ecliptic (the apparent solar
path) at the "nodes."  The lunar orbit wobbles, causing the nodes
to regress and to move around the ecliptic oppositely from the
motion of perigee over a period of 18.61 years.  The lunar period
from one node and back to that node (the "draconic month" of 27.212
days) is therefore somewhat shorter than the sidereal period. 
(Since eclipses can take place only when the Moon is near a node,
the draconic month is important in their prediction).

We also begin the week with Venus just barely past inferior
conjunction with the Sun, leaving something of a hole in the
western sky, the familiar glow of the planet now gone.  Venus is,
however, already becoming visible in the eastern morning sky, where
it will reach full brilliance in about a month.  For the moment, we
are left with Jupiter and Saturn, which still shine brightly to the
west in early evening, Saturn setting around 10 PM, bright Jupiter
an hour later, leaving us with no planets to see at all.  But do
not despair, as Mars now begins to encroach on the evening sky, the
red planet, in deep southern Ophiuchus to the east of Antares in
Scorpius, rises about midnight.  

The richness of the winter sky is escaping to the west.  Just to
the east of Orion runs the Milky Way, the disk of our Galaxy.  As
the spring constellations rise, we look perpendicular to the
Galactic plane and find far fewer stars.  As Ursa Major,
represented by the Big Dipper, passes nearly overhead (in mid-
northern latitudes) around midnight, look about 30 degrees to the
south to see the lacy sprawl of stars that makes the Coma Berenices
star cluster, the constellation that holds the Galaxy's north pole. 
Though the stars are fewer toward the Galactic pole, the lack of
obscuring interstellar dust allows a view of countless distant
galaxies.

STAR OF THE WEEK.  HAEDUS I (Zeta Aurigae).  Southwest of Capella
(the "she-goat) in Auriga lies a neat triangle of stars, the
"Kids."  In older tradition, the top star (Almaaz) is excluded, the
term belonging to the bottom two, in Latin known as the "Haedi,"
the eastern (and faintest of the set) one now called Haedus I, the
western Haedus II.  Starting at the north and going clockwise,
Bayer named the three in alphabetic order Epsilon, Zeta, and Eta
Aurigae.  At bright fourth magnitude (3.75), Haedus I is the
fainter of the pair (indeed of the three), though only because it
is -- at a distance of 850 light years -- the farther.  (Haedus II
is 220 light years away, Almaaz 2000 light years, the three Kids
only a line-of-sight coincidence.)  Quite the brilliant star,
Haedus I is visually 1700 times more luminous than the Sun.  Its
spectrum, however, shows that it is not one star but two in mutual
orbit, an orange class K (K4) supergiant (or bright giant) and a
hot blue class B (B5) star circling each other every 972.183 days
(2.66 years).  Our delight in the system lies not in the duplicity,
but in the orientation of the orbit, which lies within 3 degrees of
the line of sight.  As a result, the stars of Zeta Aurigae eclipse
each other.  Every 2.66 years, the smaller but still-bright B star
hides completely behind the larger, cooler K star, and the combined
visual light drops by 0.15 magnitudes (about 15 percent), not much,
but noticeable to the practiced eye.  (Coincidences abound in
astronomy: though the Kids have nothing to do with one another, two
of them are eclipsing binaries, the other Almaaz.)  Detailed
analysis of the eclipse and of the velocities of the stars tell
their stories.  Averaging 4.2 Astronomical Units apart, the two go
around each other in an elliptical orbit that takes them from 5.9
AU to 2.5 AU.  The K star'

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 23, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon passes through its new phase on Saturday, March 24th, and
for a couple days is invisible.  The crescent will appear in
western evening twilight the night of Monday, the 26th (though
under ideal conditions the thin crescent will be visible after
sundown the night of Friday, the 25th).  As the crescent waxes, it
will pass beneath Saturn the night of Wednesday the 28th and then
between Jupiter and Aldebaran the night of Thursday, the 29th, the
tilt of the lunar orbit taking the Moon somewhat to the south of
the two planets.  Though Jupiter and Saturn are both moving well
into the evening western sky, both will be nicely visible through
April and into May as they make their way eastward against the
stars of Taurus.

Venus, however, says farewell.  The planet will pass inferior
conjunction with the Sun, when it is between us and the Sun, on
Thursday, the 29th.  "Between us and the Sun" is pushing the
definition a bit, however, as the tilt of the orbit causes the
planet to pass a full 8 degrees north of the Sun, and causes it
both to set after sunset and rise before sunrise.  By early April,
Venus will be nicely visible in the northeastern dawn sky, and will
dominate the morning until near the end of the year.

At about the same time, on Tuesday the 27th, the asteroid Vesta
also passes conjunction with the Sun.  Vesta's proper name is 4
Vesta, as it was the fourth asteroid discovered, by Heinrich Olbers
in 1807.  The third largest asteroid, some 500 kilometers across
(15% the size of the Moon), Vesta is unusual in that it is the only
asteroid visible to the naked eye (though just barely).  

Olbers is famed as the promulgator of "Olber's paradox," which can
be traced through Edmund Halley, perhaps even to Kepler.  You can
make a profound cosmological observation any clear night.  If the
Universe is static, unevolving, and infinite, any line of sight
should wind up at a star.  Therefore the night sky should be at
least as bright as the surface of the Sun!  Yet the night sky is
dark.  The Universe can therefore not be static, unevolving, and
infinite all at the same time.  The resolution of the paradox is
that the Universe is expanding and that it is evolving, that stars
do not live forever, but are born and die.

The new Moon of course gives us the chance to admire these stars. 
By 8 PM or so, Orion is to the west of the meridian, and from mid-
northern latitudes, Gemini is close to overhead, Procyon in Canis
Minor nearly due south.  Look a bit to the east of Gemini (between
Gemini and rising Leo) to see if you can spot the famed Beehive
cluster in dim Cancer.  

STAR OF THE WEEK.  NA'IR AL SAIF (Iota Orionis).  Dangling from
Orion's Belt is the Hunter's mighty three-star sword, the trio
increasing in brightness toward the south.  "Three-star sword" is
only a traditional appellation, however, as a view with any sort of
optical aid reveals nests of stars.  The complex "middle star"
contains the famed Orion Nebula, which is lit by the tightly
compacted four-star "Trapezium" (Theta-1 Orionis), the quartet
dominated by the hot O (O6) star Theta-1 Orionis C (the Trapezium
at the top of a more-extended "Trapezium Cluster").  The bottom
"star" is dominated by the brightest of the set, bright third
magnitude (2.77) Na'ir al Saif, Arabic for "the Bright One in the
Sword," the name "Saif" (sword) wrongly transferred to Kappa
Orionis (Na'ir al Saif receiving the Iota designation from Bayer). 
A class O (O9) giant, and one of the hottest and bluest stars that
make Orion's classical figure, Na'ir al Saif shines with a
temperature of 31,500 Kelvin from a quite-uncertain distance of
1300 light years, giving it a luminosity (corrected for ultraviolet
radiation and a bit of interstellar dust absorption) of 12,600
times that of the Sun.  An O9 star should have luminosity at least
double that, suggesting that the true distance is closer to 2000
light years.  The star is a complex multiple dominated by the 15-
solar-mass O star.  At respective distances of 50 and 11 seconds of
arc lie an 11th magnitude class A or F dwarf and a 7th magnitude B
star, with true separations of at least 4400 and 20,000
Astronomical Units and orbital periods at least 75,000 and 700,000
years.  The brilliant O star also has a far more interesting hot
class B1 spectroscopic companion that orbits in 29 days at a
separation of only half an AU (or so) in an unusually eccentric
path that takes it from 0.8 AU to a mere 0.11 AU.  The collision of
their strong winds produces powerful X-rays.  Na'ir al Saif and its
close companion help reveal the power of gravity and stellar
dynamics.  Twenty-si

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 16, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon is now on something of a Friday cycle, the third quarter
reached today, Friday, March 16.  The cycle is soon to be broken,
however, as the phase period of 29.5 days slightly exceeds the 4-
week period of 28 days, causing the next new Moon to take place on
Saturday, March 24th.  As the lunar disk approaches new, its
crescent wanes toward the eastern dawn sky.  

The morning sky is filled with dim and barely accessible planets. 
On the morning of Sunday the 18th, the Moon will pass in front of
(that is, occult) the largest asteroid Ceres, the event visible
across much of the Atlantic Ocean, Africa, and the middle east (but
not, unfortunately, in the Americas).  It then passes Neptune the
morning of Tuesday the 20th and Uranus the following day, finally
passing south of Mercury (now being lost in morning twilight) on
Thursday, the 22nd.  Not to be left out, distant Pluto, to the west
of Uranus and Neptune, begins its retrograde motion on Sunday, the
18th.  The evening sky remains glorious, however.  Though the Sun
is now rapidly catching up with Venus, the brilliant planet -- to
the west at sundown -- still does not set until after the end of
evening twilight.  Higher in the sky, Saturn and bright Jupiter
march easterly through the bright stars of Taurus.  

But it is the Earth and Sun that really make the news, as the Sun
passes the vernal equinox in Pisces, crossing the celestial equator
at 7:31 AM Central Standard Time (8:31 Eastern, 5:31 Pacific Time)
on Tuesday, the 20th, near the time of sunrise.  That day the Sun
will rise due east and set due west (very noticeable for those with
east-west roads), the Earth's rotation axis will be perpendicular
to the line to the Sun, and the days and nights will be
approximately equal at 12 hours each.  The Sun also technically
rises at the north pole and sets at the south, beginning 6 months
of north polar daylight, 6 months of south polar night.  These
times and events are, however, slightly modified by the facts that
the Sun is an extended disk and that refraction in the Earth's
atmosphere raises the Sun a bit, rendering the equinox day slightly
longer than 12 hours and causing the polar Sun to have already
risen.

With the advent of spring we also begin to say farewell to the
winter constellations (Orion, Canis Major and their cohort), hello
to the spring, Leo and the Big Dipper now climbing the sky in mid-
evening, orange Arcturus glowing brightly in the east.  Just down
and to the right of Regulus in Leo, between Regulus and Procyon,
see if you can find the circlet of dim stars that makes the head of
Hydra, the Water Serpent (part of the Jason myth), the longest
constellation of the sky, which stretches below both Leo and Virgo,
finally ending near Libra. 

STAR OF THE WEEK.  TUREIS (Rho Puppis).  Within the great ship
Argo, in which Jason sailed to find the golden fleece, is a group
of stars that represents a "little shield."  The term was
erroneously applied in Greek to the star Aspidiske (Iota Carinae)
and then in Arabic to the star we now know as Rho Puppis (Carina
the hull of the ship, Puppis the stern).  Lying rather prominently
to the west of Wezen in Canis Major, Tureis shines at an easily-
visible third magnitude (2.81), and is one of the most northerly of
the brighter stars of the constellation.  From its distance of only
63 light years, this yellow-white class F (F6) giant star radiates
6 times more energy than the Sun from its 6540 Kelvin surface.  As
a giant with a mass about 1.5 times that of the Sun, it has
recently (or will soon) shut down its internal hydrogen fusion. 
Tureis, otherwise quite ordinary, makes its mark as one of the
sky's brightest "Delta Scuti" variable stars.  The Delta Scuti
stars represent the low-luminosity tail of the bright giant and
supergiant Cepheid variables that in turn are represented so well
by Mekbuda in Gemini.  Having lower masses, luminosities, and radii
than classical Cepheids (Tureis only twice the solar size), they
pulsate subtly and quickly.  Tureis changes by only about 10% over
a precisely known period of 0.14088143 days (3 hours 22 minutes 52
seconds).  Far lesser variations make the star pulsate at 0.13 and
0.16 days as well.  The variations (which influence the spectrum)
once led us to believe that the star had a close companion, but
that is no longer believed.  Though Tureis is only 6 times brighter
than the Sun, it is anomalously classed as a far grander "bright
giant," which for unknown reasons is typical of Delta Scuti stars. 
Among the Delta Scuti crowd, Tureis closely sets the low-
temperature limit.  While having no close comp

[CPS Science]Skylights (a day early)

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 9, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

We start the week with the full Moon, with the full illuminated
hemisphere of the Moon visible to us on Earth, which reveals the
great extent of the dark lunar maria.  They are huge lava-filled
impact basins that make the face of the "man in the Moon," and are
three to four billion years old.  They reveal the awesome violence
of the early Solar System, of the time when the planets were
sweeping up the debris of the smaller bodies of which they were
made.  The Moon then spends the rest of the week waning through its
gibbous phase toward third quarter, which will be reached next
Friday the 16th.  

The morning of Thursday, 15th, the Moon will appear just to the
west of Mars, the morning of Friday the 16th to the east of it. 
The red planet is now passing far (about 10 degrees) to the south
of much more distant Pluto.  While Mars invades Scorpius (and is
now to the northeast of its namesake Antares), Pluto, because of
its highly tilted orbit, lies among the stars of southern
Ophiuchus.  Mars now rises shortly before 1 AM.  Staying up until
dawn may afford a view of Mercury, which reaches greatest western
elongation, a whopping (for Mercury) 28 degrees from the Sun. 
However, the northern hemisphere morning ecliptic this time of year
lies relatively flat against the horizon, so Mercury will still be
difficult to glimpse in eastern morning twilight.  The view is far
better from the southern hemisphere.  The little planet will pass
only a tenth of a degree north of Uranus the morning of Saturday
the 10th.

The planetary view in the evening is much better.  While Venus now
descends the evening sky, it is still glorious in western twilight. 
Farther east in Taurus, Jupiter and Saturn linger in the sky
through nearly the entire evening, the pair finally setting shortly
before midnight.  

Taurus, with its marvelous Pleiades -- Seven Sisters -- star
cluster is the southern figure of a triangle of bright
constellations.  To the northwest is Perseus, whose central region
is also made of a star cluster.  To the northeast is Auriga, the
Charioteer, to which Taurus is actually connected through its
northern "horn" (one star part of both constellations) and which
contains the sky's sixth brightest (and most northerly first
magnitude) star Capella.  All three are set into the Milky Way,
that fainter part of the Milky circle that is formed by the outer
part of the Galaxy.  Perseus is filled with numerous bright hot
high mass stars from episodes of recent star formation, while
Taurus and Auriga are filled with thick dark clouds of dust in
which stars are now vigorously being born.

STAR OF THE WEEK.  PLEIONE (28 Tauri).  Mother to Taurus's
Pleiades, the famed Seven Sisters, at mid-fifth magnitude (5.09)
Pleione (Flamsteed number 28) ranks seventh among the named stars
of the cluster, just marginally behind the Pleiades' mythical
father, the god Atlas.  All nine are hot class B stars, their blue-
white similarity giving the cluster so much of its sparkle. 
Together with Sterope, Pleione is the coolest (class B8) of them,
its temperature 12,000 Kelvin.  It and Sterope are also the only
dwarfs, stars that are fully fusing hydrogen in their cores and
have not yet begun to evolve (the others either subgiants like
Merope or giants like Alcyone).  From its distance of 385 light
years, Pleione shines with a luminosity 190 times that of the Sun,
its radius 3.2 solar, its mass (from the luminosity and
temperature) 3.4 solar.  Pleione's glory lies in its spectrum, its
array of colors.  Along with Gamma Cassiopeia, Pleione is one of
the classic "Be" stars of the sky.  The "e" stands for "emission,"
and refers to emissions of hydrogen that appear at specific
wavelengths or colors (particularly one in the red part of the
spectrum, hydrogen-alpha).  A Be star's emissions come from a
surrounding ring of gas that is somehow (though no one is quite
sure) related to the star's great rotation speed, Pleione spinning
at least as fast as 329 kilometers per second at the equator, 165
times faster than the Sun, giving it a rotation period of under
half a day.  The emissions of Be stars are split by the Doppler
effect as a result of one part of the ring rotating toward us, the
other receding.  In the extreme "shell star" case, the ring also
produces absorptions from both hydrogen and from a variety of
elements caused by the ring directly blocking starlight.  The
difference in Be star styles was once thought to be a matter of
orientation, but Pleione puts the lie to the theory by switching
among all three phases, normal B star, Be star, Be shell star, the
changes taking place at intervals o

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 2, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon passes through first quarter on Friday the 2nd, and will
spend the week waxing toward full, that phase reached next Friday
March 9, a day after it passes perigee, when it is closest to the
Earth for this orbital round.  For those in the Americas, first
quarter will take place around the time twilight darkens the sky,
the Moon just to the east of Jupiter.  In the third century BC, the
great Greek astronomer Aristarchus of Samos tried to estimate the
distance to the Sun by observing the angle between the Moon and Sun
at the time of its quarters.  Only if the Sun is infinitely far
away will the angle be exactly 90 degrees.  His value of solar
distance fell short by a factor of 20 (the Sun so far away that
naked-eye measure is not possible), but his idea that the Sun was
vastly farther than the Moon was correct, and vividly reveals the
knowledge and intellectual vitality of the times.

The Moon is positioned this week between the group of bright
evening planets -- Venus (brilliant to the west), Saturn, and
Jupiter (the latter two high to the south in Taurus) -- and lonely
Mars, which does not rise until nearly 1 AM.  Venus makes special
news by beginning its retrograde, or westerly, motion against the
stars on Thursday the 7th.  Since it passed greatest elongation
from the Sun last January 16, the Sun has been catching up with it
even though both have been moving east.  The reversal in Venus's
direction means that the Sun will catch it very quickly now.  Each
evening the planet will be lower in the sky, and it will disappear
by the end of the month.  But do not despair, as it will as quickly
pop up in the morning sky.  The planet has been exhibiting an
Arctic curiosity.  The Sun is still south of the celestial equator,
whereas Venus is now well above it.  From somewhat above the Arctic
Circle, Venus sets after the Sun, as it does at lower latitudes;
but it also rises shortly before the Sun, making it both a morning
and evening "star."

Mars, by itself, is not left out of the picture, however.  On
Saturday the 4th, the red planet passes 4 degrees north of its
reddish namesake Antares in Scorpius ("Antares" meaning "like
Ares," "Ares" the Greek version of the god of war).  The two have
similar colors, are both in the Zodiac, and while Mars is now about
half a magnitude brighter than the star (and brightening as well),
they still look quite similar and are easily mistaken for each
other.

Scorpius, beautifully placed within the Milky Way, rises roughly as
Orion and his pair of hunting dogs (Canis Major and Canis Minor)
set, and gives those in the northern hemisphere hope that summer is
not all that far off.  

STAR OF THE WEEK.  MULIPHEIN (Gamma Canis Majoris).   The names
Wezen and Hadar (in Arabic form) were once applied to a pair of
stars.  Though there are candidates, no one knows which pair.  The
uncertainty was in older times expressed as an Arabic word that in
part referred to a pair of things that caused contention.  The word
itself was then taken as the pair, much mangled to Muliphein
(sometimes seen as Muliphen), and then for no good reason given to
the little star that Bayer later tagged as Gamma of Canis Major
(while Wezen was given to our modern Delta, and Hadar to modern
Beta Centauri).  Such is the logic of star names.  The mid-fourth
magnitude (4.12) star itself, 400 light years away, is a class B
(B8) blue-white bright giant, similar in color to so many stars in
the Canis Major-Orion district.  With a temperature of 13,600
Kelvin, it radiates 685 times more energy than the Sun and is 5
solar diameters across.  If the star were as close as its
constellation-mate Sirius, it would shine in our sky almost as
brightly as Venus at her best.  Muliphein's luminosity and
temperature tell of a 4.3 solar mass star that has recently ended
its hydrogen-fusing life and is now starting to evolve into a red
giant.  Rotating slowly for a class B star (only 15 times the solar
rotation speed), it, like so many class B and A stars (including
Sirius), is chemically peculiar.  Muliphein in particular is a
"mercury-manganese star" rather like Alpheratz in Andromeda. 
Muliphein is 40 percent richer in iron and chromium (compared with
hydrogen) than is the Sun, but its mercury level is more than 2000
times solar as a result of physical lofting of certain elements by
radiation forces.  The most curious thing about the star, however,
is its Gamma designation, as Muliphein is far from being the
constellation's third brightest, being beat out by several others,
including Omega!  Bayer probably proceeded from top to bottom in
the constellation, as evidenced

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 23, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

We begin the week with the new Moon passing beneath a Sun now
moving through Aquarius on its way to Pisces and spring.  By the
evening of Saturday the 24th, the Moon will be visible as a slim
crescent in evening western twilight, and will thence grow toward
first quarter, that phase to be reached next Friday, March 2.  

The evening of Monday, the 26th, the Moon will appear well to the
south -- to the left -- of brilliant Venus.  The passing angle
between Venus and the Moon is unusually large, 11 degrees.  The
orbits of all the bodies of the Solar System are somewhat tilted
relative to Earth's, the Moon's by 5.2 degrees, Venus's by 3.4
degrees.  Though the planets and Moon are all found close to the
ecliptic -- the apparent path of the Sun -- they therefore still
wander noticeably away from it.  During this week, the Moon is well
south of the ecliptic, while Venus is on the north side, the effect
exaggerated by its proximity to Earth as it prepares to head toward
conjunction with the Sun.  The champion wanderer, of course, is
highly-tilted dim Pluto, which now lies 10 degrees north of the
ecliptic in the constellation Ophiuchus.

The night of Thursday, March 1, the Moon will be positioned between
and below Saturn and Jupiter in Taurus,  passing beneath Jupiter
after moonset in North America.  Both planets are now moving in
direct motion to the east and pulling ever-farther apart.  Saturn
is noticeably on the south side of the ecliptic, while Jupiter lies
very close to it.

The winter stars are now in full force, Orion and its companions to
the south in mid-evening.  Included is the Winter Triangle of
Betelgeuse (up and to the left of Orion's three-star belt), Sirius
(the brightest star of the sky, down and to the left of
Betelgeuse), and Procyon (to the left of Betelgeuse).  Oddly,
Procyon and Sirius, the luminaries of Orion's two hunting dogs,
both have faint tiny "white dwarf" companions, dead stars that have
shrunk themselves under gravity to the size of the Earth.  

If you have had enough of northern winter, then look later in the
evening to the east and northeast to see the harbingers of Spring,
Leo and the Big Dipper in Ursa Major, climbing the sky, the Dipper
standing on its handle.  As the Earth swings a degree per day
around the Sun, the stars seem to creep a degree per night toward
the west.  Before long, for those in North America, the Dipper will
ride overhead on a warm May night, while people in the southern
hemisphere will be admiring a high view of the Southern Cross,
which is readily visible only below about 25 degrees north
latitude.

STAR OF THE WEEK.  TEJAT (Mu Geminorum).  The southwestern corner
of Gemini turns gently in a three-star tail that directs the eye to
the Summer Solstice, the position where the Sun will be on the
first day of northern summer.  At the southeastern corner of the
long rectangle that makes the constellation and at the east end of
the stellar trio lies Tejat (soft "j"), an Arabic plural for a term
of unknown meaning and one that has been applied collectively to
all three of the stars.  Though fourth brightest in the
constellation (after Pollux, Castor, and Alhena, Bayer
unaccountably assigned Tejat the Greek letter Mu.  It is seemingly
closely paired with the middle star of the trio, Propus (Nu
Geminorum), which is also referred to as Tejat Prior, rendering Mu
Geminorum Tejat Posterior.  The name has now been transferred
exclusively to Mu.  Bright third magnitude, Tejat is noticeably
reddish in color, reflecting its cool 3650 Kelvin degree surface
and class M (M3) status.  Lying 230 light years away (less than
half the distance of Propus), Tejat is a red giant that radiates
1540 times more energy than the Sun (after correction for a large
amount of infrared radiation).  While not all that much compared
with some of the stars of the naked-eye sky, the low temperature
leads to a star of great proportions, one large and close enough
for an accurate measure of its angular size, 0.0135 seconds of arc. 
>From its distance, Tejat therefore has a radius 104 times that of
the Sun, or 0.48 Astronomical Units, about half the size of the
Earth's orbit, which agrees nicely with that found from luminosity
and temperature.  A ninth magnitude "companion" two minutes of arc
away (itself a double star) is most likely a line of sight
coincidence.  Like many large giants and supergiants, Tejat is
slightly variable.  Classed as a "giant irregular," its apparent
brightness erratically wanders around by about 25 percent, between
magnitudes 2.8 and 3.0.  The star's greatest importance to
observers seems to be in calibrati

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 16, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Having just passed its last quarter, the Moon spends the entire
week in its waning crescent phase, new Moon not reached until next
Friday, the 23rd.  Watch in the morning sky as the Moon both slims
and falls farther each day toward the eastern horizon, first
through Sagittarius and then climbing northward a bit into
Capricornus before it encounters the Sun.  Note that in the
northern hemisphere that the illuminated side of the waning
crescent points downward to the left.  After the Moon passes new,
the evening waxing crescent will point down and to the right. 
Illustrators of ads and of children's books commonly get it
backwards or otherwise wrong.  While admiring the waning Moon, be
sure to look for brightening Mars, which will shortly be passing
above its namesake, the bright reddish star Antares ("like Mars,"
"Ares" the Greek version of the god of war) in Scorpius.

However, Venus, not Mars, dominates the week, as the second planet
from the Sun passes greatest brilliancy for its current evening
appearance on the night of Wednesday, February 21.  Just look to
the southwest in fading twilight.  Venus, always third in
brightness after the Sun and Moon, now reaches magnitude -4.6, 18
times brighter than the sky's brightest star, Sirius, and 8 times
brighter even than Jupiter (which, with Saturn, is high in the sky
to the south in early evening and is now making an easterly trek
through Taurus).  When Venus is on the other side of the Sun, at
superior conjunction, we see the full sunlit face.  As the planet
swings around the Sun toward the Earth, we see (through the
telescope) progressively more of the nighttime side, and the planet
becomes gibbous, and then at maximum elongation from the Sun it
looks like a quarter Moon.  Now past maximum elongation, it is a
thin crescent.  However, even though we see relatively less
daylight, the planet is getting closer to Earth, so its angular
size and brightness continue to grow -- until now, after which the
phase effect becomes more important.  Think how much brighter Earth
(which would always be near "full") would be from Venus (except of
course you would never see it from the ground because of Venus's
perpetual thick cloud cover).

Of all the bright winter constellations, Auriga, the Charioteer,
seems to get the least respect, perhaps because it is so dominated
by Orion, not in the zodiac like Gemini and Taurus, and for most in
mid-northern latitudes so high that one needs to look nearly
overhead.  There you can admire the sixth brightest star in the
sky, Capella (the "she-goat"), and the little three-star triangle
to the southwest of it that makes an asterism (an informal
constellation) called "the Kids."  The southernmost star of the
pentagonal figure (Elnath) is held in common with Taurus, and
represents the Bull's northern horn.

STAR OF THE WEEK.  ALMAAZ (Epsilon Aurigae).  Tucked to the
southwest of Capella in Auriga is a prominent narrow triangle of
stars.  Older traditions call the southern two (Hoedus I and Hoedus
II) "the Kids," though Almaaz (Bayer's Epsilon star), which shines
at mid-third magnitude (2.99), is now commonly included.  The
Arabic name, in parallel with Capella (from Latin, "the she-goat"),
means "the he-goat."  One of the weirdest and least understood
stars in the sky, Almaaz is a hot-end class F (F0, though some say
A8) supergiant located a rather uncertain 2000 light years away. 
>From a 7800 Kelvin surface a bit over an Astronomical Unit (Earth-
Sun distance) in diameter (almost half again the size of Mercury's
orbit), it radiates nearly 47,000 times as much energy as the Sun,
its mass somewhere between 15 and 19 times solar.  Surrounding it
is a gaseous ring.  Class F supergiants are relatively rare to
start with, but that is only the beginning.  Like so many stars,
Almaaz is an eclipsing double star, but one of strange and
magnificent proportions.  Nothing about it is ordinary.  The
prototype eclipser, Algol (in Perseus), dims by about a magnitude
every 2.87 days when a class K giant star passes in front of a
small but bright hot class B star.  Almaaz drops by about the same
amount, roughly to the apparent brightness of Hoedus I (Zeta
Aurigae).  However in contrast to Algol, Almaaz's eclipses are 27.1
years apart and last for an amazing two years, testimony to the
huge dimensions of the system.  Odder yet, it is the big class F
supergiant that gets eclipsed, by something that is vastly larger
than it!  And no one knows quite what.  The prevailing model is
that Almaaz is in mutual orbit with a star that is surrounded by a
thick ring of obscuring dust set nearly edge on.  A

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 9, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon, now past full, fades from the evening sky.  At first
waning through its gibbous phase, it passes third quarter the night
of Wednesday the 14th shortly before Moonrise in the Americas, and
then enters its waning crescent phase.  The morning of Thursday the
15th finds the Moon just to the north of Mars, the pair just short
of encountering the three-star head of Scorpius.  

Mercury and Uranus make an odd pair this week, as both go through
conjunction with the Sun, Uranus on Friday the 9th, Mercury on
Monday the 12th.  The two could not be more different.  Mercury, an
"inferior planet," one closer to the Sun than we are, passes its
"inferior conjunction" as it goes roughly between us and the Sun. 
Half an orbit later (as viewed from Earth) it will pass its
"superior" conjunction," when it is on the other side of the Sun
from us.  Uranus, on the other hand is a distant "superior planet"
that is outside the Earth's orbit.  Its conjunction is always on
the other side of the Sun.  When the Earth swings through half an
orbit (as viewed from Uranus), Uranus will then pass opposition
with the Sun, when the two are opposite each other in the sky. 
Though both are in solar conjunction, Mercury is moving westward
against the stars, retrograde, whereas Uranus is moving in its
normal direction, eastward.  They could hardly be more different
physically, either: Mercury is less than 40 percent the size of the
Earth, cloudless, and made of iron and rock; Uranus is four times
Earth's size, 15 times its mass, is covered with methane clouds and
haze, and is made of hydrogen, helium, water, and other light
stuff.  Unfortunately, the solar glare will make viewing them
impossible.

We CAN, however, easily view brilliant Venus, high in the
southwestern sky at twilight, and still brightening, as well as the
still-close pairing of giant Jupiter (the brighter) and Saturn,
high to the south in early evening, both still set within the
confines of Taurus (not to mention the morning appearance of Mars,
which is now rising around 1:30 AM).  

The north polar sky begins to turn our thoughts toward spring.  At
8 PM, Perseus is nearly overhead or to the north for most in North
America.  While Cassiopeia falls to the northwest, and the Little
Dipper rides beneath the pole below Polaris, the Big Dipper in Ursa
Major begins its majestic rise in the northeast, led by the four-
star "Bowl."  Watch as it climbs ever higher as the Earth rounds
the Sun and the seasons advance.  To the south, Orion the Hunter
still dominates, below him a distorted box that makes Lepus the
Hare, and below that the small flat triangle of Columba, the Dove.

STAR OF THE WEEK.  MAIA (20 Tauri).  The Pleiades, the Seven
Sisters star cluster (one of two naked eye clusters that belong to
Taurus, the other the Hyades), twinkle high in northern hemisphere
autumn and winter skies, while shining closer to the horizon in the
skies of southern hemisphere spring and summer.  Maia, a proper
name, is one of the seven mythical daughters of Atlas and Pleione. 
Shining at bright third magnitude (3.87) from a distance of 385
light years, she ranks fourth brightest after Alcyone, Atlas, and
Electra.  Except for Alcyone (Eta Tauri), the Pleiades' stars carry
only Flamsteed numbers, Maia number 20 in the west-to-east parade
of numbered naked-eye stars within the celestial Bull.  A blue-
white class B (B8) giant star, Maia radiates 660 times more energy
than does the Sun from a warm surface with a rather uncertain
temperature of 12,600 Kelvin.  Its radius of 5 1/2 times that of
the Sun gives it true giant status, although the giants in these
hotter stars are nowhere near as large as their cooler orange
cousins like Arcturus and Aldebaran (which lies in front of the
Hyades).  As a giant, Maia either has shut down its internal
hydrogen fusion or will do so very shortly, its mass of a bit over
four times that of the Sun giving the star a destiny as a massive
white dwarf.  Like the other stars of the cluster, Maia is involved
with the Pleiades reflection nebula that peaks around Merope.  Maia
appears to be a relatively slow rotator, and as such has a fairly
quiet atmosphere.  As a result, different kinds of atoms drift
downward under the pull of gravity, whereas others are lofted
upward by radiation, the effects making Maia one of the "mercury-
manganese stars," in which these two and other chemical elements
are greatly enhanced (manganese in Maia up by a factor of 160
compared with hydrogen).  The star also has a bit of a curious
history.  Fifty years ago, the great astronomer Otto Struve
suggested that Maia was slightly

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 2, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon grows through its waxing gibbous phase this week toward
full, the phase reached close to midnight in North America the
night of Wednesday, February 7, within the confines of Leo, near
Regulus.  Only 9 hours before full, the Moon also passes perigee,
its closet point to the Earth, which increases the amount of
moonlight to its maximum, 11 percent more than average, and 25
percent more than when the Moon is at apogee.  As the Moon
approaches full, it passes south of Saturn the morning of Friday
the 2nd and south of Jupiter on the afternoon of the same day. 
That night, Friday the 2nd, the Moon will appear just to the
southeast of the giant planet, the Moon and two the planets
appearing together high in the constellation Taurus.

Today, Friday the second, is a bit of a holiday (though no one
seems to get the day off).  Groundhog Day (Candlemas in Britain) is
one of four "cross-quarter" days that split the difference in time
between the solstices and the equinoxes.  Groundhog Day, in fact an
astronomical holiday, is halfway between the day on which the Sun
passes the winter solstice in Sagittarius to mark the beginning of
astronomical winter (in the northern hemisphere) and the day of
vernal equinox passage in Pisces, which marks spring.  In other
words, we are halfway there, to spring.  The other cross-quarter
days are May Eve, the night before May Day (May 1, "Beltane" in
Scotland and Ireland), All Saints' Day Eve -- Halloween -- in
October, and (in Britain) "Lammas Day," a harvest celebrating, on
August 1.  Astronomical roots, which include our common time-
keeping units, go very deep.

Nothing quite dominates the sky like Orion, his season now best
upon us.  No training is needed to find the three bright stars of
his belt (now high to the south in mid-evening), which the Arabians
referred to as the "String of Pearls."  The trio very closely rides
the celestial equator, rising due east and setting due west.  To
the upper left is the reddish supergiant Betelgeuse, to the lower
right blue-white Rigel.  Hanging down from the Belt is the three-
star Sword, the middle star of which is embedded in the great Orion
Nebula, which marks a region of past and intense star formation and
which is easily visible with the least optical aid.  Unlike most
constellations, which are random groupings of stars, most of
Orion's stars are physically related in the vast "Orion OB1
Association," an expanding group of hot blue-white stars that were
born more or less at the same time within the space now occupied by
the great constellation.  Well down and to the left, pointed to by
the Belt, is Sirius in Canis Major, the brightest star in the sky. 
If you live far enough south, below about 35 degrees south
latitude, you can also see the second brightest star, Canopus in
Carina, the hull of Jason's ship, Argo. 

STAR OF THE WEEK.  ALNILAM (Epsilon Orionis).  Three brilliant
stars mark the belt of Orion the Hunter, from right to left (west
to east) Mintaka, Alnilam, and Alnitak.  The names of all three
refer to the whole set.  The outer two are named after the "belt"
of the Arabs "Central One (a mysterious feminine figure), while
Alnilam comes from an Arabic word that aptly means "the String of
Pearls," which the trio so well represents.  The brightest of the
Belt stars (which Bayer lettered in alphabetic order, again from
right to left, Delta, Epsilon, and Zeta), and ranking fourth in the
whole constellation, Alnilam shines at bright second magnitude
(1.70).  Though all three stars have similar colors, classes, and
temperatures, Alnilam, a hot B (B0) bright supergiant, is notably
the most luminous: it is brightest even though farthest away, half
again farther than the other two (which lie at nearly the same
distance, about 870 light years).  From Alnilam's measured (though
rather uncertain) distance of 1340 light years, it spectacularly
radiates (after correction for its great amount of ultraviolet
light) 375,000 solar luminosities from its 25,000 Kelvin surface,
and is so hot that it illuminates its own (faint) nebulous cloud
from the surrounding interstellar gases.  Alnilam has served for
many years as a "standard star" against which to compare others. 
Its brilliant blue and relatively simple spectrum also provides a
fine background against which to study the gases of the intervening
interstellar space.  Like most supergiants, Alnilam is losing mass. 
A powerful wind blows from the star's surface at speeds up to 2000
kilometers per second, the flow rate two millionths of a solar mass
per year (20 million times that from the Sun).  Though seemingly
single, which

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 26, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon spends most of the week in its waxing crescent phase,
finally reaching first quarter on Thursday, February 1 a few hours
before its mid-day rise.  The night of Saturday the 27th, the lunar
crescent will be seen a few degrees below brilliant Venus, which in
the Moon's absence continues to dominate the early evening sky.  

Venus passed its greatest eastern elongation from the Sun over a
week ago, though little change can yet be seen.  Now it is
Mercury's turn, greatest eastern elongation taking place on Sunday
the 28th, making this week a good time to find the two "inferior
planets," those closer to the Sun than we are, stretched out from
the Sun at maximum advantage.  Find a clear horizon and look to the
west southwest in dusk to find Mercury, the little planet quite
bright but washed out by the thick air near the horizon and by
fading twilight.  Because the sky looks so much like a bowl above
our heads, it is difficult to appreciate the effect of the Earth's
atmosphere.  When we look right on the horizon we actually see
through 38 times more air than when we look directly overhead.  As
a result, stars and planets appear much fainter as they rise and
set than when they are high in the sky.  

Thickness of atmosphere is no deterrent to Jupiter and Saturn,
however, which are now high to the southeast at the end of
twilight, the two planets brilliant near the Pleiades of Taurus,
leading the eye to appreciate the beauty of the Seven Sisters star
cluster.  Jupiter is the brighter of the pair, and after Venus (and
of course the Moon) the brightest body of the current sky.  Saturn
is just a bit to the west of it.  Both bodies are now in normal
direct motion, Jupiter ever-so-slowly pulling to the east of the
ringed planet.

About the time Jupiter and Saturn set, around 2 AM, Mars rises. 
Now smack in the middle of the classical figure of Libra, the red
planet is being overtaken only slowly by the Earth, and will remain
a morning planet until the beginning of April, when it will finally
begin to rise before midnight.

>From mid-northern latitudes, Auriga shines high in the sky above
Orion, Perseus beginning to descend to the northwest.  However,
from there to the North Celestial Pole, closely marked by Polaris,
the sky is drab indeed.  This area is filled with one of the larger
constellations, and certainly one of the dimmer, the obscure modern
figure Camelopardalis, the Giraffe, its three brightest stars,
which are strung out in a line toward the pole, shining only at
fourth to fifth magnitude.

STAR OF THE WEEK.  MEROPE (23 Tauri).  There is little in the sky
more attractive than the marvelous cluster of the Seven Sisters,
the Pleiades of Taurus, a bright compact, fairly young "open
cluster" that is filled with bright stars of blue class B.  Most
eyes see 6 stars, some 8 (or even more).  Seven of the stars are
named for the sisters, who in Greek mythology were the daughters of
the god Atlas and mortal Pleione, who appear there as well.  Among
the classic six visible stars is fourth magnitude (4.18) Merope,
which ranks fifth within the striking group.  Of the Pleiades, only
Alcyone carries a Greek letter (Eta), the rest designated only by
Flamsteed numbers (Merope otherwise called 28 Tauri).  Merope, a
mid-class B (B6) subgiant lies, along with the rest of the cluster,
at a well-determined distance of 385 light years.  From its hot,
blue-white 14,000 Kelvin surface, it shines with a total luminosity
(allowing for ultraviolet light) of 630 times that of the Sun.  It
is a particularly fast rotator, spinning with an equatorial speed
of at least 280 kilometers per second, 140 times that of the Sun. 
Given that the star is 4.3 times the solar size, it makes a full
rotation every 18 hours.  The fast rotation affects the star's
surroundings and the spectrum.  Like others in the Pleiades crowd,
Merope is an emission-line (Be, class B emission) star, its rapid
rotation flinging out a disk of bright, emitting gas though
Merope's is quite thin compared with others such as Pleione. 
Nevertheless, it is sufficiently dense and hot (from shock waves)
to produce observable X-rays.  Merope's greatest claim to fame,
however, is not the star itself, but its surroundings.  The
Pleiades is enmeshed in a cloud of dusty gas.  The stars are not
hot enough to make the gas glow (as they are to make the Orion
Nebula).  Instead, the tiny dust grains embedded in the cloud
scatter and reflect the starlight to make the quite-blue Pleiades
reflection nebula.  It is at its brightest around Merope, so bright
as to have special names, the Merope Nebula and "IC 349," the "IC"
s

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 19, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

This week holds the new Moon, passed the morning of Wednesday, the
24th.  Six hours later, it passes apogee, when it is farthest from
the Earth.  Before that date you can see the earthlit waning
crescent in the morning hours to the east before sunrise, and
afterward the reversed waxing crescent in the west after sunset. 
The Moon will be technically visible in western twilight the night
of Thursday, the 25th, nearly in conjunction with (and below) the
planet Mercury, but both will be quite difficult to see.  By the
night of Friday, the 26th, however, the slim lunar crescent (now
well up and to the left of Mercury) will be quite visible. 
Mercury's visibility will improve until early next week, when it
reaches greatest eastern elongation from the Sun.

Earlier in the week, on Monday the 22nd, Mercury passes close
conjunction with Uranus, a quite-invisible event.  At the end of
the week (on Thursday the 25th), Uranus's near-twin, more-distant
Neptune, finally passes conjunction with the Sun to become a
morning object.  Uranus will follow in early February.  Though
Mercury will be, as usual, hard to find, Venus will not, as it
continues to blaze away in the southwestern evening sky.

In a highly unusual coincidence, both Jupiter and Saturn cease
their westerly retrograde motions not only the same day (Thursday
the 25th), but within about an hour of each other.  It would be
difficult to figure how often this happens without running actual
orbital calculations, but it must be once in hundreds, if not
thousands, of years.  Odder still, Jupiter ceases retrograde first
even though it is the more-easterly planet (ordinarily, the more-
westerly ones finish first), a result of Jupiter's being closer to
us than Saturn (4.4 as opposed to 8.6 astronomical units, the AU
the average distance between the Earth and the Sun).  Both planets
will thereafter begin to move in their normal easterly direction
against the background stars (for now, of Taurus), Jupiter pulling
away from the ringed planet as it heads for the next constellation
of the zodiac, Cancer.  

Taurus (Jupiter and Saturn's temporary home) is at its best in the
early evening around 8 PM.  Look for the vee-shaped Bull's head
made by the Hyades cluster, the Seven Sisters (Pleiades) up and to
the right.  Aldebaran, the Bull's orange eye, is only situated in
front of the Hyades and is not actually a part of it.  Taurus is
one of the few constellations that link to another, the northern
horn (which extends northeast of Aldebaran) tied to bright Auriga,
the Charioteer, which with bright Capella, stands above Orion.  The
other linking constellations are northern autumn's Andromeda with
Pegasus and summer's Ophiuchus with Serpens.  

STAR OF THE WEEK.  KEID (Omicron-2 Eridani, or 40 Eridani).  An
unassuming star, Keid (Omicron-2) of Eridanus (and more commonly
known by its Flamsteed number, 40 Eridani) seems to play second
fiddle to its somewhat brighter neighbor Beid, Keid the Arabic "egg
shells" of Beid, "the eggs."  The two stars are not a true binary,
Beid seven times farther than Keid.  Keid, however, is by far the
more interesting star.  A modest faint-fourth magnitude (4.43) to
the eye, Keid is a triple star, famed not for Keid itself but for
the much fainter companions.  The 67th closest star and the 50th
closest star system, Keid lies a mere 16.5 light years away.  Much
farther and the cool class K (K1) ordinary dwarf (Keid-A, one of
the very few visible to the naked eye) would not be visible.  Other
than proximity, Keid A has little to offer but a cool temperature
of 5100 Kelvin, a low luminosity of 0.4 times that of the Sun, and
a mass around three-fourths solar.  Such stars abound in space, but
they are so faint that few can be seen without a telescope.  A
little over a minute of arc (83 seconds) away, however, and easily
seen with only a small instrument, lies the prize of the system,
Keid-B (rather, 40 Eridani B), a tenth magnitude white dwarf, by
far the most visible white dwarf of all (though Sirius-B is more
famed), its luminosity a mere 0.008 that of the Sun.  White dwarfs
are the final products of ordinary solar-like evolution, and are
the spent cinders of the original stars' cores.  Ordinary dwarfs
(like the Sun and Keid-A) fuse hydrogen to helium in their cores. 
When the hydrogen is gone, the stars become giants and fuse the
helium to carbon and oxygen.  The outer envelope is ejected, and
all that remains is the low-mass ultradense carbon-oxygen white
dwarf.  Typical white dwarfs are only about the size of Earth and
have extraordinary average densities of a ton per cubic centimeter. 

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 12, 2001.
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The week begins with the Moon in its waning gibbous phase.  Passing
third quarter the morning of Tuesday the 16th about the time of
sunrise, it finishes the week as a waning crescent.  The morning of
Wednesday the 17th finds the Moon to the northwest of Mars (now in
Libra), the morning of Thursday the 18th to the northeast of the
red planet.

The week belongs not to Mars or the Moon, however, but to Venus,
which reaches greatest eastern elongation from the Sun shortly
after it sets the night of Tuesday the 16th.  "Elongation" is the
angle that any planet makes with the Sun.  Venus, closer to the Sun
than we are, can never get more than 47 degrees from the Sun, its
maximum angle.  The planet thus achieves its greatest visibility
this week.  Since it is now at the tangent point in its orbit as
seen from Earth, it will (through the telescope) also appear as the
quarter moon does in the sky, one half seen in sunlight, the other
half in darkness.  From here on to conjunction with the Sun on
March 29, the planet will appear as an increasingly thinner
crescent.  At the same time, it is approaching the Earth and
appearing ever-larger, and thus brighter, maximum brilliancy to be
reached on February 21.  Though the angle between the Sun and Venus
will now decrease, since the Sun is moving northward along the
ecliptic and is setting later, Venus will continue to set later as
well until early February, the "evening star" quite dominating the
early night sky.

At the other end of the planetary spectrum, Mercury passes
conjunction with Neptune (Mercury 2 degrees south) on Saturday, the 13th,
the solar glare causing the event to go unseen.  In between, the
early evening eastern sky is dominated by Saturn and Jupiter, both
still retrograding (but not for long) in Taurus.  The three
planets, Venus, Saturn, and Jupiter, act like a dotted line in the
sky that to a good approximation shows the ecliptic and the path
the Sun will soon follow as northern winter wanes and turns to
spring.

Look to the southeast around 8 PM to see Orion climb the sky.  To
the right, on the meridian to the south, winds dim Eridanus, the
celestial River that represents the "River Ocean" of classical
times.  From modest Cursa, just northwest of Rigel in Orion,
Eridanus flows west, and then curves south below the horizon of
northern latitudes, ending in brilliant Achernar, which can be seen
only south of 32 degrees north latitude.  High above, the Pleiades,
the Seven Sisters star cluster, crosses the meridian as well.  Most
eyes see 6 stars, but those with sharper vision might catch 8 or
even more, the view richly enhanced by binoculars.

STAR OF THE WEEK.  BEID (Omicron-1 Eridani).  In the middle of the
first southerly turning of Eridanus (the River) lie a seeming pair
of fourth magnitude stars, Beid and Keid, which Bayer placed far
down in the Greek alphabet as Omicron-1 and Omicron-2.  The part of
Eridanus that contains Azha (Eta Eridani) and stars south, where
the River makes its second southerly plunge, were known to the
Arabs as the "Ostrich's Nest."  Outlying Beid refers to "the Eggs"
and Keid to the "Egg Shells," all showing a wonderful celestial
mixture of cultures.  Though to the eye they may look like a real
pair, they are very much not.  Keid (Omicron-2) is very close, a
mere 16.5 light years away, where as Beid (Omicron-1) lies over 7
times farther, 125 light years distant.  To be roughly similar in
apparent brightness, Beid (mid-fourth magnitude, 4.04) must be much
the more luminous.  Indeed, it is a white class F (F2) giant, with
a temperature of 7100 Kelvin and a luminosity 28 times that of the
Sun.  Though direct measure of mass is not possible (it has no
known companion, whose orbit would determine the mass), its current
luminosity and temperature compared with theory show its mass to be
almost exactly double that of the Sun.  The star seems to have
recently left the core-hydrogen-fusing main sequence of stars, and
is now evolving with a dead (for now) helium core that is
surrounded by a shell of fusing hydrogen.  With a radius of 3.5
times that of the Sun, its rather high equatorial rotation speed of
100 kilometers per second show the star to rotate in less than 2
days.  Beid's real distinction, however, is that it is a notable
variable, a "Delta Scuti" star, the variation discovered only in
1971.  Delta Scuti stars are less-luminous versions of the Cepheid
pulsating variables (those like Mekbuda).  Unlike simple Cepheids,
however, they pulsate very subtly and rapidly with several periods
at the same time.  Beid varies by at most only a few hundredths of
a magnitude (a 

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 5, 2001
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Welcome not to the turn of the odometer (to 2000), but to the turn
of the Millennium itself, not to mention the 21st Century and the
New Year, 2001.  We begin with the Moon heading toward its full
phase, reached on Tuesday the 9th.  Having just had a solar eclipse
last Christmas Day, we get a lunar eclipse this full Moon (if
conditions are right for a solar eclipse, a lunar eclipse will
commonly precede or follow).  This one will be total and a beauty,
but for Europe, Asia, and Africa, not for the Americas.  This full
Moon will be the second highest of the year, shining from amidst
the stars of Gemini.  The highest will occur next December, when
the Moon will also be eclipsed, bookending the year.  (That eclipse
will not be visible in the Americas either, no loss, since it will
be a penumbral partial-shadow eclipse, which is essentially a non-
event).  Lunar perigee takes place only a day after full
(Wednesday, the 10th), the combination of full, near-perigee, and
near-perihelion (for Earth) producing exceptionally high ocean
tides.

While approaching full, the Moon will make beautiful configurations
with the two giant planets, Saturn and Jupiter, which are both
still in retrograde against the stars of Taurus.  The night of
Friday, the 5th, the waxing gibbous Moon will pass just beneath
Saturn, while the following night it will be down and to the left
of Jupiter, at the same time just above Taurus's brightest star,
first magnitude Aldebaran, all this action taking place above the
sky's brightest constellation, great Orion.  One can hardly ignore
Venus, however, shining brilliantly in the southwest at sundown. 
If you stay up after midnight, also note Mars, which now rises
around 2 AM and continues to brighten among the stars of Libra.

Directly north of Jupiter and Saturn, find the hero of the
Andromeda myth, Perseus, which passes roughly overhead in mid-
northern latitudes and contains a bright portion of the Milky Way. 
The constellation is especially known for naked-eye clusters.  The
most famed, the Double Cluster, lies to the northwest of the bright
string of stars that make the most prominent part of the
constellation.  Just barely visible without optical power, it is a
marvelous sight in a small telescope.  Hardly recognizable as a
cluster is central Perseus itself, making Perseus one of the
constellations that are not made just of random stars, but of those
that are at least in part physically associated.  Much the same is
true for Orion.  Though not a cluster bound by gravity, many of the
stars are in loose association, their births connected in both time
and space.  Look in particular for the famed "belt," the Arabs
"string of pearls," which nearly straddles the celestial equator
and lies above the most famed of all interstellar gas clouds, the
Orion Nebula.

STAR OF THE WEEK.  GAMMA PER (Gamma Persei).  The naming of stars
at times seems to have little to do with their brightnesses.  All
the first magnitude stars, and most of the second magnitude visible
from classical lands, have names (an outstanding exception being
Gamma Cassiopeiae), but from there on the naming is erratic.  Gamma
Persei (so-called though actually fifth brightest in Perseus)
carries no known western name even though it lies at mid-third
magnitude (2.93).  One reference calls it "Algenib," an alternative
name for Mirfak (Alpha Persei), but that is surely a mistake.  And
a pity too, as the star fascinates.  Lying about 225 light years
away, it is a close (only barely separable) double that consists of
a class G (G8) giant coupled with an ordinary main sequence class
A (A2) dwarf, which together shine 300 times more brightly than the
Sun, the yellow giant notably the brighter of the two.  Rather
overwhelmed in Perseus by Algol, the brightest eclipsing double
star in the sky, Gamma Per takes its fame from being the second
brightest eclipser, a fact only recently discovered.  The star was
long known from spectroscopic (Doppler) observations to be a double
that takes 14.6 years to orbit. Moreover, the plane of the orbit
was found to lie tantalizingly in the line of sight, presenting the
small possibility that the two might actually eclipse each other. 
Diligent observations discovered the eclipse (which produces a 30
percent dip in the light output of the system and is just visible
to the naked eye) only in 1990.  The event, in which the giant gets
in the way of the dwarf, takes somewhat under two weeks.  Analysis
of the observations shows the average separation of the stars to be
10 Astronomical Units.  The orbit, however, is highly elliptical
(almost as much as that o

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 29, 2000
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

Our Moon, Earth's ancient companion, ends the year, the century,
and the millennium (technically anyway) as a growing crescent.  On
the second day of 2001 it will pass its first quarter amidst the
dim stars of Pisces just to southeast of the Great Square of
Pegasus, thereafter waxing toward full.  As it moves against the
starry background, it beautifully encounters the sky's brightest
planet.  Tonight, Friday the 29th, the earthlit crescent Moon will
make a close pass to (falling just below) Venus, the modest star
Deneb Algedi (Delta Capricorni) roughly between the two.  The Moon
will pass beneath Saturn next Friday evening, the ringed planet now
shining to the east at sundown just above and to the right of
brilliant Jupiter.  The morning hours host only one planet now,
Mars, which continues to brighten among the stars of eastern Virgo
as the Earth slowly catches up with it.  The red planet will not
become an evening object until the beginning of April, when it
finally rises before midnight.

Once again the Earth is on stage, as it passes perihelion with the
Sun, when it is closest to the Sun on its elliptical orbit, on
Thursday, January 4 at a distance of 147,097,500 kilometers
(91,402,150 miles).  Rather obviously, since the northern
hemisphere is now in the dead of winter, the variation in distance
from the Sun -- which is not great, only about 3.5 percent -- is
not the cause of the seasons (which are produced solely by the tilt
of the Earth's axis relative to the vertical to the orbital plane). 
Aphelion, when the Earth is farthest from the Sun, will take place
this year on the United States' national holiday, the Fourth of
July, when we will be 152 million kilometers (94.5 million miles)
away.  All things being equal, the variation in distance to the Sun
should cause the Earth's southern hemisphere to have greater
seasonal extremes than the northern (Argentina's summer coinciding
with perihelion, winter with aphelion), but the effect is quite
lost in the inequality of land masses on Earth, most of the
moderating oceans being in the south.

The autumn stars, Pegasus, Aries, and the rest, begin their annual
flight to the west as the winter stars take over.  Orion is now up
as the sky darkens.  He makes his great transit across the southern
meridian around 11 PM, and serves as the centerpiece for a host of
bright stars and constellations that encircle him.  The stars of
Orion, together with those of Canis Major, Taurus, Perseus, and
others, are part of a celestial ring of bright, relatively nearby
stars called "Gould's Belt" (after B. A. Gould, a prominent
nineteenth century astronomer) that tilts slightly relative to the
Milky Way, the band of light created by the disk of our Galaxy (the
portion of the Milky Way running to the east of Orion faint and
difficult to see).  Happy New Year to all; may it bring health,
peace, and prosperity.

STAR OF THE WEEK.  MESARTHIM (Gamma Arietis).  Shining third among
the stars of the flat triangle that make the classical figure of
Aries, the Ram (for that reason gaining the Gamma designation),
Mesarthim actually takes fourth place in the constellation after
non-named 41 Arietis, which glows softly off to the northeast.  The
name (derived from Arabic) originally came from the same root as
that for the Beta star, Sheratan (meaning "the two"), but was
corrupted by mistranslations into its current form.  Famed from
history, Mesarthim is also called "the first star of Aries," as
during the ancient times when the stars were being systematically
organized, it was the closest of the Ram's stars to the vernal
equinox.  (Precession, the 26,000 year wobble of the Earth's axis,
has since shifted the equinox westward to Pisces.)  Shining at us
from a distance of 204 light years, Mesarthim is one of the classic
double stars of the sky, its two components, of nearly equal
brightness, an easily separable 8 seconds of arc apart and known
since 1664.  Both actually white, the (just slightly) fainter one
(Gamma-1, since it is the more westerly) has been called "pale
grey," which is a visual contrast effect.  Both fifth magnitude
(Gamma-1 4.83, Gamma-2 4.75), they combine to make a mid fourth
magnitude (3.9) star.  Gamma-1, a class B (B9) dwarf, is the
hotter, its temperature 11,000 Kelvin.  Gamma-2, a bit
controversial, has been classed both as an A (A1) (probably) dwarf
and as a B (B9.5) subgiant (meaning that it may be starting to
evolve), the temperature between 9200 and 9800 Kelvin.  Though
Gamma-1 is a bit dimmer to the eye, it is actually the more
luminous, as the higher temperature causes more of its light to
shine i

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 22, 2000
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon wanes through its crescent phase early in the week,
passing new on Monday, the 25th, Christmas day.  By the night of
Tuesday, the 26th, Boxing Day in Canada in the UK, it will be just
visible as a waxing crescent in the southwest shortly after
sundown.  Two days later it passes its apogee, its farthest point
from Earth.  

Venus, in the far southwest, grows ever more lustrous, while
Jupiter, on the other side of the sky, to the east, climbs yet
higher in early evening, Saturn up and to the right.  Venus will
pass 1.3 degrees north of Uranus the night of Saturday, the 23rd. 
The event will be visible in binoculars and will provide a
convenient way of finding the dim planet, the seventh out from the
Sun.  Mercury makes less-visible news as it passes superior
conjunction with the Sun (on the other side of the Sun) also on
Christmas Day, just two hours after new Moon.  

Overwhelming all, however, is a partial eclipse of the Sun that
will be visible on Christmas Day, Monday the 25th, throughout North
America.  Usually, the new Moon passes above or below the Sun; this
time new Moon occurs while the Moon is near crossing the ecliptic
(the solar path), so it will pass across the solar disk.  The event
is actually a polar partial eclipse, but with the Earth's northern
axis now turned away from the Sun, it centers over northern Canada. 
Nowhere will it be total.  Time of occurrence depends mostly on
time zone and longitude.  In the central US, it will begin around
9:45 AM (CST), be at maximum around 11:15, and end about 12:50 PM. 
In the New York City area, the times are 11:10 AM (EST), 12:45 and
2:20 PM, while near San Francisco they are 7:35 AM (PST), 8:20 AM,
and 9:15 AM.  The coverage of the Sun by the Moon depends more on
latitude: the farther north of the central US, the better the
event.  Chicago and New York see 55 percent.  The west coast does
not fare so well, however, with only 19 percent coverage in San
Francisco.  Please do NOT try to see the eclipse directly, as any
exposure of the eye to brilliant sunlight can cause retinal burns
and permanent damage.  Instead, simply make a pinhole in a piece of
cardboard and PROJECT the image onto a piece of paper.  (Do NOT
look through the pinhole!)  Such pinhole projection produces a
beautiful -- and safe -- optical image.  You can also merely stand
under a leafed tree (admittedly hard to find in winter), and see
the eclipse projected on the ground through the pinhole spaces
between the leaves (or pine needles).

The constellations of the zodiac this time of year brighten from
southwest to northeast, from dim Capricornus (through which Venus
is now sweeping), Aquarius, and Pisces, to the brighter flat
triangle that makes Aries, and into Taurus (which now holds Jupiter
and Saturn) and finally Gemini, the most northerly of them and the
container of the Summer Solstice.  Take heart, the Sun will be
there in less than six months.  A good holiday season to all.

STAR OF THE WEEK.  SHERATAN (Beta Arietis).  The most prominent
part of Aries, the Ram, historically the "first" constellation of
the Zodiac (as it held the Vernal Equinox in ancient times), is a
thin flat triangle of stars that Bayer lettered (from east to west)
Alpha (Hamal), Beta (our Sheratan), and Gamma (Mesarthim).  The
first two are also in order of brightness, just-barely-third
magnitude (2.64) Sheratan ranking second behind Hamal.  The name
originally referred to both Sheratan and Mesarthim, and evoked "two
things" that have been lost to time, the name now applied to Beta
Arietis alone.  At first, the star looks like a very ordinary,
white, mid-class A (A5) main sequence dwarf, one in the normal
process of fusing hydrogen to helium in its core.  At a distance of
60 light years, and with a temperature of 8200 Kelvin, it is
pumping 22 solar luminosities into space.  However, it keeps a
secret from the eye, a companion that has been known for a century
and that is visible only by means Doppler motions in the spectrum
(that detect line of sight movement).  While such discoveries are
not at all unusual, Sheratan stands out as a result of the
extremely high eccentricity of the orbit (0.88), the companion
trapped in a record-holding elongated path.  Moreover, the star is
an observational treasure.  The two stars are so close together
that they cannot be separated directly through the telescope; all
we ever actually see is one star (again common, as to allow
detection via the spectrum requires the stars to be close and
moving quickly).  However, sophisticated observation of Sheratan
with an interferometer, a device that makes use of the 

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 15, 2000
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon descends during the very early part of the week through
the tail end of its waning gibbous phase.  Now rising well after
sunset, it then passes through third quarter the night of Sunday
the 17th, the phase actually reached a few hours before near-
midnight moonrise in North America.  As the lunar crescent wanes,
it will pass above Mars the night of Tuesday the 19th, and by
moonrise will be seen somewhat to the northeast of the red planet. 

Mars, now in Virgo, just to the northeast of the star Spica, is
slowly brightening as the Earth (moving slightly faster around the
Sun) creeps up on it, and is now just making the transition from
second magnitude to first.  Best visibility, Mars opposite the Sun,
is still six months off, however.  For planets we have instead the
glorious evening view of brilliant Venus in the southwest in
Capricornus, and Jupiter and Saturn, hosted by Taurus, climbing the
eastern sky.  

The big event this week involves our own planet Earth, which sees
the Sun pass the Winter Solstice in Sagittarius at 7:37 AM Central
Standard Time (8:37 EST, 5:37 PST).  At roughly the time of
sunrise, the Sun will have reached its lowest point for the year,
23 degrees 26 minutes south of the celestial equator, and
astronomical winter will begin in the northern hemisphere.  The
northern end of the Earth's axis will be tilted as far as possible
away from the Sun, sunlight must spread itself out over a larger
area, and the northern hemisphere will receive its minimum solar
heat.  Since it takes some time for the Sun to climb back up north,
the weather, as we all know, will continue to chill.  The southern
hemisphere, however, now glories in sunlight, with the Sun passing
overhead at the Tropic of Capricorn, the parallel of latitude that
lies 23 degrees 26 minutes south of the equator.  As part of the
whole event, the Sun will not rise north of the Arctic Circle, and
will not set south of the Antarctic Circle (these located 23
degrees 26 minutes from the north and south poles of the Earth).

Winter: with the constellations of autumn starting to make their
transition to the west, the Great Square of Pegasus high to the
south at 7 PM, Orion and his mighty crew begin to make an impact in
the evening sky, the celestial hunter hitting the meridian to the
south around midnight.  Here we can glory in the "winter-six" of
Orion, his two hunting dogs Canis Major and Canis Minor, Gemini
(its Summer Solstice now high in the sky), Auriga, and Taurus, the
latter carrying the Hyades, the Pleiades, Jupiter, and Saturn to
the west.

STAR OF THE WEEK.  ALGENIB (Gamma Pegasi).  Though the brightest
star of Pegasus is Enif, the Epsilon star, the stars of the Great
Square are of such obvious note that Bayer gave them Alpha through
Delta.  Delta (Alpheratz), the brightest, actually belongs to
Andromeda as Alpha Andromedae, leaving Alpha, Beta, and Gamma
Pegasi (Markab, Scheat, and Algenib) ranking 3-2-4 in the
constellation and 2-1-3 in the Square, Algenib the faintest of
them.  The star's name, from Arabic, means "the side," and
originally belonged to Mirfak in Perseus (whose alternative name is
still Algenib).  Second magnitude (2.83) as viewed from Earth,
Algenib is a brilliant hot blue class B (B2) star with a high
temperature of 21,500 Kelvin.  From its distance of 335 light
years, and allowing for a lot of ultraviolet radiation, we find a
luminosity 4000 times that of the Sun (one suggestion as high as
12,000, though that seems extreme) and a radius of 4.5 solar.  The
lower figure calls for a mass 7 times solar, the higher up to 10. 
The star is now beginning slowly to evolve.  Though classed as a
"subgiant," it is probably still fusing hydrogen into helium in its
core.  It will evolve into a massive carbon white dwarf rather like
Sirius B, the upper mass limit suggesting a more advanced status as
a neon-oxygen white dwarf.  Algenib is measured by the Doppler
effect to be an especially slow rotator, only 8 kilometers per
second (4 times solar), unusual for hot class B stars, which are
ordinarily high-speed spinners.  Most likely, we are looking at the
star almost along its axis (its pole pointed at us), so that we do
not sense the real rotation.  (If a star is exactly pole-on, the
Doppler effect, which is sensitive only to line-of-sight motions,
would give zero rotation; that is, the star would not appear to
rotate at all.)  Algenib is also among the collection of Beta
Cephei stars (named after Alfirk, Beta Cephei, and which include
such luminaries as Mirzam, Hadar, and several others).  All hot
class B stars, most beginning to evolve in some way, th

[CPS Science]Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 20, 2000
Phone (217) 333-8789.
Prepared by Jim Kaler.
Find Skylights on the Web at 
 http://www.astro.uiuc.edu/~kaler/skylights.html, 
and Stars (Stars of the Week) with constellation photographs at
 http://www.astro.uiuc.edu/~kaler/sow/sow.html.

The Moon passed through its third quarter early this morning, and
tonight, Friday the 20th, will rise shortly before midnight
daylight time just past that phase to the east of the bright stars
of Gemini.   From here it wanes through crescent to new, that phase
reached the morning of Friday the 27th.  The morning of Tuesday the
24th sees the Moon to the northeast of Mars, which is now rising in
eastern Leo (CK) a bit after 4 AM daylight time.

Working our way more into evening, Jupiter, which is now rising
behind Saturn just after 8 PM daylight time, passes five degrees
directly north of Aldebaran around midnight (in North America) the
night of Friday the 20th.  The proximity of the bright stars of
Taurus to the two giant planets makes their westerly retrograde
motion easy to follow (in addition to making them a glorious sight
as they all pass nearly overhead in the early morning sky). 
Further toward evening, Venus makes a pass at another reddish star,
this one Antares in Scorpius, the planet 3 degrees north of the
star the night of Thursday the 26th.  Bright twilight will make the
passage (though hardly Venus) difficult to see.  With the westerly
of the four large outer planets (Neptune) having made the
transition to easterly motion, it is now number two's turn, as
Uranus stops retrograde and turns around toward the east on
Thursday the 26th.

The Orionid meteor shower, caused by the debris of Halley's Comet
hitting the Earth's atmosphere, peaks on Saturday, the 21st, the
shower best in the early morning hours, the meteors seeming to come
out of the constellation Orion.  The near-third-quarter Moon will
wash out the fainter meteors of the shower.  However, the Orionids,
which typically produce about 20 meteors per minute, will last for
a few more days while the Moon dims, letting us hang on to them for
a time.  

In mid-evening, around 10 PM, the southern sky is near its best in
displaying its "wet quarter," reminiscent of an ancient rainy
season (during which the Sun was probably passing through the
constellations, rendering them invisible).  Aquarius, the water
bearer, with its 4-star "Water Jar" (or "Urn"), is right on the
meridian, while Capricornus is to the right and the huge sprawl of
Pisces is to the left southeast of the Great Square of Pegasus. 
Directly below Aquarius is the lonely first magnitude star
Fomalhaut in Piscis Austrinus, the southern fish, into whose mouth
Aquarius is sometimes depicted as pouring his water.  Above the set
and north of the celestial equator and Capricornus is another "wet"
constellation, Delphinus, the Dolphin, easily noted by its small
tight irregular box of stars.  

STAR OF THE WEEK.  SADACHBIA (Gamma Aquarii).  Aquarius (the
Waterman, or Water Bearer) mostly sprawls gently south of the
celestial equator.  It is focused upon the 4-star Water Jar (or
Urn), a somewhat dim but very noticeable triangle that has a star
smack in the middle, the Urn appearing like a "Y" or even as a
small spray of diamonds that lies directly on the equator.  Though
at bright fourth magnitude (3.84), not quite the brightest star of
the Urn, and ranking sixth in the constellation (after Sadalmelik,
Sadalsuud, Delta, 88, and Zeta Aquarii, the latter also in the
Urn), Sadachbia is the only one of the Urn's stars to be named. 
The name harkens not to the Water Bearer, but to an older Arabic
constellation, and seems to mean "the Lucky Star of the Tents," the
Urn perhaps reminding the Arabs of a desert tent.  The "Sa" prefix
on the name relates obscurely to "luck" in the same way it does in
Sadalmelik and Sadalsuud.  Sadachbia is for the most part an
ordinary hot class A (A0) star much like Vega and so many others
that make our constellations.  Lying 158 light years away, it
radiates 62 times more light than the Sun from a 9500 Kelvin sphere
three times the solar diameter.  Theory and long observation of
similar stars in double systems show it to be a hydrogen fusing
main-sequence star with a mass just short of three times solar. 
Aside from its prominent location, Sadachbia has two qualities to
recommend it.  First, like so many of its stellar kin, it is a
"spectroscopic double," one whose character can be determined only
by examining the spectrum.  A star of unknown type orbits Sadachbia
every 58 days from a distance at least 0.40 astronomical units
away, about the distance Mercury is from the Sun.  Sadachbia also
has a small 12th magnitude companion 37 seconds of arc away from
it, but it is merely a line of sight coincidence.  Of more
significance, Sadachbia, like Kaus Australis (Epsilon Sagittarii),
is a failed "Lambda Bootis"