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 again over a precisely determined period of 7.176641
days (7d 4h 14m 22s). As Eta dims, it dips to spectral class G,
the temperature falling from a high of 6200 Kelvin to 5300. Like
all Cepheids, Eta Aquilae is in the process of dying. Having given
up core hydrogen fusion, it is probably fusing helium in its deep
core. In its current structure, it has become unstable, a deep
layer that valves radiation making it pulsate and change its
surface temperature and radius. Cepheid variables are among the
most important stars of the sky, as their variation periods are
strictly related to their luminosities. Once the period of a
Cepheid is found, we thus know the luminosity, which through
comparison with apparent brightness gives the distance. Cepheids
are so luminous they are easily seen in other galaxies, and are
thus the chief means by which we can determine their distances.
Eta's luminosity and general characteristics lead to a mass about
seven times that of the Sun. As it evolves, it will someday cease
its pulsations, lose most of its outer envelope, and die as a tiny
white dwarf smaller than the Earth.
****************************************************************
Jim Kaler
Professor of Astronomy Phone: (217) 333-9382
University of Illinois Fax: (217) 244-7638
Department of Astronomy email: [EMAIL PROTECTED]
103 Astronomy Bldg. web: http://www.astro.uiuc.edu/~kaler/
1002 West Green St.
Urbana, IL 61801
USA
Visit: http://www.astro.uiuc.edu/~kaler/ for links to:
Skylights (Weekly Sky News updated each Friday)
Stars (Portraits of Stars and the Constellations)
Astronomy! A Brief Edition (links and updates)
*****************************************************************
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