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 of Sheratan, Beta Arietis), the stars
moving from a maximum separation of 18 AU to a minimum of only 2
AU. The brighter giant has a mass 2.5 times that of the Sun, while
the dimmer A star weighs in at 1.9 solar masses. The giant, like
the brighter component of Capella, is a helium-fusing giant, while
the lower mass star is still -- like the Sun -- fusing hydrogen to
helium in its core. The age of the pair seems to be around 1.9
billion years. The chances of getting an eclipse visible on Earth
from such a wide separation of the component stars is quite small.
Algol eclipses every 2.87 days. Gamma Per's next eclipse is not
until 2005, but at that time the star will be so close to the Sun
that it will be very difficult to see, so you will really have to
wait until 2019!
****************************************************************
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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