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, they chatter
away with multiple short periods, varying by only a few hundredths
of a percent. Algenib itself has a principal very short period
(the only one so far found) of only 3.6 hours, during which it
changes by some 0.07 magnitudes. Algenib also has some mystery
companions. One, with a period of only 6.83 days and observable
only via the spectrograph, is perhaps 0.15 astronomical units away
from Algenib proper. Then, off in the distance, up to almost three
minutes of arc away, are two dim stars of 11th and 12th magnitude.
If actual companions, which is rather unlikely, they are both M
dwarfs with huge orbital periods measured in hundreds of thousands
of years. Nothing is known about any of them.
****************************************************************
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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