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 helium and settle down for a time again
as a smaller orange giant. Also called 36 Leonis, Adhafera has a
sixth magnitude "companion" (35 Leonis) 5 minutes of arc away.
Alas, it is just a line of sight coincidence, 35 Leonis being only
100 light years away, less than half Adhafera's distance, the two
moreover going in different directions. The fainter star is not
without its interest, however. A class G (G1.5) dwarf-subgiant, 35
Leonis has the same surface temperature and color as does the Sun.
Just beginning its evolution to the giant state, it is more massive
and 3.5 times as luminous.
****************************************************************
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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