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 Lupi has one of the
longest periods of its class. A secondary pulsation takes 0.236798
days. A 13th magnitude "companion" 28 seconds of arc away may
belong to Kakkab or may just be in the line of sight. As a
spectrally-classed giant star, Alpha Lupi has probably just ceased
its core hydrogen fusion. At 10 to 11 solar masses, the star -- a
mere 20 million years old -- is just on the dividing line of those
that will blow up and those that will turn into massive neon-oxygen
white dwarfs.
****************************************************************
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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