Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 19, 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 a day early this week.
The Moon passes through its first quarter this week the night of
Tuesday, October 23. On its way there it makes a close pass to
Mars in the middle of the American afternoon of Tuesday the 23rd,
and by evening will make a fine configuration just to the east of
the red planet, both bodies to the east of the Little Milk Dipper
in Sagittarius.
Though the sky changes only slowly from week to week, it changes
surely. At the same time each night, from one week to the next the
stars slip another seven degrees to the west, and to see the same
sight you have to look another (roughly) half an hour earlier. We
therefore lose the western stars to the Sun, the loss compensated
by the ever-earlier risings of the eastern stars. Two bright
constellations, Taurus and Gemini, representing late autumn and
true winter, and tagged with the giant planets Saturn and Jupiter,
now rise around 8:30 and 10:30 Daylight Time.
The planets of course have their own motions within these
constellations. Saturn, to the east of Jupiter, is now in
retrograde (as a result of the Earth beginning to pass between it
and the Sun) and is moving westward against the background stars.
Jupiter, on the other hand, is still in direct (easterly) motion.
It will not enter retrograde ("retro" in the trade) until November
2. As a result, the two planets are (as seen in the sky) moving
farther apart. After Jupiter enters retrograde, they will slightly
approach each other. But that is temporary. Jupiter will quickly
thereafter pull away from the ringed planet, and the two will not
be back together again for nearly 20 years, as Jupiter takes 12
years to orbit the Sun, Saturn nearly 30. As a result, Jupiter on
the average spends about a year in a given constellation of the
Zodiac, while Saturn visits each for just over two years.
Watch Cassiopeia now climb the northeastern sky opposite the Big
Dipper, her "W" beginning to go over the pole like a splayed "M."
Unlike the Dipper, all of whose are named, Cassiopeia, bright as it
is, has few that are. Following behind is bright Perseus, whose
central concentration of stars is actually a wide cluster. Between
the two, those under a dark sky can make out the "Double Cluster in
Perseus," the only example of two clusters, undoubtedly born at the
same time from the same interstellar cloud, moving through space
together. Eventually, as a result of gravitational forces from the
Galaxy, they will separate. Through such forces, and as a result
of simple "evaporation" (stars just leaving), each will
individually mostly dissolve, as will the bright stars of the
Pleiades in Taurus.
STAR OF THE WEEK. RHO CAS (Rho Cassiopiae). Cassiopeia is full of
bright stars, yet precious few have proper names. Even very bright
Gamma Cassiopeiae has none, at least in western lore. Pity then
the seemingly lesser stars, which have no chance at all. At least
in one spectacular case, however, the "lesser" tag is totally
wrong. Look to fifth magnitude (4.54, just over the line from
fourth) Rho Cas, way down on the Bayer Greek Letter list.
Estimated to be an amazing 8000 light years away, Rho Cas, greater
even than a supergiant, is a class G (G2, some say F8)
"hypergiant." Dimmed by two magnitudes by interstellar dust, still
it shines at near-fourth magnitude, radiating 550,000 times more
light than the Sun from a surface measured at 7300 Kelvin, the
star's energy mostly pouring out in the visible part of the
spectrum. The temperature and luminosity tell of a distended
surface 450 times larger than the Sun, one 4.3 Astronomical Units
across, 40 percent larger than the Martian orbit. Rotating at
least at 29 kilometers per second, Rho Cas could take up to two
years to make a full spin. Though it has no companion from which
to gauge its mass, the immense luminosity suggests roughly 40 times
solar. Theory shows that hydrogen-fusing dwarf stars from 10 to
about 60 solar masses evolve from blue class O first to become blue
supergiants and then into red class M supergiants. From around 40
to 60, however, they loop back, turning from red supergiants back
into much hotter and smaller blue supergiants. Higher than 60,
they bump into a wall and stay as blue supergiants. Rho Cas now
seems to be on its way back from being a red supergiant, when it
may have been some five times larger. If so, it is bouncing
against the "yellow evolutionary void," in which stars become
unstable and do not like to linger. And Rho Cas certainly is
unstable. It is an irregular variable, or at best a semi-regular,
and seems to have multiple periods of 820, 350, 510, and 645 days.
But these change, so the star may really be quite unpredictable.
In the summer of 1946, Rho took a dive from 4th to 6th magnitude
and, more remarkably, altered its spectral class. Pumping a huge
amount of gas into an expanding thick atmosphere, it seemed to
chill to become a cool M star. A year later, it was returning to
normal. The star did not so much dim as cool, the lower
temperature causing it to place much of its radiation in the
invisible infrared. In its more stable state, Rho Cas still blows
a 10 kilometer per second wind at a loss-rate of a hundred
thousandth of a solar mass a year, a hundred million times the flow
rate of the solar wind. It does not have much time left before it
grows its iron core and blossoms into the sky as a stunning
supernova. Thanks to Brian Heard who suggested this star.
****************************************************************
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)
*****************************************************************
--
This is the ISTA-talk mailing list.
To unsubscribe:
<mailto:[EMAIL PROTECTED]>
For more information:
<http://www.ista-il.org/ista-talk.asp>
To search the archives:
<http://www.mail-archive.com/ista-talk@lists.csi.cps.k12.il.us/>
