[NEIU Physics Students] [Fwd: Skylights]

[Gregory Anderson]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, August 25, 2006. 
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.

See "The StarGazer" at a planetarium near you: visit
      http://www.astro.uiuc.edu/~kaler/sg.html

Support science literacy by joining the Astronomical Society of the
Pacific, among the world's premier providers of astro education and
recognized by the Independent Charities of America as one of the
top charitable organizations in the US.  Get the outstanding
astronomy magazine Mercury and a variety of other benefits.  Call
1-415-337-1100, then press 1.

"Astronomy: Stars, Galaxies, and the Universe," an audio course on
CD with 100 page study guide narrated and written by Jim Kaler, is
now available from Recorded Books.

"Vault of the Heavens: Exploring the Solar System's Place in the
Universe," an audio course on audio CD with 100 page study guide
narrated and written by Jim Kaler, is available from Barnes and
Noble.

The evening Moon is upon us this week as it grows in the waxing
crescent phase (the nighttime side aglow with Earthlight) to first
quarter the last night of the month, that phase passed the evening
of Thursday, August 31.  If there are two full Moons in a month,
the second is called a "blue Moon" (in part because it has no
formal monthly name).  August 2006 was a month for two first
quarters, but somehow "blue quarter" does not seem to work as well. 
The night of Sunday the 27th, the crescent will be just to the
southwest of Spica in Virgo.  For a better sight, be sure to look
the night of Wednesday the 29th, when a fatter crescent will pass
five degrees south of Jupiter (the angle between the front bowl
stars of the Big Dipper).  Then the night of Thursday the 31st, the
first quarter takes on (passing just south of) Antares in Scorpius. 
The stellar passages are so close that the Moon will actually
occult Spica and Antares as seen from parts of the southern
hemisphere.

Planetary events feature first Venus and Saturn, which come into
close conjunction with each other the night of Saturday the 26th,
but out of sight for North America.  The morning of the 26th, Venus
will rise above Saturn (in twilight, around 5 AM), while the
following morning they will have reversed themselves.  Venus,
always bright, is sadly disappearing from view, while the ringed
planet will rapidly rise out of dawn.  Next comes Mercury, which
passes superior conjunction with the Sun the night of Thursday the
31st as it heads towards a poor apparition in the evening sky. 
Finally, Jupiter is planetary king of evening, indeed all there is
(Mars well out of the way in bright twilight), the giant of the
Solar System not setting until about 10:30 PM.

There is still plenty of time to admire Scorpius, which appears low
to the south as evening begins.  To the west lie more southerly
Lupus (the Wolf) and Centaurus (the Centaur), the latter directly
south of Arcturus.  While effectively invisible now from temperate
North America, the constellation still presents a fine sight from
the temperate southern hemisphere, much as the Big Dipper and
Bootes do for us back in the north.  Look then to the east of
Scorpius to find Sagittarius (quickly identified by its upside-down
Little Milk Dipper), which holds a special place in the brightest
part of the Milky Way.

STAR OF THE WEEK: ZETA CEN (Zeta Centauri).  Though the second
brightest third magnitude star (after Menkar, Alpha Ceti), Zeta
Centauri is really too far south to have received a classical
proper name.  Lying among a wonderful collection of similar hot
blue stars, it helps give Centaurus (the Centaur) its marvelous
sparkle.  While just one of many such stars, it somewhat stands out
for being almost exactly east of the great globular cluster Omega
Centauri (by roughly five degrees), the grandest of them all and
that some suggest is the remnant of a small galaxy that merged with
ours.  In almost any other constellation, this hot (21,090 Kelvin)
class B (B2.5) subgiant would be notable.  From a decent distance
of 385 light years, with allowance for a lot of ultraviolet
radiation and a six percent dimming by interstellar dust, Zeta Cen
shines with the brilliance of 7100 Suns, which with temperature
yields a radius 6.6 times bigger than the solar value.  The theory
of stellar structure and evolution then gives a hefty mass of 9
solar.  Though the star is classed a subgiant, which implies that
it has (or will soon) run out of core hydrogen, it is really
clearly a dwarf that, with an age of 20 million years, is about
halfway through core hydrogen fusion.  With a stellar wind
appropriate to its high luminosity, it is losing mass at a rate
about 500 times that of the Sun.  As do many stars of its class, it
is a fast rotator, spinning at the equator with a speed of at least
225 kilometers per second, which gives it a rotation period of less
than 1.5 days.  The rotation probably flattens the star somewhat
into an oval shape, though no such deviation has yet been measured. 
Not alone, it possesses a companion that orbits in a mere 8.02
days, implying serious proximity.  Detected only spectroscopically
(through back and forth movements of the big star, Zeta Cen A),
nothing at all is known about Zeta Cen B.  At minimum, it orbits
with a distance of 0.08 AU, about 20 percent Mercury's distance
from the Sun.  If its mass is as much as half that of Zeta-A's, the
separation would be 0.19 AU.  The luminosity of such a star would
reduce that of Zeta-A to 6800 Suns but have little effect on its
calculated mass.  But companionship ends there.  Once thought to be
a member of the huge Upper Centaurus-Lupus association of hot blue
stars, it now seems not to be.  Zeta Cen lies at the 8-10 solar
mass dividing line between stars that blow up as supernovae and
those that make massive white dwarfs.  Perhaps it will develop just
far enough to turn itself into a rare neon-oxygen white dwarf, the
result of the fusion of a more-normal carbon-oxygen core.  Perhaps
a devastating explosion will eject the companion to make a "runaway
star" like Zeta Ophiuchi.  Only time will tell.



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Jim Kaler
Professor Emeritus of Astronomy   Phone: (217) 333-9382
University of Illinois            Fax: (217) 244-7638
Department of Astronomy           email: kaler at astro.uiuc.edu
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)
       The StarGazer (a new planetarium show)
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