[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, November 19, 2004.
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

Interested in Astronomy and Astro Education?  Join the Astronomical
Society of the Pacific (an international organization) to get the
outstanding astronomy magazine Mercury and a variety of other
benefits.  Call 1-415-337-1100, then press 1.

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

Since last week fully held the waxing crescent Moon, this one holds
the waxing gibbous.  First quarter took place around midnight as
Thursday the 18th turned to Friday the 19th to begin our week.  The
Moon will then pass its full phase during the day on Friday the
26th, and will rise just short of full -- and therefore just before
sunset -- the night of Thursday the 25th.  At the beginning of our
week, Friday the 19th, the Moon also passes south of Uranus in
Aquarius.  With the Sun approaching the Winter Solstice in
Sagittarius, the fattening Moon will be climbing the northern part
of the ecliptic, giving us plenty of bright Moonlight later in the
week.

Saturn, in eastern Gemini, is now seriously encroaching on the
evening scene.  Rising around 8:30 PM, it stands high to the east
at midnight.  This is also a week for Mercury, which reaches its
greatest elongation to the east of the Sun on Saturday the 20th.
The apparition is not very good, however, as the little planet will
still be very low in southwestern evening twilight and will be
quite difficult to find.

Even with such evening activity, the morning sky remains
undefeated, as it still holds the twin glories of Jupiter and
Venus, Jupiter much the higher at dawn, Venus the brighter.  With
Jupiter rising ever earlier and Venus (preparing to swing in back
of the Sun) rising later, the two are separating fast, the giant
planet now rising just before 3 AM, Venus not until almost 4:30.
By the end of the year, about the time Jupiter makes its passage
into the evening sky, Venus will be rising as twilight begins to
lighten the eastern morning sky.

As the Moon brightens, the stars seem to fade away, overwhelmed by
Moonlight.  The brighter ones are still there to view, however.  By
late evening, Orion is climbing the eastern sky with stars bright
enough to withstand the Moon's glow.  Then just an hour after
Saturn rises, so does Sirius (in Canis Major), the brightest star
of the sky, which makes it the champion twinkler as well (the
twinkling effect caused by the Earth's turbulent atmosphere).  Look
to the northwest of Orion to find Taurus with its Pleiades and
Hyades star clusters, and to the northeast of him to Gemini and
back to Saturn.  Obscure constellations seem to fade away, lost to
moonlight.  Among the least of these is a jagged row of stars that
passes nearly overhead for mid-northerners and that make Lacerta,
the celestial lizard, which finds itself surrounded by Cygnus,
Pegasus, and northern Andromeda, the row pointing northward to
Cepheus and the famed variable star Delta Cephei.

STAR OF THE WEEK: 10 LAC (10 Lacertae).  Surrounded by Cepheus,
Cygnus, Pegasus, and Andromeda, we pay poor Lacerta, the Lizard,
little heed, all its stars faint and obscure, its brightest star,
Alpha Lacertae, just fourth magnitude.  Yet treasures it does hold,
including an "OB association," a collection of loosely organized,
unbound hot class O and B stars called "Lacerta OB I."  Among the
chief members is one of the few naked-eye class O (09) dwarfs stars
(indeed one of the few O stars of any kind) in the sky, Flamsteed's
number 10.  O stars are the hottest and rarest of the stellar
types; of all stars, only 0.00007 percent fall into the class.
That we see as many as we do is testimony to their inherent
brilliance.  And 10 Lac, one of the best, is no exception.  The
star's apparent fifth magnitude (4.88) faintness is the result of
large distance (parallax measures bring it in at between 1000 and
1060 light years) and a bit of absorption by interstellar dust in
the Milky Way, without which 10 Lac would climb almost to fourth
magnitude (4.53).  Being hottest, O stars are also the bluest of
the lot.  Through the telescope, 10 Lac shines like a blue-white
diamond, the star used as a spectral standard against which to
compare others.  From a surface heated to a quite-amazing 32,000
Kelvin, it radiates a with a luminosity of 26,800 Suns (the
majority of the light in the invisible ultraviolet), from which we
derive a radius 4.7 times that of the Sun and a great mass of 16
times solar.  An equatorial rotation speed of at least 31
kilometers per second gives it a rotation period less than eight
days.  Typical of such stars, 10 Lacertae is losing mass through a
wind that blows at a rate of a tenth of a millionth of a solar mass
per year, a million times the rate of the solar wind.  The spectrum
suggests that the star actually rotates much faster, but is viewed
nearly pole-on and is surrounded by a thin gaseous disk, though the
contention is argued.  The star is somewhat deficient in carbon,
nitrogen, and oxygen compared with the Sun, but that is typical, as
the Sun actually appears to be a bit rich in heavy elements as
compared with its immediate Galactic surroundings.  A tenth
magnitude class A companion lies a minute of arc and at least
20,000 Astronomical Units away.  If that is the actual distance,
the orbital period would be over half a million years.  From there,
10 Lac would shine with the light of 4 full Moons.  Most likely,
however, the fainter star is not attached to 10 Lac proper.
Falling close to the "zero-age main sequence" of dwarf stars, 10
Lacertae was recently born.  Such stars do not live very long.  In
only 10 million years, 10 Lac will have used up its internal
hydrogen fuel, and will eventually explode as a supernova.


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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@xxxxxxxxxxxxxx
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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