[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 14, 2005.
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.

While last week was about as busy as could be, this week is quite
the opposite, nothing much actually "happening," which is not bad
since it allows one simply to enjoy the quiet sky.  One "big
event," such as it is, is that we begin the week with the Moon in
its waxing crescent phase moving through Aquarius and Pisces, then
passing its first quarter about midnight the night of Sunday the
16th, about the time of Moonset in North America.  The rest of the
week sees the Moon in its waxing gibbous phase climbing northward
toward Taurus and Gemini.  The night of Wednesday the 19th, the
Moon will lie close to the Pleiades star cluster in Taurus, though
its brightness will make the cluster's stars hard to see.  The
passage reveals just how angularly small the Moon really is.
Though the Pleiades seems small to the eye (a bit over a degree
across), you could "fit" two full Moons across it!  The Moon's
apparent large angular size is but an illusion (as anyone who tries
to photograph a Moonrise with an ordinary camera quickly
discovers).

The planetary week begins with Mercury and Venus still in close
companionship, only half a degree apart, but now closer to the
horizon in morning twilight and more difficult to see, the pair not
rising until after dawn begins.  Much more readily visible is Mars.
Moving quickly to the east against the background stars, the planet
now lies rather well to the east of Antares in Scorpius, the color
similarity quite obvious.  Though Mars is just now crossing the
boundary from second to first magnitude, Antares is still a bit
over half a magnitude (65 percent) brighter.  Now a bit to the west
of the meridian at dawn find Jupiter, which -- though Venus is
brighter -- still dominates the sky, the giant planet now rising
shortly before midnight.  The prize for best visibility still goes
to Saturn.  Having just passed opposition with the Sun, and rising
just before Sunset, the planet is with us practically all night,
shining brightly in Gemini to the south of Castor and Pollux, the
trio making a truly lovely celestial sight.  If you have a new
telescope, by all means take a look at the planet to admire its
wide-open rings.  Nearby will be a tiny "star," Saturn's largest
satellite Titan, which is the focus of the Cassini-Huygens space
probe.

Perhaps the best sight is Comet Machholz.  Just visible to the
naked eye (and far better in binoculars), this visitor from the
frozen depths of the Solar System will spend the week passing
against the stars of Perseus.

Standing directly north mighty Orion is one of the brightest
constellations of the sky, Auriga, the Charioteer, which contains
the most northerly "first magnitude" star, Capella (the "She-Goat")
which -- as sixth brightest in the sky -- actually ranks as
magnitude "zero."  Right next to it is a thin triangle of stars
that represent her "kids."  In the faint Milky Way near the Auriga-
Taurus boundary is the Galaxy's "anticenter," where we look
directly away from the Galactic nucleus in Sagittarius, the latter
home to a gigantic black hole of over three million solar masses.

STAR OF THE WEEK: AE AUR (AE Aurigae).  As Mu Columbae speeds to
the south away from Orion's Trapezium, from where it was hurled
(rather from near where, since the Trapezium was not yet born), AE
Aurigae (in Auriga, the Charioteer) zips away to the north (perhaps
in myth drawn by the speeding Chariot?), the two separating at some
200 kilometers per second.  The idea is that around two million
years ago two massive binary stars collided close to where the
Trapezium now finds itself in the middle of Orion's Sword.  Two
stars were exchanged to make the odd eccentric double Iota Orionis,
while the other two were ejected to create the classic "runaway
stars."  At sixth magnitude (nominally 5.96), near the limit of
naked-eye visibility and rather lost among a clutch of nearby
stars, AE is not very impressive.  The two-letter name implies
variability, and indeed the star does vary between magnitudes 5.4
and 6.1.  The apparent faintness is a product of great distance
(near 1450 light years, just a bit farther than Mu Col), and a
significant amount of absorption by interstellar dust, which if it
were not there would make AE Aur 0.8 magnitudes brighter, boosting
it to mid-fifth.  Not surprisingly, Mu Col and AE Aur are similar,
both class O (O9.5) hydrogen-fusing dwarfs, AE's temperature
measured at 36,500 Kelvin (anomalously high for its class; 33,000
might be better).  In either case, the star is very luminous,
between 26,000 and 33,000 times the luminosity of the Sun
(depending on the temperature and the allowance for ultraviolet
radiation) and massive (at least 17 times solar, greater than Mu
Col's mass) with a radius about 5 times that of the Sun.  A rather
slow rotation speed of under 40 km/s implies that the rotation pole
is more or less pointed at us.  Like the Pleiades in Taurus, AE is
now passing through an unrelated interstellar cloud of gas and dust
that it is illuminating (a chunk of which was apparently mistaken
for a companion star).  Hot stars, with their simple spectra, make
ideal backgrounds for the study of the spectra of intervening
interstellar gases (which are superimposed on the stellar spectra),
and AE is no exception.  Indeed its very high speed across the line
of sight of 90 km/s relative to the Sun allows the fine structure
of the interstellar medium to be probed as the star passes in back
of the wispy gasses, causing the interstellar spectrum to vary.
There is really nothing all that unusual in such stars; some 15
percent of O and B stars are runaways, some caused by binary
interchange, others -- like Zeta Ophiuchi -- caused by one of the
stars of a binary exploding off-center and ejecting the other.
AE's fate is to explode as well...but as a single star which will
someday be no more but for a lonely neutron star left behind.





****************************************************************
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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