[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 23 2007. 
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.

Support science literacy by joining the Astronomical Society of the
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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
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.

NEW! The Cambridge Encyclopedia of Stars.

Skylights resumes its normal weekly schedule, but is presented two
days early.

Beginning the week as a fat crescent, the Moon passes its first
quarter early, on Sunday, March 25, about the time of Moonrise in
North America.  Watch as it rises ever higher in the daylight
eastern sky.  It then spends the rest of the week waxing in the
gibbous phase towards full, which it will hit next April 2.

As it travels, the brightening Moon will make a fine passage to the
north of Saturn the night of Wednesday, the 28th, close enough to
actually occult the planet as seen from northern Britain,
Scandinavia, and the Atlantic.  The next night, that of Thursday
the 29th, it's Regulus's turn, the Moon passing by about the same
angle to the north of the star, Regulus also occulted as seen by
roughly the same parts of the world.

Continuing its dramatic ascent relative to the Sun, not setting
until just after 10 PM Daylight Time, brilliant Venus quite
dominates the early evening sky.  As it sets, look to the south to
see Saturn crossing the meridian still to the west of Regulus in
Leo.  Moving slowly retrograde, the ringed planet has almost, but
not quite, made it back into Cancer.  To the west of Saturn lies
the Beehive Cluster, which the planet passed last year, but which
the bright Moon renders difficult to see.

In the morning hours, you can admire Jupiter, which now rises
around local midnight and transits the southern meridian in mid-
dawn.  To the east of the giant planet lies Mars, whose rising for
the next month will continue to track the onset of morning
twilight.  The event impossible to witness, Mars will pass only a
degree south of much more distant and fainter Neptune on Sunday the
25th.  Though lower than Mars, Mercury is more than twice as
bright, and might be glimpsed in brightening eastern sky.

As Orion flees to the west, he is being chased by one of the dimmer
constellations of the sky, the modern figure of Monoceros, the
Unicorn, which lies to the east of him.  Most of the constellation,
which contains a number of the riches of the Milky Way, lies
between the two Dogs, Canis Minor and Canis Major.  Below Canis
Major float Puppis and Carina of Argo, the ship of the argonauts,
which now sails into the sunset as well.

STAR OF THE WEEK: ALPHA MON (Alpha Monocerotis).  By general rule
and common wisdom, Alpha stars are supposed to be the brightest
stars within their constellations.  This Greek letter listing,
invented by Johannes Bayer for his Uranometria of 1602 is, however,
more than occasionally violated, as it is in Monoceros (the
Unicorn), in which Beta Mon (no star in the constellation has a
proper name) is the luminary.  Other rule violators abound:
Aquarius, Cancer, Capricornus, Cetus, Corvus, Crater, Delphinus,
Draco, Gemini, Hercules, Libra, Orion, Pegasus, Pisces, Saggita,
Sagittarius, Triangulum, and Ursa Major; and that's just the
ancient constellations.  But in Monoceros's case, it's pretty
forgivable, since it's almost impossible to distinguish the
brightest among the top three, such as they are since no star tops
fourth magnitude.  In order the "Big Three" are fourth magnitude
(3.92) Beta Mon, Alpha Mon (3.93), and Gamma Mon (3.98), a mere one
percent separating Alpha from Beta.  That Beta comes out on top is
a bit of a cheat, since through the telescope it is seen to be
triple, each one just fifth magnitude, notably dimmer than Alpha. 
Luckily for our Alpha it also escaped even having its very name
stolen, as in some older star catalogues, Alpha and Gamma were
swapped for each other, which would have lowered Alpha's status
even more.  Physically, Alpha Mon is yet another yellow-orange
class K (K0) "clump giant," a helium-fusing giant star similar to
a great many others of its class (so-called as they clump together
in a graph of temperature vs. luminosity).  At a distance of 144
light years, it radiates at a rate 60 times that of the Sun from a
coolish surface of 4815 Kelvin, which together yield a radius 11
times solar and a mass of 2.5 solar.  With a total age of about 1.4
billion years, the star ran out of core hydrogen fuel some 250
million years ago.  Alpha Mon's metal content is about 20 percent
lower than that of the Sun (which is not very unusual).  The
rotation speed, hence period, is unknown.  While appearing rather
ordinary, Alpha Mon's very ordinariness is a distinct advantage. 
The star is known for its spectral stability, and serves
astronomers as a standard for determining the velocities of other
giant stars that are not quite so well behaved.  Like all of its
breed, once the helium runs out, it will lose its outer hydrogen
envelope and retire as a dense white dwarf about the size of Earth.


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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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