[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 15, 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.

The Moon passes its first quarter on Saturday the 16th somewhat
before its daytime Moonrise in North America, and then waxes
through gibbous as it heads to its full phase next week.  Only four
hours after it passes the quarter, it also passes apogee, where it
is farthest from the Earth.

Tides are caused by both the Moon and the Sun, which act
independently of each other.  (Their gravity "stretches" the waters
of the Earth, so that as our planet rotates beneath the tidal
bulge, the water at the shore goes up and down.)  At the quarters,
the weaker solar tide fills in the lunar, so the tides (called
"neap tides") are neither as high nor as low as they are at full
and new Moons.  Tides are also very sensitive to distance, and with
the quarter Moon at apogee, this neap tide will be especially weak.

The night of Friday the 15th finds the not-quite-quarter in central
Gemini, just up and to the right of Saturn, the two, along with the
constellation's bright Castor and Pollux, making a fine sight.  The
following night, with the Moon to the east of Saturn, will be
almost as good.  By the night of Tuesday the 19th, the Moon will
have arrived in central Leo.  Near the end of the week, the night
of Thursday the 21st, the brightening Moon will lie just to the
west of Jupiter, which itself is now just east of Porrima (Gamma
Virginis) in Virgo.  The ringed planet now crosses the meridian to
the south well before sunset, and sets around 2 AM daylight time,
leaving the morning to Jupiter (which crosses the meridian around
midnight and does not set until dawn begins to brighten the eastern
sky) and Mars.  Slowly brightening, while moving quickly to the
east against the background stars, the red planet (along with
Neptune, in Capricornus) rises just before 4 AM (Daylight Time).

Among the naked eye planets, that leaves us with Mercury and Venus. 
On opposite sides of the sky, Mercury rises midway through morning
twilight and is quite difficult to see.  Setting shortly after the
Sun, Venus is virtually impossible to find against the bright sky. 
By next month, it will begin to make an appearance.

Spring chases Orion and Canis Major (with brilliant Sirius)
westward.  Down and to the left of the Great Dog is the broken ship
Argo, which is separated into Puppis (the Hull) immediately
southeast of Canis Major, Vela (the Sails) farther to the
southeast, and Carina, the Keel, which contains the second
brightest star of the sky, Canopus, and extends far to the south,
its limit only 15 degrees from the southern pole.

STAR OF THE WEEK: MIAPLACIDUS (Beta Carinae).  In spite of intense
study, some stars refuse to yield their true characters.  Here is
the opposite, one that seems to be about as completely understood
as reasonably possible.  One of the few stars of the far southern
hemisphere to carry a proper name, Miaplacidus -- Beta Carinae in
Carina, the Keel of the Ship Argo -- is also one of the brightest
of the sky.  Almost first magnitude (bright second, 1.68), it ranks
28th in the list of bright stars, and in its constellation is
beaten out only by brilliant Canopus, which ranks second.  It and
Atria (in Triangulum Australe) are in addition the two brightest
stars closest to the South Celestial Pole, each offset from it by
about 20 degrees, both circumpolar (never setting) from latitudes
below about 20 degrees south (and sadly invisible from latitudes
above 20 degrees north).  While the root of the name means "placid"
or "gentle," the word is not well understood, some references
citing "gentle waters" (appropriate for Argo), others stating
"unknown."  In any case, the name is of modern origin, since the
ancients had no way of seeing this deep southern star.  Physically,
Beta Car is a relatively common, white class A (A2) subdwarf, one
(from its class alone) that has just given up core hydrogen fusion. 
At a distance of 111 light years, with a temperature that from
various sources averages 9100 Kelvin, Miaplacidus shines with the
light of 210 Suns, which yields a radius 5.85 times that of the
Sun.  A direct measure of angular diameter (1.59 thousandths of a
second of arc) together with the distance gives a radius of 5.83
solar, showing that all the parameters must be correct.  A rapid
rotation velocity of at least 139 kilometers per second gives a
rotation period under 2.1 days.  The theory of stellar structure
and evolution shows the star to have a mass between 3 and 3.1 times
solar, and that in accord with the spectral class, at an age of 350
million years, it is indeed giving up core hydrogen fusion and
instead has a core of nearly pure helium.  Helium fusion to carbon
will begin in a mere TK years, after the star becomes a red giant. 
While a number of class A stars are surrounded by dusty disks that
imply planet formation has taken place, Miaplacidus seems devoid of
such.  Indeed, there is no evidence for any kind of companion at
all.  The only mystery we are left with is the name.


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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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