[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 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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"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.

Ever have one of those weeks where nothing much happens?  But it's
not so bad, since we always have quiet contemplation of the
nighttime sky.  The biggest thing is the Moon waxing from first
quarter the night of Friday, February 23, through gibbous toward
full, which is not actually reached until the evening of Saturday,
March 3.  Not all is lost, though, as the Moon will make a very
close passage just to the north of Saturn the night of Thursday,
March 1, one well worth watching.

Then we might anticipate a total eclipse of the Moon as it passes
through its full phase the early evening of Saturday the 3rd.  It
will best be seen in Europe and Africa.  In North America, the Moon
will rise with the eclipse already in progress.  Totality will be
seen roughly to the east of the Mississippi, while the far west
will see increasingly little of it, the west coast quite losing
out.

Saturn, slow of motion and in retrograde, is getting just a bit
farther each night to the west of Regulus in Leo.  Already up by
sunset, it crosses the meridian to the south around 11 PM, while
setting in mid-dawn.  The big evening show, though, belongs to
Venus, which is gloriously visible in evening western twilight and
is now the classic "evening star."  Each night appearing higher and
higher, the planet does not now set until nearly an hour after
twilight ends.  And as winter turns to spring, its aspect will
continue to get better.

While Venus and Saturn reign over the evening hours, Jupiter rules
the morning.  Now rising around 2 AM, the giant planet is close to
crossing the meridian to the south in growing dawn.  Look for
Antares of Scorpius to the west of it.  Mars, still rising in
growing morning twilight, remains elusive.

Early evening presents us with a spectacular winter sky, with stars
so bright even the near-full Moon can't quite wash them out.  Look
for mighty Orion about half way up the southern sky, with Gemini to
the upper left of him, Taurus above and to the upper right.  In the
deep chill of winter, it's warming to see the Summer Solstice at
the border between the latter two, the point where we will find the
Sun on the first day of summer.

STAR OF THE WEEK: Al ZIRR (Xi Geminorum).  Or Alzirr, as in "star
name English," the Arabic article is often run together with the
noun.  The name means "the button," though the button of exactly
what is not known.  At the rather prominent foot of the eastern of
Gemini's twins, the name is probably not part of the Greek
tradition, but one of the ancient Arabic.  But who knows, as the
scholarship on it is lacking, as for that matter is the science, as
little is actually known about this rather obvious third magnitude
(3.36) star, which is far better known by its Greek letter name, Xi
Geminorum.  Fairly nearby, Al Zirr is a relatively lower-luminosity
class F (F5) giant (really subgiant) only 57 light years away
(though that is still 68 percent farther than the constellation's
luminary, Pollux).  From that distance, our Sun would still be
visible to the naked eye, albeit at a mere sixth magnitude (6.05)
and quite lost amidst the Milky Way's stars in what we here on
Earth call Scutum (the Shield).  Distance and a temperature of 6460
Kelvin lead to a luminosity 11.1 times that of the Sun and a radius
of 2.7 solar.  The theory of stellar structure and evolution then
yield a mass between 1.5 and 1.6 times that of the Sun (depending
on the exact state of evolution) and an age of around 2.5 billion
years (just over half the solar age).  While formally classed as a
giant, Xi Gem is really a subgiant that has just given up (or is
about to give up) its core hydrogen fusion, more-massive stars
having shorter hydrogen-fusing lifetimes.  The star is actually
right at a critical point in the stellar spectral sequence where,
as we go to warmer stars, fusion begins to run on the "carbon
cycle" and where we hit the "rotation break," where stars begin to
rotate much faster than does the Sun.  (The Sun's fusion is by the
"proton-proton chain," in which hydrogen builds to helium in a
direct three-step process; in the more efficient carbon cycle,
carbon is used as a nuclear catalyst.)  Consistent with being at
the rotation break, Al Zirr has a rather rapid lower limit to its
equatorial rotation speed, 68 kilometers per second (34 times that
of the Sun), which gives it a rotation period of less than two
days.  (We can know only lower limits because the tilt of the
rotation axis is not known.)  The rapid rotation, along with an
outer layer in a state of convection, lead to a fair degree of
magnetic activity, X-ray radiation, and a two-level outer corona,
one with a temperature of two million Kelvin (much like that of the
Sun), the other of eight million.  Xi Gem is listed as a "Delta
Scuti" star, one that subtly oscillates with multiple periods, but
one whose character is also quite unexplored, consistent with the
lack of other knowledge about the star.  The star (which has no
known companion) is now preparing to become a much more luminous
red giant, and like all others of its modest mass, will eventually
die as a modest white dwarf.


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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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