[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 2, 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.

We begin the week, the night of Friday, March 2, with the Moon just
shy of full, that phase reached the evening of Saturday the 3rd. 
The remainder of the week sees the Moon then waning in its gibbous
phase toward third quarter, which is not passed until the evening
of Sunday the 11th.  Because of the eccentricity of the lunar orbit
and the gravitational pull of the Sun, the quarters (with "full" as
"second quarter") do not quite exactly "quarter" the 29.5 day phase
cycle (7.4 days).  Here we have a rather long stretch of 8.2 days
between first and full.  The night of Friday the 2nd, watch for the
Moon rising just to the east of Regulus in Leo, with Saturn on a
line to the west of the pair.

This full Moon is special, as the night of Saturday the 3rd it will
pass through the shadow of the Earth, allowing many of us to see a
total eclipse.  The event is seen in its entirely in Africa and
Europe.  The far northeastern coast of North America sees the Moon
rising in the east while it is entering the dark umbral shadow,
while the southern seaboard and midwestern US and Canada see it
rising during totality, which will present an eerie sight indeed. 
West of the midline of the continent, viewing gets worse, totality
missed altogether, while the west coast essentially sees nothing at
all.  Totality begins at 5:44 EST, while mid-totality is achieved
at 6:21 EST, 5:21 CST.  Totality ends at 6:58 EST, 5:58 CST, and
the show is over (except for the dim penumbral phase with the Moon
in partial shadow of the Earth) at 8:12 EST, 7:12 CST, 6:12 MST.

While admiring the eclipse, be sure to turn around to the west to
take in one of the great gifts the sky has to offer, brilliant
Venus, which is now not setting until a full hour after evening
twilight is done with.  Saturn, now well up by sundown, will make
a nice sight to the west of the eclipsed lunar disk.  You'll find
the ringed planet about midway up to the south by 10:30 PM.  Then
wait until 1:30 AM for the rising of Jupiter (to the northeast of
Antares).  The giant planet is nearly to the meridian by the time
it disappears in bright dawn.  For awhile we can still forgo Mars.

Two of the great sights of the sky lie in Taurus, which in turn
lies above and to the right of Orion.  The head of the celestial
Bull is made of the vee-shaped Hyades star cluster, which is 150
light years away.  Aldebaran, which marks the Bull's eye, is not a
part of it, but instead lies a bit under halfway between us and the
cluster.  Farther up and to the right are the Pleiades, the Seven
Sisters, which appears smaller because of its greater distance of
425 light years.  The telescope shows a huge number of such
clusters dotting the immediate area.

STAR OF THE WEEK: HYADUM I (Gamma Tauri).  Much and deservedly
famed are the Hyades (in mythology half-sisters to the Pleiades),
a nearby cluster that makes the vee-shaped head of Taurus the Bull. 
Right at the point of the "vee" lies just-barely-fourth magnitude
(3.65) Gamma Tauri, whose Latin name of Hyadum I indicates it to be
"first of the Hyades."  Consistently, in Arabic lore the star was
"premier" too, as it was known as Awwal al Dabaran, "first of the
followers of the Pleiades," as it led first magnitude Aldebaran
(Alpha Tauri), whose name means more simply "the follower."  We now
understand Gamma Tauri (by which it is far more commonly known) to
be one of the four ageing giants of the middle-aged (650 million
year old) cluster.  Of them, Gamma ranks second in brightness, just
after Ain (Epsilon, 3.53) and just before Hyadum II (Delta-1, 3.76)
and Theta-1 (3.84).  Along with its fainter giant brothers, Gamma
is class K0, while brighter Ain comes in a bit ahead at G9.5
(though all are usually lumped together as the "K giants").  While
the directly-measured distance is 154 light years, it is probably
more accurate to take the Hyades' average distance of 151 light
years for Gamma as well.  With a temperature measured at 4970
Kelvin, "Hyadum the First" is then found to radiate at a rate 79
times greater than does the Sun, from which we can derive a radius
12 times solar, and from the theory of stellar structure and
evolution, a mass of about 2.6 Suns.  Though the star has a
reputation of varying a bit, up to 0.1 magnitude, there is no
confirmation, nor is anything known of the variation period.  The
rotation is so slow that the equatorial spin speed is difficult to
measure, various estimates giving 2.4, 1, and under 1 kilometer(s)
per second.  Adopting an average of 1.7, the rotation period could
be as long as 150 days.  Yet the star has a surrounding
magnetically-active outer layer, perhaps a corona, with very highly
excited X-ray emissions from iron, oxygen, and neon, which is odd,
since cool-star magnetic fields are believed to require much more
rapid spins for their creation.  Like the other Hyades giants,
Gamma Tau is richer in metals than the Sun, its iron content
(relative to hydrogen) up by about 25 percent.  And like the rest
of its gang, it will eventually slough off most of its outer
hydrogen envelope and expire as a white dwarf with about two-thirds
of a solar mass.



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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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