[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 17, 2004.
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, in its perpetual cycle around the Earth, passes through
its first quarter on Saturday, December 18, shortly before its
daylight Moonrise in North America, at which time we see half the
sunlit face of the Moon, while the other side -- that facing away
from the Sun -- is in darkness.

The morning sky remains a marvel, with all five ancient planets on
display in order of their distance from the Sun.  Near the horizon,
Mercury glows in bright twilight, below and to the left of
brilliant Venus (which now rises in the southwest just before
twilight begins).  Up and to the right of Venus, you can spot
reddish Mars in Libra.  Higher still is obvious Jupiter, in Virgo,
which now rises around 1:30 AM.  Quite a bit farther over in the
western sky is the most distant of the planets known since ancient
times, Saturn, in Gemini.  Rising just after twilight ends, Saturn
is the lone bright planet in the evening sky, though the
progression continues with faint Uranus and Neptune (which now set
in mid-evening).  Wrapping around to the other side of the Sun (and
into the morning hours again), the progression ends with currently
invisible Pluto.

All that is left out is the Earth, which will actually take center
stage when the Sun passes through the Winter Solstice in
Sagittarius at 6:42 AM Central Standard Time (7:42 EST, 5:42 MST,
4:42 PST, 3:42 Alaska, 2:42 Hawaii) on Tuesday, December 21st,
about or before the time of sunrise.  At that moment, the Earth's
axis is tilted directly away from the Sun, astronomical winter
begins in the northern hemisphere, the Sun is overhead at the
Tropic of Capricorn, is circumpolar (not setting) at the Antarctic
Circle, and barely makes it to the horizon at the Arctic Circle.
Though the Sun will now begin its long, six-month, trek to the
north, the days in the mid-northern hemisphere will continue to
chill as the northerly movement is at first so very slow such that
the solar heating rate remains low until spring approaches.

The stars of winter join in and are fully with us, Orion now up
above the eastern horizon as evening twilight comes to an end.  Up
and to the right of him is an icon of the passage from autumn to
winter, Taurus the Bull, whose head is formed from the Hyades star
cluster and the first magnitude star Aldebaran (which is not
actually a part of the cluster, but a foreground star).  Taurus is
perhaps best known for containing the exquisite, more distant
cluster called the Seven Sisters, or Pleiades, of which six stars
are readily visible.  Directly above the Pleiades, to the north, is
bright Perseus, whose most famous star is the eclipsing double
Algol, the "Demon Star."

STAR OF THE WEEK: CELAENO (16 Tauri).  Above the "vee" of Taurus's
head, more or less pointed to by Orion's Belt, lies one of the most
absorbing of celestial sights, from Greek mythology the Seven
Sisters, the Pleiades.  From our modern view they are collectively
a beautiful young open cluster of hundreds of stars topped by 10
bright blue-white class B stars that lie 430 light years away, the
pack led by Alcyone, Eta Tauri.  While six of the stars are readily
visible with the naked eye, there have to be seven, these daughters
of Atlas and Pleione.  Hence perhaps the various stories of the
"Lost Pleiad."  Which one depends on which version.  Some say
Merope (23 Tauri), others say Electra (17 Tau), while still say it
must have been Celaeno (16 Tau).  The parents of these sisters,
Atlas (27 Tau) and Pleione (28 Tau), rank second and seventh in
apparent brightness, Atlas confusingly part of the set of visible
six.  Of the sisters, Electra ranks second and Merope ranks fourth,
so neither seems "lost."  So, as the faintest of the trio (nearly
sixth magnitude, 5.46), and in agreement with the Bright Star
Catalogue, we pick Celaeno as our Lost Pleiad.  (Oddly nobody chose
Sterope, the faintest of them all.)  If nearly 0.3 magnitude of
dust absorption could be removed from the line of sight, this class
B (B7) subdwarf would appear at magnitude 5.19.  From the star's
distance, its temperature of 13,200 Kelvin (from which we calculate
the amount of ultraviolet light), and a correction for the light
from a close companion (see below), we calculate a luminosity 240
times that of the Sun, which leads to a radius 3 times solar and a
mass 3.7 solar.  The minimum rotation speed of 185 km/s in turn
leads to a rotation period of less than 19 hours.  Though called a
subdwarf (which implies that it has given up core hydrogen fusion),
Celaeno is actually a well-advanced hydrogen fusing dwarf.  At that
mass, the hydrogen fusing lifetime is 225 million years, which is
nicely consistent with the age of the cluster itself, only 130
million years.  Interferometry reveals our "Lost Pleiad" to have a
companion a factor of six fainter at a separation of just 0.0062
seconds of arc, which at the distance of the Pleiades corresponds
to a class A3 star about just under one Astronomical Unit away,
which in turn implies a period of just under half a year.  Welcome
them both back home.

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