[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 3, 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 week begins with the Moon just shy of its third quarter,
allowing the skies to darken once again.  After the passage of the
phase on the night of Saturday, December 4, rather well before
Moonrise in North America, the Moon wanes in its crescent, becoming
ever thinner in the early pre-dawn sky.

Watch the Moon as it passes by stars and planets, marking them out
one by one.  The morning of Wednesday the 8th, our companion will
appear just down and to the left of first magnitude Spica in Virgo.
Look for Jupiter higher in the sky in western Virgo to the other
side of Spica.  The next morning, on Thursday the 9th, the Moon
will help form a line that leans down and to the left, the Moon on
top, Mars in the middle, and Venus below.  To the left of the Moon
will be much fainter Zubenelgenubi in Libra.  By the morning of
Friday the 10th, the positions will be reversed, the Moon now below
Venus as they all climb out of eastern dawn.  Of the three planets,
Venus (at minus fourth magnitude) is by far the brightest, minus-
second magnitude Jupiter next, Mars (second magnitude) by far last.
Earlier in the week, the morning of Sunday the 5th, Venus and Mars
come into conjunction, creamy-white Venus 1.3 degrees to the north
of the more distant red planet.

Jupiter, well to the west of the Venus-Mars duo and now rising
around 2 AM, slowly moves toward the evening sky.  The evening
itself is now graced by Saturn in Gemini, which clears the
northeastern horizon around 7:30 PM.  In contrast, innermost
Mercury is quite invisible as it passes inferior conjunction with
the Sun the morning of Friday the 10th as it passes from evening to
morning.

'Tis the Earth in the news, though.  While the shortest day in the
northern hemisphere falls at the passage of the Sun over the Winter
Solstice (on December 21), the earliest sunset takes place on
Tuesday the 7th, the offset the result of the eccentricity of the
Earth's orbit and the 23.5 degree tilt of the terrestrial axis,
which together put the Sun rather well ahead of its average,
smooth-motion position.  While the morning Sun will keep rising
later until well into winter, the evening Sun will begin to set
later, giving us more light on the evening drive home.

High above in northern mid-evenings find Cassiopeia, the Queen of
the ancient legend of Andromeda and Perseus.  To the west is
Cassiopeia's husband, King Cepheus, which is more difficult to find
as it is not marked out by a slew of bright stars.  Beneath the two
figures find Polaris, the North Star, about which they nightly
whirl.  Directly opposite, and below the pole, lies the Big Dipper
of Ursa Major, whose bowl is now tipped to "hold water," all of the
figure circumpolar and visible from the far northern US and most of
Canada.

STAR OF THE WEEK: NU CEP (Nu Cephei).  Talk about an
underappreciated star!  Nu Cephei (of no proper name, in Cepheus,
the King) lies more or less on a line between Alderamin (Alpha
Cephei) and the constellation's most famous star, variable Delta,
which is the main focus of attention.  While an amazingly luminous
class A (A2) supergiant (with a temperature of 8400 Kelvin, though
there are measures up to 9100), this fourth magnitude (4.29) star
is also lost beside other grand supergiants (Mu and VV Cep, to name
two) such that much less honor is paid to what is really due.  The
last of Nu's problems is that it is significantly dimmed by
interstellar dust, which if it were not in the way would bring it
to bright third magnitude (2.82).  Class A supergiants such as this
one are quite rare (the brightest by far being Deneb in Cygnus).
As such all are far away, which compromises distance measurement.
Nu Cep's formal parallax gives a distance of 5100 light years, but
with a huge uncertainty of some 75 percent.  That distance is
certainly much too high.  Factoring in the formal uncertainty, the
star should however be at least 2900 light years distant.  From its
rough distance and motions, Nu is a member of the sprawling,
expanding (gravitationally unbound) association of O and B stars
known as Cepheus OB2 (which was once thought to contain Mu as
well), which from the analysis of all the prominent members yields
a much lower distance of 2000 light years.  The parallax limit
would then place the star outside the association, suggesting that
it too is wrong.  If nothing else, the whole affair shows that
there is still a lot of work to do in the distance game.  If at
2000 light years (which seems the more likely), the star shines
with a luminosity 22,000 times that of the Sun, its diameter 70
times solar, which would make it just shy of the size of Mercury's
orbit.  If at 2900 light years, the figures would go up to nearly
50,000 solar luminosities and about 100 solar diameters (about half
the size of Earth's orbit).  Grand as it is, however, Nu pales
beside Deneb, which radiates some 160,000 Suns into space
(explaining why Nu Cep is sometimes downgraded to a "lesser
supergiant" status).  Nevertheless, our star has a mass of between
12 and 16 times that of the Sun and is only about 15 million years
old.  It may be making a transition to becoming a larger and redder
supergiant with a now-quiet helium core, or it may be a bit older
and now fusing its helium into carbon.  From its surface blows a
150 km/s wind with a mass-loss rate of a tenth of a millionth of a
solar mass per year.  Like most supergiants of its class, Nu's most
likely fate is to grow an iron core and someday explode.


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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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