[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 30, 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.

NEW: Astronomy: Stars, Galaxies, and the Universe, an audio course
on CD with 100 page study guide narrated and written by Jim Kaler,
is now 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.

'Tis the year's end, and a Happy New Year to all, as this Saturday
night the clock rolls over to 2006.  The year ends, and Skylights'
week begins, with the new Moon on Friday, December 30, giving us a
reverse "blue moon" effect (wherein there are two full moons in a
month), December 2005 having two NEW Moons.  The rest of the week
is spent with the Moon in its waxing crescent phase.  In
celebration of the new year you can get your first look at the Moon
the night of Sunday, January 1, when it will appear low in the
southwest in twilight to the left of Venus, the brilliant planet
now setting ever earlier in twilight and rapidly disappearing from
view.  Early in the lunar cycle the nighttime side of the Moon will
be bathed in Earthlight.  On Monday the 2nd and Tuesday the 3rd,
the Moon will respectively (and invisibly) pass to the south of
Neptune and Uranus.

As Venus vanishes, the early night sky is taken over by Mars (still
in Aries), which transits the meridian to the south just before 8
PM, and Saturn (yet in Cancer), which rises about an hour earlier. 
Though Mars has faded some from its earlier glory, it still shines
at the minus first magnitude, and is brighter than all but the two
brightest stars, Sirius and Canopus.  Saturn (at magnitude zero
brighter than all but the top three stars, number three Alpha
Centauri) then transits the meridian at 2 AM, followed nearly an
hour later by the simultaneous setting of Mars and the rising of
Jupiter, whose glowing brightness no star can match.  Now in
western Libra, Jupiter falls well to the east of Virgo's Spica.

Far fainter, the asteroid Vesta (the fourth discovered), will pass
through opposition with the Sun on Thursday the 5th.  At faint
sixth magnitude in central Gemini, it is just barely visible to the
naked eye (given excellent vision in a dark location), making it
unique among such bodies (the asteroids a collection of planetary
debris mostly between Mars and Jupiter).

Meteor showers come from the debris of comets, not asteroids.  Keep
your eye out for one of the major showers of the year, the
Quadrantids, which peak the morning of January 3, and that come out
of the defunct constellation Quadrans, which lies near the Big
Dipper.

On New Year's Day, the Moon will pass through perigee, where it is
closest to the Earth.  Just three days later, at about 9 AM CST on
Wednesday the 4th, the Earth then passes through perihelion, where
it is closest to the Sun, 147 million kilometers (91.4 million
miles), 1.7 percent closer than average.  Since perihelion takes
place in the heart of northern winter, solar distance clearly plays
no significant role in the seasons, which are caused by the 23.4
degree tilt of the Earth's rotational axis relative to its orbital
axis.  Since ocean tides depend on the inverse cube of the
distances between the Earth and the Moon and Sun, they will be
especially high (and low) at this new Moon.  Tides slow the Earth's
rotation, and to synchronize clocks with precisely-kept "atomic
time," a "leap second" will be added at midnight as the Old Year
passes to the New (giving 61 seconds to the last minute).

Orion and his cohort of bright constellations are now making a
serious impact on the nightly sky, Sirius in Canis Major rising as
twilight ends.  Watch for them as they cross the sky during the
wintery nights.  Meanwhile in the northeast, the Big Dipper is
rising.  The Little Dipper, however, is at its low point beneath
the ever-constant pole.

STARS OF THE WEEK: HR 306 and HR 8938 Cep (HR 306 Cephei and HR
8938 Cephei), a two-for-one polar special.  These two north polar
stars, both in Cepheus (but closer to Ursa Minor than to the King's
classic figure), highlight several issues: first "selection," in
which nature shows you what she wants; second that the sky is not
in two dimensions, but in three; third that age and mass are
crucial in stellar life; fourth that measurement errors are always
with us.  While the vast majority of stars are dim-bulb class M red
dwarfs, the constellations are made of intrinsically luminous,
though rather rare, stars.  Nearly all the stars in these pages are
more luminous, many much more, than the Sun.  They have to be,
otherwise you would not see them.  NO red dwarf is visible to the
naked eye.  A look through all the polar stars, those here within
a six-degree-wide box around Polaris, shows the same thing.  While
at least this set is objective (that is, not selected by whimsy),
all are (in total power) still brighter than the Sun.  The
telescope reveals a very different world of much weaker stars.  We
also think of stars as plastered on the 2-D screen of the sky.  But
they are in 3-D.  What does the sky look like from another star? 
HR 306 (the name from the "Bright Star Catalogue") is a sixth
magnitude (6.25) class K (K2) giant 326 light years away that has
precious little known about it.  Estimating the temperature at 4450
Kelvin (to get the amount of infrared light) leads to a luminosity
of 45 times that of the Sun, a radius of 12 solar, and a low mass
of one or two solar.  Indeed, HR 306 is what our Sun may look like
as it starts to die in five or six billion years.  HR 8938 is a
somewhat brighter, still-sixth magnitude (5.58), class A (A7)
subgiant 307 light years off with an estimated temperature of 8000
Kelvin, a luminosity of 44 Suns, and radius of 3.5 solar.  With a
mass of 2 1/4 Suns it is (consistent with its subgiant status)
giving up core hydrogen fusion, its age about 900 million years. 
The luminosities of the stars are remarkably alike: the K giant's
is high because it is well into the death process, the A subgiant's
because it is the more massive of the two.  What would they look
like if you could fly to either star?  From the measures, they are
only 20 light years apart.  From either, the other would hardly
hide out as they do from here: each would appear at magnitude zero,
rather like Vega does to us.  (From 8938, 306 would be at magnitude
0.21; from 306, 8938 would be at minus 0.32.)  But each distance
measure is afflicted with a five percent uncertainty.  They could
thus be at nearly the same distance and be much brighter in each
others' skies.  Much of the effort in astronomy is in lowering
measurement errors to make the sky more precise -- and more
knowable.



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