[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 24, 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.

Wishes for a fine holiday season to all.

The last full week of the year begins with a bright Moon in its
waxing gibbous phase just short of full, that phase reached on the
morning of Sunday, December 26, shortly after sunrise and moonset
for continental North America.  As a result, the near full Moon
will rise just before sundown Christmas night, and will rise just
after sundown the following evening.  A day later, on Monday the
27th, the Moon will go through apogee, where it is farthest from
the Earth.  The remainder of the week the Moon wanes in its gibbous
phase toward the third quarter, which will be passed on January 3.
Around midnight the night of Monday the 27th, the apogean Moon will
pass five degrees to the north of Saturn, between the planet and
Gemini's Pollux.

The year ends with sense of grand symmetry.  Saturn, which we have
watched moving ever more into evening skies, now rises during
twilight, while Venus -- which we have seen lowering in morning
skies, rises just as twilight begins.  In between, Jupiter rises
shortly after midnight.  During the early part of the week, the
bright planets remain strung out in order from the Sun, starting
with Mercury, which is low in southeastern morning twilight, then
passing through Venus, then to Mars in Libra.  Continue on to
Jupiter, which, as the sky brightens, is high toward the south in
Virgo and to the west of the star Spica, then on to Saturn in
Gemini.

The progression ends the night of Tuesday the 28th, when Mercury
and Venus swap places.  Mercury, climbing upward in dawn to reach
greatest western elongation on Wednesday the 29th, passes
conjunction with Venus on the 28th, when the two are just over a
degree apart, Venus to the south of fainter Mercury.  Watch as the
two inner planets play tag with each other.  They will reverse
their positions again on January 13 of next year, when the outward
planetary progression resumes.

Four classic northern constellations surround Polaris and the North
Celestial Pole.  To the east, above mighty Orion, is Auriga with
bright Capella.  Moving then to the west, we find three major
constellations of the Perseus myth: Perseus himself (the Hero of
the story), Cassiopeia (the Queen), and then rather far over, her
husband, Cepheus (the King), which is practically invisible when
the sky is flooded with bright moonlight.  To the south of
Cassiopeia lies her daughter Andromeda, while between Auriga-
Perseus and the pole is the faint modern figure of Camelopardalis,
the Giraffe.

STAR OF THE WEEK: 53 PER (53 Persei).  Perseus, the Hero of the
Andromeda myth, is so luminous as a constellation that only
brighter stars carry proper names and Greek letters.  The
remainders must do with Flamsteed (or other catalogue) numbers,
fifth magnitude (4.85, on the bright side) 53 Persei among them.
Even its class B (B4, subgiant) status is reduced as a result of
all the other hot stars in the constellation.  Part of its dimness
is due to interstellar dust that lies along the line of sight.  If
it were not there, 53 Per would jump to fourth magnitude status
(4.34).  At a distance of 465 light years, this hot (16,200 Kelvin)
star, which radiates much of its light in the ultraviolet, shines
with the light of 1100 Suns, from which we derive a radius of 4.2
times solar and a mass 5.7 times solar.  The combination shows that
53 is not really a subdwarf (which implies that it has just given
up core hydrogen fusion), but a more ordinary dwarf about halfway
through hydrogen-burning with an age well under 70 million years.
The measured minimum equatorial rotation velocity is only 15
kilometers per second, which implies a rotation period of less than
14 days; most likely the star is spinning faster than that with an
axis that is more or less pointed at the Earth.  Like many stars of
its class it is somewhat deficient in metals, having about half the
content of the Sun.  Why even bother with this star, when there are
more glorious ones around it?  Because 53 Persei is the prototype
for a set of rather rare subtle variables called both "Slowly
Pulsating B (SPB) Stars" or in fact "53 Per stars."  It chatters
away with a variation of about two-hundredths of a magnitude (about
two percent), far below the eye's ability to perceive.  A "non-
radial" variable, 53 pulsates with several periods at the same
time, some parts of the star moving outward while other parts move
inward (the effect also seen through Doppler shifts in the spectrum
that show the surface to be oscillating).  There are two principal
periods, 2.16 and 1.66 days, the two switching in dominance, though
four other periods (2.12 days for example) are recognized as well.
A more famous version of the set is the set of Beta Cephei stars
(or Beta Canis Majoris stars), subdwarfs and dwarfs that lie at the
hot end of class B.  The 53 Per stars represent the lower-
temperature class B extension into the hydrogen-fusing dwarfs.  The
pulsation is driven by a layer of ionizing metals deep within the
star, the layer valving the outflowing stellar heat.  Variation, it
seems, is almost everywhere if we look for it hard enough.


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