[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 10, 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.

We pass through new Moon this week, on Saturday December 11th, when
it will be completely invisible.  Only 20 hours after the new
phase, the Moon goes through perigee, the combination bringing
especially high tides to the coasts.  By the evening of Sunday the
12th you might get a glimpse of an extremely thin waxing crescent
during fading twilight.  By the next and successive nights, the
crescent will be obvious, the dark side glowing with earthlight.
On Wednesday the 15th, the Moon will pass south Neptune in
Capricornus, the next day south of Uranus in Aquarius.

That planetary progression is merely the (excuse the expression)
"tip of the iceberg."  We have an unusual situation here.  Mercury
passes inferior conjunction with the Sun on Friday the 10th, making
it the first planet in the morning sky to the west of the Sun.
Pluto comes into conjunction with the Sun on Monday the 13th, and
until then it is the last planet in the evening sky to the east of
the Sun.  Thus for that short interval, the planets are all arrayed
outward from the Sun in actual order: Mercury, Venus (ok, we drop
Earth), Mars, Jupiter, Saturn, Uranus, Neptune, and ... PLUTO!  It
means nothing whatever physically.  It's just a very rare event,
and kind of curious.  There is no effect on anybody, or any other
planet, whatsoever.  Mercury is quite invisible, while Venus, Mars,
and Jupiter grace the morning sky in a nice row.  Saturn (in
Gemini) now rises just an hour after the end of twilight to give us
an evening planet, and while Uranus and Neptune are not readily
visible, they sure are there over in the far west at sundown; as is
ultradim Pluto, which is so close (in angle) to the Sun that it is
not visible through any telescope at all.

Of more interest perhaps is one of the best meteor showers of the
year, the Geminids.  Appearing to radiate from the constellation
Gemini, they peak the night of Monday December 13-Tuesday December
14, when an observer under a dark sky might see as many as 100
meteors per minute.  The Moon, near new, fully cooperates.  The
Geminids are debris flaked from what was once thought to be an
asteroid, 3200 Phaeton, which is apparently really a dead comet,
one with its volatiles burned off by the Sun.

Moving off to the southwest in mid-evenings is Fomalhaut of Piscis
Austrinus, the Southern Fish.  To the south and a bit west walks
Grus, the Crane, while to the southeast is a bright second
magnitude star that marks the top of another bird, the imaginary
Phoenix, which is supposed to have recreated itself from its own
ashes.  Farther down yet on a nearly straight line from Fomalhaut
through Phoenix, and visible only from the far southern US, is
Achernar, at the end of the river Eridanus.

STAR OF THE WEEK: GAMMA PHE (Gamma Phoenicis).  The majority of
evolving stars are rather settled giants of classes G and K, which
are quietly fusing their core helium into carbon and oxygen.  Here
and there though one finds a redder class M giant that, far from
being settled, is in some kind of transitional state, though
exactly what state may not be immediately evident.  At the northern
tip of the southern constellation Phoenix, the Firebird, is second
magnitude Ankaa, Alpha Phoenicis, in brightness followed by third
magnitude Beta, which are respectively typical K and G giants.  To
the east of Ankaa however, is third-ranked third magnitude (3.41)
Gamma Phe, the rarer class M (M0) giant that is our focus.  At a
distance of 235 light years, the star radiates at a rate 575 times
that of the Sun, most of its light coming out in the infrared, as
the star's surface is rather cool, only 3900 Kelvin -- an estimate
based on class, as oddly there are no actual measurements of it.
Like most such stars, Gamma Phe is large, its temperature and
luminosity giving a radius 53 times solar, a quarter of an
Astronomical Unit (AU), 65 percent the size of Mercury's orbit.
The theory of stellar structure and evolution yield a mass only 25
percent greater than that of the Sun, so the star gives some sense
of what is going to happen to us sometime in the next several
billion years.  Having a somewhat greater mass, though Gamma is
evolving faster than will the Sun, and is now a bit over 5.5
billion years old -- the Sun has almost that much time still left
to it as core-hydrogen burner.  The actual status of Gamma Phe is
arguable.  It might be brightening with a dead helium core or it
might have just started fusing helium to carbon and is dimming as
it prepares to settle in as a quiet class K giant.  However, Gamma
Phe is also a semi-regular or irregular variable star that
erratically (there is no known period) changes between magnitudes
3.39 and 3.49, which marginally implies that it has ceased helium
fusion and is brightening for the second time, now with a quiet
carbon core.  Far more certain is that the star has a companion
(revealed spectroscopically) with a period of 193.85 days.  Nothing
else is known about it.  If it is a low mass dwarf, the two average
just over 0.65 AU apart, whereas if it is as massive as the Sun,
the companion is 0.8 AU from its redly-glowing, dying mate, which
is destined to slough off its outer envelope and become a shrunken
white dwarf.



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