[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, November 17, 2006. 
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.

Support science literacy by joining the Astronomical Society of the
Pacific, among the world's premier providers of astro education and
recognized by the Independent Charities of America as one of the
top charitable organizations in the US.  Get the outstanding
astronomy magazine Mercury and a variety of other benefits.  Call
1-415-337-1100, then press 1.

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

Coming in December: The Cambridge Encyclopedia of Stars.

To those in the US (and elsewhere too), a Happy Thanksgiving.

We begin as always with the ever-popular Moon (at least unless you
are trying to see faint stars), which begins the week as a slim
waning crescent in the morning sky and then goes through new on
Monday, November 20th.  The morning of Saturday the 18th finds it
just to the east of Spica in Virgo, which is now in the act of
clearing the Sun.  By the evening of Wednesday the 22nd you can
find the narrow waxing crescent in western twilight, the Moon then
growing in phase during the remainder of the week.  It's a good
time to admire the glow of Earthlight illuminating the lunar night.

Most of the bright planets remain bunched near the Sun.  Jupiter is
the champion no-show, as it passes conjunction with the Sun (far to
the other side of it) on Tuesday the 21st, thereafter becoming a
morning planet (though not visible until mid-December).  Mercury at
least makes the attempt.  After a beautifully observed transit
across the Sun last Wednesday, it pops into the morning sky heading
toward greatest western elongation next Saturday the 25th.  By the
end of the week it will be visible in eastern morning twilight. 
The little planet is visited by the waning crescent Moon the
morning of Saturday the 19th, the event essentially invisible in
bright dawn.

The planetary sky is thus dominated by Saturn.  Moving ever more
into the evening sky, the ringed planet (all the outer planets
having rings, Saturn just having by far the brightest) now rises an
hour before midnight.  Look for it in the morning sky to the west
of Regulus in Leo.  As a planetary coda, Uranus (in Aquarius)
ceases retrograde motion on Monday the 20th, and begins to move in
its normal easterly direction against the background stars.

It's worth a look to see if you can see any Leonid meteors (named
after the seeming direction of origin in the constellation Leo),
the shower peaking near the morning of Saturday the 19th.  The
debris flaked from Comet Tempel-Tuttle is, however, best seen from
Europe and Africa.  Here we might see a few over a few-day period.

As northern hemisphere winter closes in, we might contemplate the
Vernal Equinox (where the Sun will be on the first day of Spring),
which now rides high in Pisces.  The invisible point, where the
ecliptic (the apparent path of the Sun) crosses the celestial
equator, lies just down and to the left of the faint "Circlet,"
which represents the body of the western of the two mythical
fishes.  The rest of the constellation sprawls in a huge "vee" to
the east and then to the north, wrapping around the Great Square of
Pegasus.

STAR OF THE WEEK: IOTA PSC (Iota Piscium).  Among the classic and
best-loved celestial asterisms -- informal constellations -- is the
Circlet of Pisces, the body of the western fish.  And where would
the Circlet be without Iota?  Though at mid-fourth magnitude (4.13)
not very bright (much like the rest of the huge constellation), it
not only helps complete the ragged circle, but sits at the juncture
between the Circlet and the line of stars that begins the fish's
tail and continues with the ribbon that joins it to the eastern
fish.  (The Circlet provides something of a guide to the Vernal
Equinox, which lies a few degrees to the southeast of it.)  More
importantly, Iota Psc intrigues as one of "us," as a star rather
similar in properties to the Sun, one of those we look at and
wonder if it too has planets.  A yellow-white class F (F7)
hydrogen-fusing dwarf with a temperature only a bit warmer than the
Sun (6300 Kelvin), Iota Psc shines with a luminosity 3.3 times
solar, from which we derive a radius 1.5 times that of our own
star.  Place our Earth at a distance of 1.8 Astronomical Units from
it and we would feel very much at home.  Stellar structure theory
tells of a star that carries 1.25 times the mass of the Sun, one
that, with an age of 3.7 billion years, is closing in on the end of
its projected 4.3 billion year hydrogen-fusing lifetime (higher
mass stars living shorter lives).  Among the bigger differences is
the shorter rotation period of under 13 days (about half that of
the Sun), which certainly stirs up some significant magnetic
activity (yet to be explored).  Though listed as "variable," there
is precious little evidence of it.  Indeed, the star is used a
spectroscopic standard for velocity measures.  But are there
planets?  None has as yet been detected.  Neither has any
circumstellar debris that might suggest something of an asteroid
belt.  Perhaps unsurprisingly.  Planet-holding stars tend to be
metal-rich as compared with the Sun, while Iota goes in the other
direction, being slightly metal-poor, its iron content relative to
hydrogen 70 percent of solar.  While once thought to have one or
two dim stellar companions at separations of 70 and 280 seconds of
arc, both are simply line-of-sight coincidences, the star at the
juncture of the Circlet seeming very much alone.


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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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