[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

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

See "The StarGazer" at a planetarium near you: visit
     http://www.astro.uiuc.edu/~kaler/sg.html

Support science literacy by joining the Astronomical Society of the
Pacific, among the world's premier providers of astro education and
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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.

The Moon, going through its phases every 29.5 days, passes its
third quarter on Sunday, November 12th, during the day and about
the time of Moonset.  The pair of previous days have the Moon as a
waning gibbous, whereas the rest of the week sees it as a waning
crescent.  In the middle of the week (Wednesday the 15th), the
crescent coincides with apogee, at which time the Moon will be a
bit over five percent farther from Earth than average.

Earlier in the week, the Moon will make a very nice passage past
Saturn and through Leo.  The ringed planet, in "quadrature" with
the Sun (90 degrees to the west of it), now rises in late evening,
around 11:30 PM.  On the night of Saturday the 11th (the morning of
Sunday the 12th), the quarter will, as it climbs the eastern sky,
be above (to the west of) Saturn (which in turn is above, to the
west of, Regulus).  The following night (and morning), the Moon
will have passed the planet, and will appear just below (or east
of) Regulus.  The morning of Tuesday the 14th, the lunar crescent
passes south of the Lion's main body.

Other than Saturn, the planetary sky lacks luster.  Mercury, Venus,
Mars, and Jupiter are all bunched near the Sun in a sort of
alignment (which has no effect on the Earth whatsoever).  The lack
leaves the evening with dim Uranus (in Aquarius) and dimmer Neptune
(just barely in Capricornus). 
The big event, and it will not be much for North America, is the
Leonid meteor shower, which emanates from the direction of Leo, and
which will build during the week to a peak very early next week. 
The leavings of Comet Tempel-Tuttle, the meteors can produce
powerful storms every 33 years just after the comet passes the
Earth.  While we long since passed the big peak, some 100 meteors
per hour are still expected, though mainly as seen from Europe and
Africa.  Here, we may see a few.

Among the lesser of the constellations of the Perseus myth, both in
story and in brightness, is Cepheus (the King, Cassiopeia's
husband).  Not in interest, however.  Cepheus not only contains the
prototype of the Cepheid variables, Delta Cephei, it has a
prominent star with a planet (Errai, Gamma Cep), two of the largest
stars known (Mu and VV Cep), and is filled with distant hot blue
stars that are dimmed by the interstellar dust of the Milky Way.

STAR OF THE WEEK: 19 CEP (19 Cephei).  A trio of little (so they
seem) fifth magnitude stars that lie in a curved row in south-
central Cepheus lead to the eye to the magnificent eclipsing binary
and monster star, VV Cephei.  From south to north, 19 Cephei
(magnitude 5.11), 20 Cep (5.28), and 18 Cep (5.31) epitomize
stellar differences, the two "bookends" providing an almost
startling contrast near opposite ends of the stellar temperature
scale.  Flamsteed 18 is a class M (M5) red giant 450 light years
away, whereas 19 is a hot (31,500 Kelvin), blue class O (O9) "class
I" supergiant (though a "lesser" "Ib" version) that is so far away
that there are no direct distance measures.  The middle star, 20
Cep, as a class K (K4) giant 320 light years off, also has middling
properties.  While 18 Cep is clearly reddish, distant 19 Cep -- our
focus here -- is dimmed by over a full magnitude by interstellar
dust, which reddens the starlight to white.  Were the dust not
there, "19" would appear as fourth magnitude (3.96) and nicely
outshine its seeming (quite coincidental) mates.  The direct
parallax of 19 Cep gives a whopping distance of 5400 light years,
but the associated error is so large -- almost as big as the
measure itself -- that such a distance cannot at all be trusted. 
Were it true, "19" would have to be a bright "Ia" supergiant and it
clearly is not.  From the star's traditional membership in the
Cepheus OB2 association, the distance is "only" 2400 light years. 
However, one detailed study removes the star from the association. 
The supergiant class alone then implies a greater distance of 1700
light years.  We just don't know.  In either case, the star is very
luminous and massive.  If at the shorter distance, it is 180,000
times brighter than the Sun (allowing for a lot of ultraviolet
light) with a radius of 14 solar and a mass of 30 solar.  If at the
longer distance, the numbers are 345,000, 20, and 35 solar.  Oddly,
in either case, 19 Cephei appears to be a very luminous class O
dwarf that is still fusing hydrogen into helium in its core rather
than a supergiant with a dead helium core.  A projected rotation
speed of 33 kilometers per second provides little constraint, the
star rotating in under a month.  Whatever the parameters, 19 Cephei
is heading for disaster.  Only five or so million years old, it is
furiously losing mass at a rate of two millionths of a solar mass
per year (millions of times that in the solar wind) at a speed of
2000 kilometers per second.  When the core hydrogen runs out, the
star will swell into a red supergiant and explode as a supernova. 
It was long ago assigned a pair of dim 11th magnitude companions at
angular separations of 20 and 59 seconds of arc, but they are
almost certainly just line-of-sight coincidences.  (Thanks to
Reginald Quinto, who suggested this star.)


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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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