[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 20, 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
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 soon: The Cambridge Encyclopedia of Stars.

The week begins with the waning crescent Moon one day shy of new,
rendering it quite invisible, the new phase reached the night of
Saturday the 21st.  We then get to see the waxing crescent low in
western evening dusk the night of Tuesday the 24th.  Watch as it
climbs in successive nights ever higher out of twilight, heading
toward first quarter early next week.  If you can spot the Moon
early on the night of the 24th, you might also look for Jupiter and
Mercury to the right of the crescent, Jupiter the higher of the
two.  You'll need a flat horizon and probably binoculars.  The
bright star Antares will be up and to the left of the Moon, while
the following night (Wednesday the 25th) look for the star to the
right of the crescent.

The week (if extended by a day) has interesting and rather curious
parings.  The two planets that bracket the Earth, Mars and Venus,
both come into conjunction with the Sun, Mars from the direction of
the evening sky on Monday the 23rd, Venus (in superior conjunction,
in back of the Sun) from the morning sky on Friday the 27th.  Mars
will then become a morning planet, Venus an evening planet, both
symmetrically clearing twilight shortly after year's end.  Crossing
apparent paths, the two quite invisibly also come into conjunction
with each other on Tuesday the 24th (though widely separated by
nearly a full Astronomical Unit: the distance between Earth and
Sun).  The largest and smallest of the ancient planets, Jupiter and
Mercury, then come into conjunction with each other in mid-week, on
Wednesday the 25th.  If you want to see a bright planet, all that
is readily available is Saturn, which rises to the west of Regulus
in Leo around 1:30 AM Daylight Time.

While admiring Saturn, you might look before dawn for the Orionid
meteor shower (the meteors coming from the direction of Orion),
which will be at its peak the mornings of Saturday the 21st and
Sunday the 22nd, giving us (in a dark sky) 20-25 meteors per
minute.  These flakings of Halley's Comet will continue in
diminished numbers throughout the week and into next week as well.

'Tis the season for the constellations of the Perseus myth, led by
Pegasus (with its Great Square climbing the eastern sky in early
evening) and, well to the north, by Cepheus, the King and father of
Andromeda.  The "W" of the most famed of the set, Cassiopeia, the
Queen (and mother), is then seen beautifully climbing in the
northeast.

STAR OF THE WEEK: VV CEP (VV Cephei).  Two of the most magnificent,
and largest, stars of the sky lurk close together and rather
anonymously within the dark interstellar dust clouds of Cepheus
(the King): Herschel's Garnet Star (Mu Cephei) and the
extraordinary variable and binary, VV Cephei.  Both are huge red
supergiants.  Mu Cep stands at only fourth magnitude (4.08), VV
fainter at fifth (4.91).  Were it not for the dimming effects of
the dust, they would respectively shine at second (1.97) and third
(2.91) and might have been parts of the formal constellation.  Mu
Cep has a current estimated radius somewhere between 1200 and 1650
times that of the Sun, or 5.6 to 7.7 Astronomical Units, bigger
than the orbit of Jupiter.  Though VV may well top it out, the
uncertainties preclude accurate assessment.  Above all, VV Cep is
a terrific example of a mass-exchanging eclipsing binary, in which
a distorted, swollen red class M (M2) bright supergiant orbits with
a much fainter class B (B8) blue-white hydrogen-fusing dwarf (one
that has been classified as hot as O8), which tidally distorts its
vastly bigger, and much less dense, companion.  The pair orbits
with a period of 20.4 years.  Separated by 25 Astronomical Units
(80 percent the distance between Neptune and the Sun), a high
eccentricity takes them between 17 and 34 AU apart.  When the blue
star goes in back of the supergiant, the visual light dips by about
20 percent.  The supergiant is so huge that the blue dwarf is
eclipsed for the better part of a year, 250 days.  The binary is
hard to study, as the interval between eclipses gives only a couple
of them in a working astronomical career.  Analysis of the spectrum
and the eclipses give radii for the supergiant between 1600 and
1900 solar (7.5 and 8.8 AU).  The bigger estimate gives us a star
92 percent the size of Saturn's orbit, making it the largest known. 
The temperature, not well known, falls between 3300 and 3650
Kelvin.  Radius and temperature give a luminosity that falls
somewhere between 275,000 and 575,000 times that of the Sun, which
in turn give masses between 25 and 40 times solar.  These figures
can be checked through visual brightness, distance, and
temperature.  Much too far away for parallax, the distance is
estimated through the star's membership in the Cepheus OB2
association of hot blue stars, which gives 2400 light years to
within about 20 percent.  However, one study claims the star does
not belong to the association at all, so in truth we really do not
know.  At 2400 light years, the temperature range (from which we
estimate the amount of infrared light) gives similar luminosities
of between 163,000 and 535,000 Suns, a radius between 1000 and 2200
Suns (4.7 and 10.4 AU), and an accordingly similar mass range.  The
smaller values (from the higher temperature) seem the more
reasonable, which then do not accord well with the values from the
spectrum and the eclipses.  VV Cephei A (the red supergiant),
however, is not spherical.  It instead seems to be distorted into
a teardrop shape and to fill its tidal surface, from which it sends
matter into a disk around the smaller, much hotter, companion,
resulting in overestimates of average dimension (and making the
basic concept of radius problematic).  The star may also be much
farther away, perhaps twice the distance of Cepheus OB2.  The
companion is even more mysterious, as we are not even certain of
its class and mass, which is probably a few times that of the Sun. 
The mass exchange, which could be as high as a few hundredths of a
solar mass per year (and which must alter the evolution of the
lesser star), is probably at the heart of sudden changes in orbital
period.  The flowing matter makes the two into "emission line
stars."  Typical of supergiants, VV Cep is also a pulsating semi-
regular variable that changes by a few hundredths to a few tenths
of a magnitude with recognized periods of 58, 118, and 349 days
plus one of 13.7 years.  While the various parameter ranges are
unfortunately large (showing how hard it is to study such rare
stars), it is clear that the supergiant (now probably fusing helium
into carbon in its deep core) will "soon" blow up as a grand
supernova, perhaps ejecting its companion back into the cosmos as
a single star that had quite a career behind it.  VV Cephei is
included in Jim Kaler's "The Hundred Greatest Stars."  Thanks to
Jose Rodriguez, who suggested it.



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