[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 29, 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.

The Moon, having survived its eclipse by the Earth's shadow, takes
the whole week to wane through its gibbous phase, and then ends the
period at the third quarter, that phase reached the night of
Thursday, November 4, about the time of Moonrise in North America.
With the Sun about halfway to its low point at the Winter Solstice
in Sagittarius, the quarter will be on the downslide of the
ecliptic and just to the west of the Sickle of Leo.  In the middle
of the week, Tuesday the 2nd, the Moon passes apogee, where it is
farthest from the Earth.  The waning gibbous passes Saturn during
the day of Wednesday, November 3, so it will be just to the west of
the ringed planet the night of Tuesday the 2nd, and to the east of
it the following night.

While not really an astronomical event, note that Daylight Savings
Time in the US ends on Sunday, October 31.

Saturn, now rising about 10 PM Standard Time, is our only evening
naked-eye planet.  To see more of the planets, you must wait for
the morning, at which time you can see Venus and Jupiter rising
just after 3:30 AM.  To rise nearly simultaneously, they must be
close together (in angle as seen from Earth, hardly in distance,
Jupiter five times farther away).  And indeed, they will come into
conjunction with each other during the day on Thursday, November
4th.  This marvelous event, the meeting of the two brightest
planets, will be nicely on display the mornings of Thursday the 4th
and Friday the 5th.  At their closest, the two will be only 0.6 of
a degree apart, brighter Venus passing to the north of giant
Jupiter.  Just to the left of the pair look for Porrima in Virgo,
which will appear as a much fainter third conjunctive companion.
Way down below the two you might glimpse Virgo's Spica rising out
of brighter twilight, and then down and to the left of it, Mars,
which is beginning to make its appearance, rising just after dawn
begins.  Spica and Mars are in conjunction the morning of Sunday,
October 31, with the planet three degrees to the north of the star.

As you admire the morning planets, note Orion now well to the west
of the meridian at dawn as he heads toward his annual evening show.
As Jupiter and Venus rise, Sirius in Canis Major, Orion's larger
Dog, crosses the meridian to the south.  The evening stage is set
with the heart of the stars of fall.  With Cygnus and the summer
gang heading off to the west, the Great Square of Pegasus is seen
high to the south, while Andromeda stretches off to the northwest
as its star-streams connect with Perseus, the Hero of the great
story of Cassiopeia, herself seen crossing the meridian around 10
PM as the Big Dipper crosses beneath the pole.

STAR OF THE WEEK: ZETA CYG (Zeta Cygni).  The sky's great bird,
Cygnus the Swan, flies with outstretched wings to the south down
the path of the Milky Way.  Each wing is marked by one star, the
northwestern one by Delta Cygni, the southeastern by Gienah
(Epsilon).  The latter wing, however, can also be stretched farther
to third magnitude (3.20) Zeta Cygni (of no proper name), a
"yellow" class G (G8) giant star.  Shining at us from a distance of
150 light years from a surface with a well-determined temperature
of 4980 Kelvin, Zeta radiates 119 solar luminosities into space.
The radius of 14.7 times that of the Sun derived from temperature
and luminosity agrees nicely with the value of 14.0 found from
direct measure of the angular diameter coupled with distance, the
two lying well within the errors of measurement of each other.  The
star's most likely status is that of a core-helium-fusing "clump
giant" (so-called because of the large number of stars with similar
characteristics that gather into a clump when graphed on a diagram
of luminosity vs. temperature), though it might also be in
transition to gianthood with a quiet helium core.  If the former,
the mass is around 3.0 solar with an age of about 400 million
years; if the latter, the mass is a bit higher.  Zeta Cyg, like
Zeta Capricorni, Alphard, and several others, is also a mild
"barium star" with a superabundance (about triple) compared to the
Sun of that chemical element, as well as other heavy elements, in
its atmosphere.  Rather than making the elements themselves through
nuclear reactions, such giants have been contaminated by once-more-
massive companions that evolved first, made the stuff (by neutron
capture deep in their nuclear-burning hearts), and then sent it on
through mass-loss processes to their then-less-massive companions
before turning into much lower mass white dwarfs (the lower mass
the result of the same mass loss).  And sure enough, Zeta Cyg has
a faint companion.  Spectroscopic observations reveal an orbiting
star with a period of 6489 days (17.8 years).  An assumption of a
typical white dwarf mass yields an average separation of 11
Astronomical Units, the modest eccentricity taking the stars from
as close as 8 AU to as far as 13 (some 30 percent farther than
Saturn is from the Sun).  And sure enough once again, Hubble Space
Telescope observations reveal a little blip, the white dwarf
itself, which is almost lost in the glare of the much brighter glow
of Zeta Cyg itself.  A detailed theoretical study shows that the
system started with stars of 3.0 and 2.5 solar masses.  The greater
of these lost most of its mass to space in the act of becoming the
white dwarf, sent half a solar mass to the lesser (including heavy
chemical elements that it had manufactured), which increased the
initially lower mass star to three solar masses and is the star we
see today as Zeta Cyg.  The system thus vividly demonstrates the
dynamic binary interactions that can change the course of an
individual star's history and fate.

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