[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for short week starting Saturday, October 23, 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 starts its week in the waxing gibbous phase as it heads
toward full, the phase reached the night of Wednesday, October 27,
when there will be a magnificent lunar eclipse that is beautifully
timed for continental North America.

On that night, the Moon will pass just to the north of the central
part of the Earth's shadow.  The "penumbral phase" (when the Moon
falls into partial shadow of the Earth) is only barely visible and
is here ignored.  The Moon begins to enter total shadow (called the
"partial eclipse phase" because only part of the Moon is obscured)
at 8:14 PM CDT.  (Add an hour for EDT, subtract an hour for MDT, 2
hours for PDT; subtract another hour if you are on standard time).
The eclipse then becomes total at 9:23 PM CDT.  Mid-eclipse, when
the Moon is as dark as possible, falls at 10:04 PM CDT.  The events
then reverse, with the Moon beginning to leave the total shadow at
10:45 PM CDT, the show over at 11:54 PM CDT.  While Alaskans will
see the eclipse, Hawaiians will see only the last parts of it.

Since the Earth's atmosphere scatters sunlight into the realm of
total shadow, the Moon does not become completely dark during
totality.  The brightness of the eclipsed Moon depends on the state
of the terrestrial atmosphere, particularly on the degree of
volcanic activity prior to the eclipse, as dust and aerosols dim
the amount of light getting through.  The colors on the Moon can be
quite lovely, especially if the eclipse is viewed with binoculars
or a telescope.

As anticlimax, Neptune, deep in Capricornus, ends retrograde motion
on Sunday the 24th.  The morning sky near dawn does much better
with the planets, where brilliant Venus clearly rules.  However,
look below Venus to find bright Jupiter, which is now clearing the
horizon in modest twilight.  In the other direction, to the west,
find Saturn, which is now rising just after 11 PM Daylight Time.
Though still in Gemini, Saturn is very close to the border with
Cancer, and is nicely pointed to by Gemini's Pollux and Castor.

As the Moon dims into totality, watch the stars come back out.
Since the Sun is now well past the autumnal equinox, the fully
eclipsed Moon will be the same degree to the east of the Vernal
Equinox, and south of the classic figure of Aries.  To the
southwest lies Neptune's Capricornus, one of the dimmer Zodiacal
constellations, while the Summer Triangle of Deneb, Vega, and
Altair moves off to the northwest.

STAR OF THE WEEK.  ZETA CAP (Zeta Capricorni).  Dipping into the
eastern lower half of mostly faint Capricornus, find the modest
fourth magnitude (3.74) "yellow" class G (G4) supergiant ("yellow"
referring not so much to actual color but to the part of the
spectrum where much of its light emerges) Zeta Capricorni.  The
"supergiant" tag implies that the star must be fairly distant to
shine so faintly, and sure enough, it measures 122 light years
away.  The temperature of 5050 Kelvin (not that much cooler than
our Sun) allows for a small correction for some infrared light,
which with distance yields a luminosity 490 times that of the Sun,
and a resulting radius 29 times solar.  These are distinctly
smallish values for a star called "supergiant," and more like those
of a "bright giant."  Since the term comes from simple examination
of the spectrum, however, it remains in use, showing that some
stars are not quite what they are at first made out to be.
Combination of temperature and luminosity suggest a star of about
four solar masses, which makes it under 200 million years old, one
that is now most likely fusing helium into carbon in its deep core
and will "soon" (on an astronomical timescale) begin to brighten
once again to close to real supergiant status.  The interest in
Zeta Cap lies not in these characteristics, however, but in its
chemical composition and in its classification as a "mild barium
star," one rich in that heavy element, and rich in other elements
as well that are created under high temperature conditions through
capture of neutrons (Zeta Cap enriched in such elements as
zirconium and the "rare earths" by a factor of 10).  In some stars,
chemical enrichment comes from interior nuclear reactions, the
newly-formed elements raised upward by convection.  In others,
however, they come from contamination from a companion that has
since died and left a legacy of its own nuclear reactions brought
to the current giant star through mass exchange.  Zeta Cap and all
other barium stars (epitomized in the naked-eye sky by Alphard)
fall into the latter category, the earlier (once more massive and
brighter) star now reduced to a dead cinder, a white dwarf.  While
the white dwarf is unseen, measures of Zeta Cap's spectrum (through
the Doppler effect) reveal a companion in a 6.5 year period at a
separation of around 6 Astronomical Units.  Another "companion," a
12th magnitude star at a separation of some 20 seconds of arc, is
also a white dwarf, but the alignment is merely a line-of-sight
coincidence, again showing that not all things are what they at
first seem to be.



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