Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, September 21, 2001.
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.
Our Moon passes its first quarter early in the week, on Monday the
24th, brightening as it heads toward its full phase next week.
That same night it will make an especially good pass at Mars (now
in Sagittarius), when in the early evening it will be only 2
degrees directly to the north of the red planet. The configuration
will provide a fine chance to see the lunar motion, as the Moon
moves through its own diameter in about an hour. The night of
Wednesday the 26th, the Moon will visit Neptune, the night of
Thursday the 27th Uranus, both planets within the confines of
Capricornus.
The morning sky remains glorious, with Saturn, brighter Jupiter,
and yet-brighter Venus all strung out on a line from high in the
sky toward the eastern horizon, the three planets roughly defining
the ecliptic -- the plane of the Solar System -- rather like a
dotted line. Saturn (still in Taurus), now rising around 10:30
Daylight Time, passes a watershed during this round of evening
visibility when it begins retrograde -- backward -- motion the
night of Wednesday the 26th, as the Earth prepares to pass in
between the Sun and the ringed planet. Jupiter (beautifully set
amidst the stars of Gemini), rising around 12:30 AM Daylight Time,
prepares to move into evening as well.
It is planet Earth, however, that takes center stage, when it
crosses into the sky's southern hemisphere at the autumnal equinox
in Virgo at 6:04 PM Central Daylight Time (7:04 Eastern, 4:04
Pacific) on Saturday the 22nd, and astronomical autumn begins in
the northern hemisphere (and spring in the southern). On that day
the Sun will rise very close to due east and set close to due west
all over the Earth except at the poles. At the north pole, the Sun
will officially set (though because of its finite diameter and
atmospheric refraction it will still be visible); at the south pole
scientific stations, the Sun will officially rise and be up for the
next six months. At the Earth's equator, the Sun will pass through
the zenith, the point overhead.
As we move into fall, the summer constellations begin to make their
exit to the west, Scorpius appearing low in the southwest, giant
Ophiuchus, the Serpent Bearer, standing above the scorpion more to
the west. To the east look for the Great Square of Pegasus, which
has already risen by nightfall. To the left of the Great Square,
runs the graceful string of stars that makes most of Andromeda, and
coming up in the northeast is the "W" of Cassiopeia. Still, the
memory of summer lingers through the Summer Triangle of Vega,
Deneb, and Altair, which in early evening is nearly overhead in
mid-northern latitudes.
STAR OF THE WEEK. ZETA OPH (Zeta Ophiuchi). Just over the line
into third magnitude (2.56), and third brightest star within the
constellation Ophiuchus (the Serpent Bearer), Zeta Ophiuchi is
oddly not graced with a proper name, which is odder still since it
is in the middle of the line of stars that make the bottom border
of the constellation, the others, from west to east being Yed Prior
(Delta Ophiuchi), Yed Posterior (Epsilon), and Sabik (Eta) (though
Zeta has been known to share that name with just-barely-brighter
Eta). Too bad too, as Zeta Oph is truly the magnificent star, a
blue-white class O (though at 09.5 just barely) hydrogen-fusing
"dwarf" (a strange term for a star with a diameter 8 times that of
the Sun). The star (which is very slightly variable), however,
does not LOOK so blue-white. Zeta Oph is one of the brighter stars
in the sky to be significantly affected by absorption and reddening
of its light by passage through interstellar dust (which lies
everywhere within the Milky Way). At a distance of 460 light
years, the star is deeply involved with dust gas clouds (and even
illuminates one of them), and is used as a background light source
with which to examine the stuff of interstellar space. If the dust
were not in the way, Zeta would shine at almost first magnitude.
>From its distance and great temperature of 32,500 Kelvin (from
which we can account for the star's fierce ultraviolet light), we
calculate a magnificent luminosity of 68,000 times that of the Sun,
and from that a mass of 20 times solar, the star about in the
middle of its short (8 million years) hydrogen-fusing lifetime.
Like most luminous stars, Zeta Oph is losing mass through a strong
wind that in this case blows at about 1600 kilometers per second at
a rate of about a hundredth of a millionth of a solar mass per
year. The star's only fate seems to blow up as a supernova. Among
Zeta Ophiuchi's most interesting properties is that it is one of
the sky's most famed "runaway stars," stars that used to be
together and are now fleeing from a once-common point. The prime
examples are Mu Columbae and AE Aurigae, which are running away
from Na'ir al Saif (Iota Orionis) after an exchange and expulsion
when two massive double stars encountered each other. Zeta
Ophiuchi, on the other hand, seems to have been expelled from a
double star system when its one-time and clearly more-massive
companion exploded and is now a tiny "neutron star" about the size
of a small town. The explosions that make supernovae are
apparently off-center, so that when one of the stars goes off and
is blasted like a bullet to one side, the other one can, if
conditions are right, be shot off as well. Zeta Ophiuchi is now
single, however, so that the scene cannot repeat itself.
****************************************************************
Jim Kaler
Professor of Astronomy Phone: (217) 333-9382
University of Illinois Fax: (217) 244-7638
Department of Astronomy email: [EMAIL PROTECTED]
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)
Astronomy! A Brief Edition (links and updates)
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