Jim Kaler
Sun, 10 Jun 2001 17:57:06 -0700
Skylights, University of Illinois Department of Astronomy.
Astronomy News for the short week starting Sunday, June 10, 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.
The Moon, beginning the week in its waning gibbous phase,
passes third quarter the evening of Wednesday, the 13th, and
thereafter wanes through crescent, every day rising later after
midnight. It passes south of Neptune, in Capricornus, on Sunday
the 10th, and south of Uranus, in the far eastern end of the same
constellation, the following day.
The two planets outward from the Earth provide a wonderful
contrast. Jupiter, the Solar System's giant, and ordinarily the
second brightest of planets, passes conjunction with the Sun on
Thursday the 14th, and will be quite invisible. Mars, on the other
hand, is now at its best, passing opposition to the Sun the day
before, Wednesday, the 13th. On opposition night, Mars will rise
at sunset, set at sunrise, cross the celestial meridian to the
south at midnight (1 AM daylight time), and have its greatest
angular retrograde (westerly) speed. It will also about as close
to the Earth as possible during this orbital round, 68.6 million
kilometers, or 42.6 million miles. Unfortunately, the red planet,
which now shines brighter than any star in the sky (and is now
exceeded only by the morning's Venus, the Moon, and the Sun), will
also be at about its most southerly position (just to the west of
Sagittarius), and for northern observers about as low as it can
get. Turbulence in the Earth's atmosphere then makes detail more
difficult to view by telescope. After opposition, Mars will begin
to rise before sunset, will move more and more into the early
evening sky, and will be with nicely us for the remainder of the
year. Mars passes opposition to the Sun every 780 days. The orbit
is so eccentric that opposition distance vary considerably. The
next one will be better. The minimum distance is 56 million
kilometers (35 million miles), the maximum almost twice as great.
Only Venus comes closer.
Mars actually lies in far southern Ophiuchus between Sagittarius
and Scorpius, providing a good chance to compare the color of the
planet with its namesake Antares (Ares the Greek version of the war
god), which is nicely visible to the far right of Mars.
As spring slowly blends into summer, these beautiful
summertime constellations, which contain the Milky Way, will begin
to overtake the more drab spring skies, which now still dominate in
the early evening, Virgo and Spica nearly due south as night falls.
Watch as they slip away a degree per day to the west as the sky
reflects the degree-per-day motion of the Earth around the Sun. If
your skies are dark, you might make out the dim box of stars that
forms Libra, the scales, the zodiacal constellation that lies
between Virgo and Scorpius.
STAR OF THE WEEK. ZANIAH (Eta Virginis). An old term -- before
we understood that stars do move -- referred to the "fixed stars,"
the phrase really meant to distinguish real stars from the
"wandering stars," the planets. The names applied to stars
sometimes appear about as fixed as the stars themselves, that is
they, or at least some of them, move around too. "Zaniah," the Eta
star of Virgo, the Virgin, refers an angel, the "angel of a-awwa"
(the meaning unknown, and with apologies to Arabic readers for
leaving out the necessary accents), and was originally applied to
Porrima (Gamma Virginis), and later fell to dimmer (mind-fourth
magnitude, 3.89) Eta. In a rather special place, Zaniah, to the
west of Porrima, is squeezed between the celestial equator and the
ecliptic. Of the brighter stars that make the constellation
figures, Zaniah is one of the closest to the equator, only 2/3 of
a degree to the south of it, and only 10 degrees to the east of the
autumnal equinox, the point where the Sun crosses the equator on
its way south in September. Zaniah, 250 light years away, is
classed as an A (A3) subgiant, the latter meaning that the star
seems to be about to give up its central hydrogen fusion, if it has
not already. From its surface temperature of 8800 Kelvin (and of
course its distance) we can calculate a luminosity 130 times that
of the Sun. The star's status and properties, however, are
seriously compromised by its seeming triple nature. None of the
components can be resolved by eye at the telescope. Ultrashort
imaging (to avoid smearing of the image by twinkling) in addition
to occultations by the Moon reveal a pair of stars (one fourth
magnitude, the other fifth) separated by but 0.12 seconds of arc,
or around 10 Astronomical Units. One of them, probably the
brighter, is revealed by the spectrograph to be a much closer
double with a period of 72 days and an average separation of only
half an astronomical unit. This very close double is then orbited
by the third outer star. It would seem impossible for a planet to
survive the gravitational onslaught of the trio. All three seem to
be class A stars, and one or more may be slightly variable. Though
individually, each would become a relatively massive white dwarf,
the proximity of the close pair will probably disrupt the flow of
evolution as the more-massive of them will expand first and will
encroach upon the other, a common scenario among close pairs.
****************************************************************
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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