Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, July 27, 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.
We begin the week with the Moon passing its first quarter. For the
next seven days it will wax through gibbous, getting brighter each
night as it moves at its 13 degree-per-day pace against the
background stars. Since the Sun is now moving through the
northerly constellations of the Zodiac (this week in Cancer), the
Moon -- as it approaches full -- passes through the southerly
constellations, bottoming out in Sagittarius the night of
Wednesday, August 1.
Aside from the Moon, the evening sky is dominated by reddish Mars,
the next planet out from the Earth. Shining from southern
Ophiuchus (between the classic zodiacal constellations of Scorpius
and Sagittarius), the planet is slowly picking up easterly speed
against the background stars. The tilt of its orbit has sent it
several degrees below the ecliptic (the apparent solar path), while
the tilt of the lunar orbit has sent the Moon just north of it. As
a result, as the Moon approaches Mars the night of Sunday the 29th,
the Moon will be some 6 degrees to the north, just greater than the
spread between the front bowl stars (the "Pointers") of the Big
Dipper (which in the evening is now descending the sky to the
northwest). The morning, on the other hand, is dominated by the
mythological opposite of the "god of war," Venus, the "goddess of
love and beauty," which is strikingly lovely in the morning hours
to the east before dawn.
It is dim Neptune's week, however, as the farthest of the large
planets passes through opposition to the Sun on Monday, July 30, as
it moves retrograde in Capricornus. Discovered in 1846 as a result
of its gravitational influence on Uranus, Neptune has yet to make
a full orbit since found, as it takes 165 years to make a complete
circuit of the Sun. (It will come full circle in 2011.) Of the
traditional planets of the Solar System (including tiny Pluto,
which now hangs out in Ophiuchus 15 degrees almost exactly to the
north of Mars), only Neptune and Pluto require a telescope to see,
Uranus (somewhat to the east of Neptune) faintly visible to the
naked eye.
As July moves into August, we find Scorpius on the meridian to the
south as evening descends. Part of a "double constellation," the
scorpion's claws stretch out to the west as the bright stars of
Libra (the celestial scales), Zubenelgenubi the southern claw,
Zubeneschamali the northern. Two and a half thousand years ago the
"Balance" held the autumnal equinox (the point where the Sun
crosses the celestial equator on its way south). Precession, the
26,000 year wobble of the Earth's axis, has long since moved it
westerly into Virgo, just as it has moved the vernal equinox (where
the Sun moves north of the equator) from Aries into Pisces.
STAR OF THE WEEK. ZUBENHAKRABI (Sigma Librae = Gamma Scorpii).
There are a small number of "linking stars" that traditionally
belong to two constellations. Alpheratz, Alpha Andromedae, is also
Delta Pegasi, while Elnath, Beta Tauri, is also Gamma Aurigae (the
two alternatives no longer used since the stars are within the
modern boundaries of Andromeda and Taurus). Zubenhakrabi is more
extreme. The star, whose name means "the scorpion's claw," was
originally part of Scorpius along with the more-traditional "claws"
Zubeneschamali and Zubenelgenubi (which refer to the "northern" and
"southern" claw). As such, Bayer called it Gamma Scorpii. But it
was so far west of Scorpius proper and so much a part of Libra that
in the nineteenth century B. A. Gould (who in 1851 founded the
"Astronomical Journal" (one of the world's premier research
publications) gave it to Libra as Sigma Librae, which is how it is
referred to today. To add to the confusion, early in that century
Elijah Burritt coupled the name "Zubenhakrabi" to Eta Librae! The
star is as interesting as the story of its name. Notably fainter
than Zubenelgenubi or Zubeneschamali, third magnitude (3.29)
Zubenhakrabi is a cool class M (M3) rather-luminous red giant.
>From a distance of 290 light years, it radiates 1900 solar
luminosities from a reddish 3600 Kelvin surface that is swollen to
a radius 110 times that of the Sun (0.52 astronomical units, which
would take the star about halfway between the orbits of Mercury and
Venus). It is a subtle "semi-regular variable" (in the astro-
trade, an "SRb") that changes its brightness by only 0.16
magnitudes over a 20-day period. This dying 2 (?) solar mass star,
with its dead carbon-oxygen core, is expanding and brightening as
a giant for the second time (the first brightening was with a dead
helium core) fueled by internal nuclear-burning shells of helium
and hydrogen. It is on its way to becoming a much larger,
brighter, Mira-type "long-period variable" that will eventually
slough off its outer envelope, its now-quiet carbon-oxygen core to
become yet another of the white dwarfs (a lower mass version of
Sirius-B) that flock around us. The star has been well-examined
for various quirks, but comes away clean, with no indication of a
surrounding dusty shell or any anomalies in surface chemical
composition, although both may well pop up in the distant future.
****************************************************************
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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