Skylights, University of Illinois Department of Astronomy.
Astronomy News for the 5-day period starting Sunday, August 19
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.
During this shortened period, the Moon waxes through its crescent
phase, first quarter reached next Saturday, August 25th. The night
of Wednesday, the 22nd, the Moon will make a nice pass to the north
of the star Spica in Virgo, which is now seen well to the southwest
as twilight draws to a close.
In spite of the growing Moon, the evening still belongs to Mars,
which shines brightly just to the west of south as the sky darkens.
Follow its progress as it ever-so-slowly (but with increasing
speed) pulls to the east of reddish Antares in Scorpius, the star
not very far to the right of the red planet. Three planets ride
the morning sky, the leader Venus. Moving to the east against the
stars of Gemini, the brilliant planet passes seven degrees to the
south of Pollux, Gemini's brightest star, on Wednesday, the 22nd.
To the west of Venus, find bright Jupiter, and rather well to the
west of Jupiter is Saturn, the two giant planets of course far
beyond Venus, Jupiter almost 5 times farther away, Saturn nearly
eight.
The night of Tuesday, the 21st, the Winter Solstice in Sagittarius
will be directly south at 9 PM Daylight Time. The Solstice, the
most southerly point of the ecliptic, marks the position of the Sun
on the first day of northern winter. Just rising exactly in the
east at that time will be the Vernal Equinox, the point at which
the solar path crosses the equator.
Immediately to the left of the equinox, which lies 23.4 degrees
south of the celestial equator, is a small fuzzy spot visible to
the naked eye, a large cloud of interstellar gas and dust, the
Lagoon Nebula, or Messier 8. Binoculars will make the cloud
immediately jump from the background. Almost directly north, and
nearly overhead at mid-northern latitudes, lies one of the most
ancient constellations of the sky, Hercules, the celestial memorial
to the great hero, the figure originally known as the "Kneeler."
Toward the northwestern corner lies another fuzzy spot, the
greatest of the northern globular clusters, Messier 13, the "Great
Cluster in Hercules." The telescopic view of the cluster, which
contains somewhere around a million stars, is stunning. To the
west of Hercules is the semi-circle that makes Corona Borealis, the
Northern Crown; its southern hemisphere counterpart, Corona
Australis, the Southern Crown, lies south of Sagittarius. To the
east of Hercules find Lyra, the Harp, brilliantly marked by Vega,
the fifth brightest star in the sky.
STAR OF THE WEEK. ZETA HER (Zeta Herculis). With the exception of
the brightest stars, star names were handed out more by position
than brightness, as attested to by Zeta Herculis. At bright third
magnitude (2.81), just barely the second brightest star in the
constellation Hercules (right behind Kornephoros, Beta Herculis),
Zeta Herculis was ignored by the ancients. Even Bayer rather
ignored it by giving it the sixth letter in the Greek alphabet, the
Alpha designation going to faint-third-magnitude Rasalgethi clearly
because of its position in the Hero's head. In spite of the star's
lack of public prominence, it has a lot to recommend it. Zeta Her
is actually double, a modestly bright third magnitude star orbited
by a sixth (5.53) companion only a second of arc or so away. The
brighter star, Zeta Her A, is a class G (G2) subgiant with the same
temperature (5780 Kelvin) as the Sun (which is also a G2 star).
With a mass some 50 percent greater than the Sun, however, and
beginning its evolution toward gianthood (its core hydrogen fusion
likely shut down), Zeta Her A is 6 times more luminous than the Sun
with a radius 2.5 times as large. Nevertheless, the star gives a
good idea of what the Sun would look like from a great distance, in
Zeta Her's case 35 light years. The companion, a cooler class G
(G7) hydrogen-fusing dwarf with a luminosity only 65 percent that
of the Sun and a mass about 85 percent solar, orbits with a period
of 34.5 years at a mean distance of 15 Astronomical Units (over 50
percent farther than Saturn is from the Sun). A rather high
eccentricity takes the two as far apart as 21 AU and as close as 8
AU. Under such conditions, planets would very likely be
impossible. Astronomers have identified a number of extended
"moving groups" of stars that seem to have some common origin (the
most famed the stars that are related to the Ursa Major cluster).
The "Zeta Herculis moving group," of which the star is the leader,
contains stars as far removed as Perseus, Lupus, and Octans, the
dim constellation that surrounds the South Celestial Pole.
****************************************************************
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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