Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 20, 2000
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 passed through its third quarter early this morning, and
tonight, Friday the 20th, will rise shortly before midnight
daylight time just past that phase to the east of the bright stars
of Gemini. From here it wanes through crescent to new, that phase
reached the morning of Friday the 27th. The morning of Tuesday the
24th sees the Moon to the northeast of Mars, which is now rising in
eastern Leo (CK) a bit after 4 AM daylight time.
Working our way more into evening, Jupiter, which is now rising
behind Saturn just after 8 PM daylight time, passes five degrees
directly north of Aldebaran around midnight (in North America) the
night of Friday the 20th. The proximity of the bright stars of
Taurus to the two giant planets makes their westerly retrograde
motion easy to follow (in addition to making them a glorious sight
as they all pass nearly overhead in the early morning sky).
Further toward evening, Venus makes a pass at another reddish star,
this one Antares in Scorpius, the planet 3 degrees north of the
star the night of Thursday the 26th. Bright twilight will make the
passage (though hardly Venus) difficult to see. With the westerly
of the four large outer planets (Neptune) having made the
transition to easterly motion, it is now number two's turn, as
Uranus stops retrograde and turns around toward the east on
Thursday the 26th.
The Orionid meteor shower, caused by the debris of Halley's Comet
hitting the Earth's atmosphere, peaks on Saturday, the 21st, the
shower best in the early morning hours, the meteors seeming to come
out of the constellation Orion. The near-third-quarter Moon will
wash out the fainter meteors of the shower. However, the Orionids,
which typically produce about 20 meteors per minute, will last for
a few more days while the Moon dims, letting us hang on to them for
a time.
In mid-evening, around 10 PM, the southern sky is near its best in
displaying its "wet quarter," reminiscent of an ancient rainy
season (during which the Sun was probably passing through the
constellations, rendering them invisible). Aquarius, the water
bearer, with its 4-star "Water Jar" (or "Urn"), is right on the
meridian, while Capricornus is to the right and the huge sprawl of
Pisces is to the left southeast of the Great Square of Pegasus.
Directly below Aquarius is the lonely first magnitude star
Fomalhaut in Piscis Austrinus, the southern fish, into whose mouth
Aquarius is sometimes depicted as pouring his water. Above the set
and north of the celestial equator and Capricornus is another "wet"
constellation, Delphinus, the Dolphin, easily noted by its small
tight irregular box of stars.
STAR OF THE WEEK. SADACHBIA (Gamma Aquarii). Aquarius (the
Waterman, or Water Bearer) mostly sprawls gently south of the
celestial equator. It is focused upon the 4-star Water Jar (or
Urn), a somewhat dim but very noticeable triangle that has a star
smack in the middle, the Urn appearing like a "Y" or even as a
small spray of diamonds that lies directly on the equator. Though
at bright fourth magnitude (3.84), not quite the brightest star of
the Urn, and ranking sixth in the constellation (after Sadalmelik,
Sadalsuud, Delta, 88, and Zeta Aquarii, the latter also in the
Urn), Sadachbia is the only one of the Urn's stars to be named.
The name harkens not to the Water Bearer, but to an older Arabic
constellation, and seems to mean "the Lucky Star of the Tents," the
Urn perhaps reminding the Arabs of a desert tent. The "Sa" prefix
on the name relates obscurely to "luck" in the same way it does in
Sadalmelik and Sadalsuud. Sadachbia is for the most part an
ordinary hot class A (A0) star much like Vega and so many others
that make our constellations. Lying 158 light years away, it
radiates 62 times more light than the Sun from a 9500 Kelvin sphere
three times the solar diameter. Theory and long observation of
similar stars in double systems show it to be a hydrogen fusing
main-sequence star with a mass just short of three times solar.
Aside from its prominent location, Sadachbia has two qualities to
recommend it. First, like so many of its stellar kin, it is a
"spectroscopic double," one whose character can be determined only
by examining the spectrum. A star of unknown type orbits Sadachbia
every 58 days from a distance at least 0.40 astronomical units
away, about the distance Mercury is from the Sun. Sadachbia also
has a small 12th magnitude companion 37 seconds of arc away from
it, but it is merely a line of sight coincidence. Of more
significance, Sadachbia, like Kaus Australis (Epsilon Sagittarii),
is a failed "Lambda Bootis" star. Lambda Bootis stars are
otherwise normal stars near class A that have unusual deficiencies
in heavy elements. No one really knows why. One theory is that
during their extreme youth, these rare stars accreted interstellar
matter that was itself highly deficient in heavy elements as a
result of the condensation of such elements onto dust grains (which
is an observational fact). Sadachbia was once considered such a
star, but later investigations showed instead that the chemical
composition was more or less normal. While thus not an actual
example of a Lambda Boo star, Sadachbia still allows the
illustration of the myriad odd stellar beasts that populate the
sky.
****************************************************************
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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