Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, June 15, 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 disappears from the sky toward the end of the week, as it
passes its new phase on Thursday, the 21st. As the lunar crescent
wanes during the early morning hours, it will make a nice
configuration with brilliant Venus the morning of Monday, the 18th.
The following morning, Tuesday the 19th, the slimming crescent will
be found beneath the Pleiades star cluster in Taurus and up and to
the right of Saturn, which is now just clearing the glare of the
Sun. Little Mercury, however, is not so lucky (or perhaps it is we
who are not), as this smallest of the inner planets passes inferior
conjunction with the Sun on Saturday, June 16.
The big date takes place 5 days later. First, the Sun will cross
the Summer Solstice in Gemini at 2:38 AM Central Daylight Time
(1:38 AM EST, 12:38 PST), marking the first day of astronomical
summer in the northern hemisphere, astronomical winter in the
southern. At that moment, the northern end of the Earth's axis
will be tipped in the direction of the Sun, the Sun will shine
overhead at the Tropic of Cancer, will be circumpolar (not setting)
at the Arctic Circle, and will not rise at the Antarctic Circle
(technically anyway: atmospheric refraction and the extended
diameter of the Sun will still make it visible). The Sun will then
be as far north as it can get, 23.4 degrees from the equator. For
the next 6 months, solar movement will be southerly.
On the same date, Thursday the 21st, the new Moon will exactly
cover the Sun to produce a total solar eclipse that will be visible
along a path through the South Atlantic Ocean and across southern
Africa. None of it will be visible in North America, though
eastern South America will see a bit of a partial eclipse. The
geometry of eclipses requires at least two solar eclipses a year.
The June event is the only total eclipse. One other, on December
14, is annular (that is, the Moon will be too far away to
completely cover the Sun), and will be visible principally through
the Pacific Ocean.
None of the news of the Sun can eclipse the current glory of Mars,
however. Moving retrograde between Sagittarius and Scorpius, the
planet is nicely up in the southeast at the end of twilight. Just
look for the brightest thing you can see! Though Mars passed
through opposition with the Sun on June 13, its eccentric orbit
causes it to get slightly closer to us until -- again -- Thursday,
the 21st, when the red planet will be 67,344,000 kilometers
(41,846,000 million miles) from us, and at its best for viewing.
Even a small telescope can show polar caps and dark markings. To
the right of Mars is bright Antares in Scorpius. If you are not
too far north, the figure of the celestial scorpion, curved tail,
stinger, and all, is quite obvious.
STAR OF THE WEEK. GIRTAB (Theta Scorpii). Girtab, Bayer's Theta
star within Scorpius, the celestial scorpion, stands out in almost
any way you can look at it. The name alone is unusual, in that it
comes to us from Sumerian rather than Arabic or Greek, and means
simply "the scorpion." Moreover it carries an additional name,
"Sargas," also Sumerian, whose meaning is unknown. We'll stick
here with Girtab, though Sargas is commonly used as well. Though
the most southerly extent of the Sun is in neighboring Sagittarius,
Scorpius is the most southerly of the zodiacal constellations. And
bright second magnitude (1.87) Girtab is the most southerly BRIGHT
star in the Scorpion, closely anchoring the southern curve of the
scorpion's tail. At almost exactly 40 degrees below the celestial
equator (and beat out only by much dimmer third magnitude Eta
Scorpii, and then by only 1/6 degree), Girtab is invisible north of
50 degrees north latitude. The star's southerly position has
allowed northern observers to use its visibility as a test of the
night-sky brightness near the horizon. Scorpius is filled with
bright blue-white stars of class B. As a yellow-white class F (F1)
bright giant, Girtab is again an exception. From its distance of
272 light years, the star radiates 960 times more energy than the
Sun from a surface with a temperature of 7200 Kelvin, its radius 20
times solar, making it a true giant indeed. Though its equatorial
rotation speed is high (over 50 times that of the Sun), the large
size still gives it a fairly long rotation period of 10 days (or
less). Girtab is unusual too in our knowledge of its evolutionary
status. There is no question that the star, which weighs in with
a mass 3.7 times that of the Sun, is rapidly evolving with a dead
helium core toward lower temperatures. One hundred million years
ago, it WAS a blue class B star, one that would have fit right in
with its current scorpian neighbors. As the star swells and the
surface cools, it should in under a million years become a Cepheid
variable like Mekbuda, and then become a red giant 5 times brighter
than it is now, at which point the helium in its core will begin to
fuse to carbon and oxygen, setting the stage for it to become a
massive -- and distinctly single -- white dwarf.
****************************************************************
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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