Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 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.
The Moon, in its perpetual journey around the Earth, passes through
its first quarter this week, when it is 90 degrees to the east of
the Sun, on Monday, April 30, thereafter waxing toward full. Two
days after the quarter, it passes perigee, its closest point to the
Earth (typically 363,000 kilometers, or 226,000 miles, 5.5 percent
closer than average). The night of Friday, the 27th, the crescent
will occult, or cover, two modestly bright stars of Gemini. Eta
Geminorum will be occulted for east and central North America
around 9:30 Central Time (add an hour for the east coast), while Mu
Geminorum (Tejat) will be occulted for westerners around 10:45
Pacific Time. The exact time depends on location.
As May begins the giant planets slip away to the west. On Sunday
April 29th, Saturn sets just as twilight ends around 9:30 PM
daylight time, though Jupiter lingers another hour. Mars, however,
is quickly rising ever-earlier, and is now up in the southwest in
Sagittarius by midnight. In the morning, Venus rises in the east
just minutes after twilight commences. On Friday May 4th, this
closest of all planets reaches greatest brilliancy for its current
morning appearance. Since Venus has just passed between us and the
Sun, we are still mostly seeing its nighttime side, and the planet
appears in the telescope as a small crescent.
Twilight is caused by sunlight that illuminates the Earth's
atmosphere after sunset or before sunrise. There are three
definitions. As the Sun sets, "civil twilight" ends when the Sun
reaches 6 degrees below the horizon and it becomes too dark for
ordinary outdoor activities. At 12 degrees we reach the end of
"nautical twilight," when the seagoing navigator can no longer see
the ocean horizon (against which to measure the altitudes of stars
to determine latitude and longitude). At 18 degrees, the end of
"astronomical twilight," the sky becomes as dark as it can get.
That is the time for the astronomer to get to work. The sequence
is repeated backwards in the morning. Professional observatories
run on "-18 degrees."
As twilight ends, the Big Dipper of Ursa Major is now high overhead
for those in the mid-northern hemisphere, the Little Dipper of Ursa
Minor pointing up to meet it. If far enough north, above 35
degrees north latitude, you can see Cassiopeia swing beneath the
pole opposite the Dipper. To the west of the Dippers and around
toward Cassiopeia is an immense area of sky so devoid of bright
stars that the ancients invented no constellations within it. Not
until after 1600 did astronomers need to organize the sky better
and fill in the blanks, giving us (among many others) the obscure
and large northern figures of Camelopardalis ("the Giraffe," which
lies between Auriga and the Pole) and Lynx (the obvious Lynx),
which sprawls in a long line from east of Auriga to nearly under
the Big Dipper.
STAR OF THE WEEK. BETA CAM. Camelopardalis, the Giraffe, a huge
"modern" northern constellation invented only about 400 years ago,
is so obscure that no star within it carries a proper name, and
only three carry Greek letter names. Only four of its stars are
even as bright as fourth magnitude, the brightest, Beta
Camelopardalis, in the middle of the range (4.03). Nobody points
out the dim figure at astronomy open houses. Yet the ghostly
Giraffe is not without its highlights, one of which is its
luminary, Beta. This great star looks faint only because of its
large distance of 1000 light years. In truth it is a class G (G0)
yellow-white supergiant (though of a somewhat fainter variety) that
shines with a luminosity 3300 times that of the Sun from a surface
just a bit cooler than solar (about 5500 Kelvin) that would stretch
three-tenths of the way from the Sun to the Earth. The star is far
enough away, and close enough to the Milky Way, that it is dimmed
some 15 percent by interstellar dust. With a mass around 7 times
that of the Sun, Beta Cam is only around 40 million years old.
Lying just over a minute of arc away is a companion that is itself
double, about which nothing is known except that the brighter is
class A, the dimmer probably F. Separated by at least 25,000
Astronomical Units, the small double takes at least a million years
to orbit the supergiant (which from the little double would shine
with the brightness of four full Moons). Beta Cam is also a double
mystery. It is most likely making the transition from being a
hydrogen fusing dwarf (of hot class B) to a larger helium-fusing
red giant. Whatever its status, it falls into a zone of
temperature and luminosity in which stars become unstable and
pulsate as Cepheid variable stars (like Mekbuda in Gemini, and even
Polaris). Beta Cam, however (like Draco's Rastaban and some
others), does not vary, though some multiple pulsations are present
with periods of tens of days. No one knows why the star is so
stable. But is it? During aircraft observations of meteors in
1967, Beta Cam was seen suddenly to flash, brightening by about a
magnitude over only a quarter of a second. A variety of "flashes"
have been seen from two dozen stars, including Enif (Epsilon
Pegasi) and Cursa (Beta Eridani). Beta Cam is an X-ray source,
suggesting some kind of solar-like magnetic behavior, and perhaps
the star popped something akin to a solar flare. So keep your eye
on the celestial Giraffe (if you can find it!).
****************************************************************
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)
*****************************************************************
--
This is the ISTA-talk mailing list.
To unsubscribe:
<mailto:[EMAIL PROTECTED]>
For more information:
<http://www.ista-il.org/ista-talk.asp>
To search the archives:
<http://www.mail-archive.com/ista-talk@lists.csi.cps.k12.il.us/>
