Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, September 7, 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, waning early in the week through its gibbous phase,
passes its third quarter shortly after it sets during the day on
Monday the 10th. It will thereafter wane as a crescent as it loops
through the most northerly constellation of the Zodiac, Gemini.
Just before the Moon reaches the quarter, it will occult, or pass
over, Saturn, now in Taurus. Unfortunately, the event takes place
in daylight for a good fraction of North America. On the west
coast, however, the sky will still be dark enough however, the
occultation beginning in California around 5 AM Daylight Time
(specific times need a city-by-city timetable). Hawaiians will get
a grand view around 1 AM. Two days later, on Wednesday the 12th,
the Moon will do the same to Jupiter. The Americas lose out even
more, however, this event visible best in Europe. At least on the
night of Sunday the 9th we can see the Moon approach Saturn, then
be on the other side the night of the 10th, and then watch the same
act the nights of the 11th and 12th as the Moon approaches, then
recedes from, Jupiter.
The brightness of the Moon (which shines only by reflected
sunlight) at different phases depends on the degree to which
mountains (really crater walls) throw shadows, and on the local
reflectivity of the rock. The full Moon is 8 or so times brighter
than the first quarter. Although the same area appears illuminated
at both first and third quarters, the first is twice as bright as
the third, principally because of the extent of a huge dark lava
plain called Oceanus Procellarum (the "Ocean of Storms"), the
largest dark area visible to the naked eye. (The allusion to
oceans and seas on the Moon comes from a time when people thought
they might really exist -- the dark areas are lava-filled impact
basins, not ocean basins. The Moon has no significant water).
The two planets that flank the Earth remain firmly set as opposing
brilliant jewels that grace the early evening and morning. Look
for bright reddish Mars to the just to the west of south just after
dusk, the planet setting around midnight, and then for even-
brighter white Venus, which rises around 4 AM, well before dawn.
If you are in a dark location, Venus may lead your eye to one of
the more subtle sights of the Solar System, the "zodiacal light,"
which is caused by the scattering of sunlight from dust particles
that lie in the plane of the solar system (and thus through the
constellations of the Zodiac). The best time to see the phenomenon
in the northern hemisphere is in autumn mornings and spring
evenings. Before the beginning or morning twilight, the zodiacal
light (sometimes called the "false dawn") is now nicely visible as
a faint cone of light standing upward from the eastern horizon.
STAR OF THE WEEK. ALPHA SCT (Alpha Scuti). Eighty-eight formal
constellations grace the sky (along with many more informal ones
plus figures that have long-since fallen into disuse). Over half
come down from ancient times. The rest are of "modern" origin,
that is, created between about 1600 and 1800 as astronomers
furiously competed with "filling in the blanks" between the ancient
constellations. Lying in the Milky Way between ancient Aquila and
Sagittarius is the modern constellation Scutum, the Shield, which
honors the Polish king John Sobieski for his defense of Vienna in
1683. Though the stars are faint, the northern part of the Shield
stands out mightily as a bright patch of the Milky Way. None of
its stars carry proper names, even the brightest of them, which is
known simply as Alpha Scuti, abbreviated as an unpronounceable
"Alpha Sct." Alpha Scuti, which shines at only fourth magnitude
(3.85), is yet one more orange class K giant, though one with a bit
of a difference, at class K3 slightly cooler than most, its
temperature measured at 4280 Kelvin. From its distance of 175
light years it radiates 134 solar luminosities from a surface
swollen to 21 times that of the Sun, about a quarter the size of
the orbit of Mercury. Decidedly single, Alpha Scuti is reported to
be slightly variable, its brightness varying by about 10 percent.
No one seems to know the period of oscillation, however (if indeed
it has one), or what class of variable it might be. The most
interesting aspect of the star is its ambiguous evolutionary
status. As solar-type stars age after they cease hydrogen fusion
in their cores, they (1) brighten as they become giants with dead
helium cores; (2) shrink a bit as they begin to fuse helium to
carbon; (3) fuse carbon; (4) brighten even more with dead carbon
cores; (5) lose their outer envelopes and become dim white dwarfs
(like Sirius B and Procyon B). Alpha Scuti, a 1.7 solar mass star
that is at least 2 billion years old, could be in state 1, 2 or 4,
which somewhat overlap. That the star is variable suggests that it
might indeed be in state 4, and beginning to brighten for the last
time.
****************************************************************
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 CPS Science Teacher List.
To unsubscribe, send a message to
<[EMAIL PROTECTED]>
For more information:
<http://home.sprintmail.com/~mikelach/subscribe.html>.
To search the archives:
<http://www.mail-archive.com/science%40lists.csi.cps.k12.il.us/>
