Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, August 10 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 next Skylights will appear Sunday, August 19. As the week
begins, the Moon is nearly in its third quarter, the phase passed
the night of Saturday, the 11th. During the remainder of the week
it will wane in the crescent phase until Saturday the 18th, when it
will pass new. Only three hours after new, the Moon passes
perigee, the coincidence causing near-maximum ocean tides. The
last glimpse of the crescent will be in twilight the morning of
Friday, the 17th. <br><br>
As the crescent wanes, it will appear to the west of Saturn (still
in Taurus to the east of the Hyades) the morning of Monday the
13th, and to the east of the ringed planet the following morning.
The morning of Wednesday the 15th, the Moon will appear up and to
the right of Jupiter (the planet in western Gemini). During the
day, around 3 PM Central Time, the Moon will actually occult (pass
over) Jupiter, the event visible through a telescope. (Saturn's
occultation on the 13th is visible only in Africa and India). The
most striking pairing will occur the morning of Thursday the 16th,
when Moon will appear nearly to brush brilliant Venus, the event
well worth getting up at dawn to see. While less dramatic, the
evening still holds bright Mars, the planet still just a bit to the
east of Antares in Scorpius. Finally, distant Uranus, now in
retrograde in eastern Capricornus, passes opposition to the Sun on
Wednesday, the 15th. <br><br>
The week, however, as always this time of year belongs to the famed
Perseid meteor shower, which will be at maximum the morning of
Sunday the 12th. Meteors are caused by small rocks from space that
heat and streak through the Earth's upper atmosphere. The Perseid
shower typically sends 50 to 100 meteors per hour (the show best
seen well after midnight). Unfortunately, the third quarter Moon
will light the sky this year, taking out the fainter meteors.
Nevertheless, the brighter ones will shine through. The meteors
are the stony debris from Comet Swift-Tuttle, which on its 130 year
orbit last passed by the Earth in 1992. The direction of the
meteoroid stream coupled with the direction of the Earth makes the
meteors seem to come from the constellation Perseus. The best
place to look, however, is straight up.<br><br>
When observed at the same time of night, the stars slip slowly past
us by a degree each successive day as the Earth orbits the Sun. To
the south at 9 PM Daylight Time find the tail of Scorpius, to the
east the figure that makes Sagittarius. Above Sagittarius is a
bright patch of Milky Way coupled to modern constellation Scutum,
the Shield. Below Sagittarius are a few stars that make the modern
constellation Telescopium, the Telescope, whose stars appear
overhead to those near 50 degrees south latitude. To the far
north, the head of Draco, the Dragon, is high in the sky, and near-
overhead for those near latitude 50 degrees north. <br><br>
STAR OF THE WEEK. ALPHA TEL (Alpha Telescopii). The modern
constellations, those that survive, honor two cognate scientific
instruments, Microscopium, the Microscope and Telescopium, the
Telescope, the first south of Capricornus, the latter south of
neighboring Sagittarius. Neither constellation is very bright, the
luminary of Telescopium barely fourth magnitude (3.51) Alpha
Telescopii, which like most of the stars in modern constellations
has no formal proper name. Though the brightest star of its
constellation, it is nearly as obscure as the constellation itself,
mentioned in only about one scientific paper a year. A class B
(B3) blue subgiant (more about that below) almost exactly 250 light
years away, this quite-luminous star shines with the light of
almost 900 Suns from a surface heated to 18,400 Kelvin. The
luminosity and temperature tell that the star has a mass six times
that of the Sun. In opposition to the subgiant spectral class,
which suggests that the star is beginning to evolve from being a
core-hydrogen-fusing dwarf, the luminosity and temperature strongly
suggest that the Alpha Tel is fairly young and has a long way to go
before any kind of evolution sets in. The star seems decidedly
single. Its most significant characteristic, other than being a
rather hot class B star, is that of chemical peculiarity. There
are several classes of such stars in which various chemical
elements are enriched or depleted as a result of diffusion of atoms
(some settling below the star's gaseous surface, others lofted
upward by radiation, the effect sometimes coupled with magnetic
fields). The most prominent are the "Am" metallic line stars
(Sirius, for example), the "Ap" (for "class A peculiar," like Cor
Caroli and Alioth) stars that have strong magnetic fields, and the
"mercury-manganese stars" (Alpheratz), which obviously have
enhancements of these elements. Alpha Tel is a member of the rare
class of "helium-rich stars" whose oddness is also diffusion-
related. (Many highly evolved stars, like the Wolf-Rayet companion
in Regor, are much more helium-enriched as a result of the loss of
their hydrogen-rich envelopes; Alpha Tel is not one of these.)
Otherwise its composition seems more-or-less normal. For a class
B star it is rotating slowly, only 35 kilometers per second
(suggesting that its pole is more-or-less pointing at us). Some 7
million years from now, it really will lose its outer hydrogen
layers and become a massive white dwarf like Sirius-B.
****************************************************************
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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