Jim Kaler
Fri, 31 Aug 2001 05:28:46 -0700
Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, August 31 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.
Precious little actually "happens" this week. The Moon brightens
and heads to full, the phase reached early, the night of Sunday,
September 2. Almost exactly one day before technical "fullness,"
the Moon also passes its apogee point at a distance of 406,330
kilometers (252,480 miles) from the Earth, the maximum distance
minimizing high ocean tides. As autumn comes on, the full Moons
fall farther to the north, this one near the Aquarius-Pisces
border. Traditionally, the September full Moon is called the
"Harvest Moon." The ecliptic, the apparent path of the Sun, is in
the evening near its flattest to the horizon, and as a result the
delay in moonrise from night to night is at a minimum, the effect
flooding the fields with bright moonlight after sundown. The
October full Moon, the "Hunter's Moon," actually fills the bill
better, as this year it occurs quite a bit closer to the beginning
of fall on September 22, when the Sun will cross the autumnal
equinox.
In spite of the week's quietness, the sky still keeps on turning,
stars and planets rising and setting daily and shifting to the west
an extra degree every day as the Earth goes around the Sun. In the
evening, bright Mars hangs to the south-southwest after dark, while
Saturn (much fainter, but still brighter than any star of the
northern hemisphere) begins to make its move into the evening sky,
now rising just before midnight daylight time. About an hour and
a half later, bright Jupiter (which has pulled well the west of
Venus) lofts itself over the northeastern horizon; Venus saves
itself for around 4 AM, rising just over an hour before morning
twilight commences.
Though bright moonlight floods the nightly scene early in the week,
you can still admire the glorious Summer Triangle, which consists
of Vega (in Lyra), now high in the sky for northerners as night
falls, Deneb (in Cygnus) to the east of Vega, and Altair (in
Aquila) to the south of the pair. When the Moon finally disappears
from early evening, if you live in a dark site you can see the
Milky Way flooding southward through the Triangle. As the Milky
Way leaves Aquila, it brightens through the modern constellation
Scutum (the Shield), and then even more so through Sagittarius, the
Archer, whose arrow points toward the center of the Galaxy. In the
other direction, the Milky Way flows from Cygnus into the coming
autumn constellations, through Cassiopeia and then into Perseus.
Already the flying horse of the Perseus myth, Pegasus and its Great
Square, are climbing the eastern sky in late evening. There are
only three weeks to go until the beginning of autumn in the
northern hemisphere, spring in the southern.
STAR OF THE WEEK. RUKBAT (Alpha Sagittarii). Alpha stars are,
according to logic, tradition, and expectation, supposed to be the
brightest in their constellations. Sure there are exceptions --
Alpha Orionis (Betelgeuse) is slightly fainter than Beta (Rigel) --
but they are minor and understandable. Rukbat, of Sagittarius, is
among the most dramatic of counter-examples, an Alpha star that
lies at mid-fourth magnitude (3.97) and is hard to see from any
lighted town. (Sagittarius's brightest star, second magnitude Kaus
Australis, Epsilon Sagittarii, is actually the brightest.) The
name, which from Arabic refers to "the Archer's Knee," clearly
indicates that the star's residence is Sagittarius, and that it is
not some interloper. (It even has a second name, "Alrami," that
means "the Archer.") Not only did Bayer assign it "Alpha," but in
his great star atlas of 1602 (the "Uranometria"), he draws it
vastly brighter than it really is (as he does also-dim Arkab, the
Beta star). No one knows why. Rukbat is very far south, indeed
not even visible north of 50 degrees north latitude, so Bayer may
have had difficulty in knowing its brightness. An alternative
speculation might be that Rukbat has simply faded over the past 500
years, but rather ordinary class B (B8) hydrogen-fusing dwarfs do
not do that. Rukbat, 170 light years away, radiates 112 solar
luminosities from its 12,370 Kelvin blue-white surface, the star
2.3 solar diameters across. Its temperature and luminosity give an
ambiguous status. Rukbat may indeed be a dwarf, one of 3.2 solar
masses; but it may also be near the end of its hydrogen-fusing
lifetime, and at 3 solar masses may be becoming a growing
"subgiant." Practically ignored in the scientific literature
(mentioned in but one publication per year), the star still has a
few things to recommend it. Rukbat's spectrum indicates that it
may have a companion -- a careful search for one, however, turned
up empty. The star is also a weak source of X-rays. More
important, Rukbat is a "Vega-like" star that is surrounded by a
cloud (probably in the form of a disk) of dust that is likely the
remnant of the star's formation and that for all we know has
spawned planets. If so, given Rukbat's proximity to the
termination of its hydrogen-fusing life, any that are close-in do
not have long to survive.
****************************************************************
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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