Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Sunday, August 24 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 passes through its first quarter early in the week, on
Saturday, the 25th, thereafter waxing toward its full phase,
brightening as it goes through Scorpius and Sagittarius, bottoming
out at its most southerly position of the month the night of
Tuesday, the 28th. The night of Saturday, the 25th, the Moon will
pass 12 degrees north of Antares in Scorpius, while the following
night it will be approaching its passage to the north of Mars. The
night of Thursday the 30th, it is Neptune's turn to be visited.
Mars is now transiting the meridian to the south in bright
twilight; by darkness it has moved into the southwest, where, as
seen in the early evening, it will remain throughout the year. For
observers in the mid-northern hemisphere, setting time has moved to
just after midnight daylight time. But as Mars prepares to set,
Saturn rises in the northeast, followed around 2 AM by Jupiter,
which has now pulled rather far to the west of Venus, the brilliant
"morning star" rising before the onset of twilight until the end of
October (by which time Saturn and Jupiter will be rising in early-
to-mid evening).
Not that anyone will notice, but Pluto ceases its westerly
retrograde movement against the stars of southern Ophiuchus this
week, on Saturday the 25th. Three days later, Ceres, the largest
asteroid (570 miles -- 910 kilometers -- wide and also invisible
without a telescope), does the same thing. The orbits of asteroids
are commonly more highly tilted than are those of the planets
(Pluto excepted). Ceres, now beneath the Little Milk Dipper in
Sagittarius, is about as far south as it gets, some 8 degrees below
the ecliptic (the apparent path of the Sun).
As August heads towards September, the sky's fifth brightest star,
Vega in Lyra, passes nearly overhead in early evening for those in
mid-northern latitudes. A bit farther north (and a bit west) is
the much fainter head of Draco, the Dragon. A line drawn south
from Vega passes through the line of stars that makes the tail of
Serpens the Serpent (the only constellation that comes in two
parts, the head and tail divided by Ophiuchus), then much farther
down back to Sagittarius, which sits atop Corona Australis, the
Southern Crown. Once the Moon gets out of the way, you can admire
the great star clouds of the Milky Way that seem to blanket
Sagittarius, the celestial archer, one of two mythological centaurs
in the sky, the other Centaurus, a much larger constellation now
escaping to the west, its southern portions far below the horizon
for most people in the northern hemisphere.
STAR OF THE WEEK. MENKENT (Theta Centauri). Centaurus is
dominated by its two brightest stars, Rigil Kentaurus (Alpha
Centauri, third brightest star in the sky and the nearest star to
the Earth) and first magnitude Hadar (Beta Centauri). Though these
two shine brilliantly to the lucky residents of the southern
hemisphere, neither is visible from mid-northern latitudes, so if
those that live there wish to know Centaurus, they must begin with
the third brightest star in the constellation, Menkent, to which is
assigned the rather lowly letter Greek letter Theta (Gamma Centauri
rather oddly coming in second). The name comes from an Arabic word
for "shoulder" (of the Centaur), to which is attached the Latin
abbreviation for "Kentaurus" for Centaur, tying Menkent back to the
constellation's luminary, Rigil Kentaurus. Menkent, at mid-second
magnitude (2.06) is but four percent fainter to the eye than
Polaris. Much closer than Polaris, however, only 61 light years
away, it is intrinsically much less luminous. At the warm end of
class K (K0), the star is a near-clone of the northern hemisphere's
Pollux, just fainter to the eye than Pollux because it is 80
percent more distant. Menkent, with no known or even suspected
companions, is about as pure a sample of its class you can come by.
>From its 4780 Kelvin surface, this yellow-orange star radiates at
a luminosity 60 times that of the Sun, the star's radius 11 times
solar. Well along in its evolution, Menkent is now fusing helium
into carbon and oxygen in its deep core. The star's only offbeat
property is its rather high "proper motion," its speed across the
line of sight. Approaching us at only one kilometer per second, it
is speeding past us at 65 kilometers per second, about twice
"normal," suggesting that the star really belongs to the outer part
of the Galaxy's disk and is only visiting the solar neighborhood.
****************************************************************
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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