Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, July 6, 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 falls "between quarters" this week, spending the entire
seven day period in its waning gibbous phase, full moon having
taken place last Thursday, July 5th, the third quarter not being
passed until next Friday, July 13. As it moves through the
constellations of the Zodiac, it passes south of Neptune on
Saturday the 7th, south of Uranus on Sunday the 8th (both planets
in Capricornus), and its apogee (where it is farthest from the
Earth) on Monday the 9th.
The planets put on quite a show for us this week. In the evening,
Mars, four degrees south of the ecliptic and about as southerly as
it can get, glows a brilliant red just to the east of the bright
reddish star Antares in Scorpius. The planet, brighter than the
brightest star and comparable to Jupiter, lies above the great
curve of stars that make the Scorpion's tail. Since Mars moves in
orbit only a bit slower than Earth, we will have it with us in the
evening for the rest of the year.
The big event, however, takes place the morning of Friday, the 13th
(a lucky day!) when Venus, Saturn, and the bright star Aldebaran of
Taurus all gather together into a tight equilateral triangle below
the Pleiades star cluster (Saturn down and to the left of Venus,
Aldebaran below, to the south of Saturn). Venus is now at its
earliest rising of the year, coming up just before 3 AM daylight
time, about an hour ahead of twilight. As the sky grows light, you
might then glimpse Jupiter and Mercury just above the east-
northeast horizon, Mercury (the fainter) about two degrees down and
to the right of Jupiter. The pair comes into conjunction the day
before, on Thursday the 12th. Three days before, on Monday the
9th, Mercury passes its greatest western elongation, the little
planet 21 degrees to the west of the Sun.
Even the asteroids get into the act, as Ceres, the largest of them
(though only 900 kilometers across and invisible to the naked eye)
passes opposition with the Sun on Saturday, the 7th. Only number
4 in discovery order, 500-kilometer wide Vesta (the third largest
of them), can be seen without optical aid, and then just barely.
Nearly 100,000 of the critters have been discovered. Vast numbers
of tiny ones small enough to hold in your hand hit the Earth every
day as meteorites.
Two crowns ride the nightly sky, one in the north, the other in the
south. As the sky darkens, look 30 or so degrees north of the
equator (nearly overhead for mid-northern latitudes) to find the
gentle curve that makes Corona Borealis, the Northern Crown. If
you are far enough south, around 1 AM you can watch the passage of
Corona Australis, another curve of stars that makes the Southern
Crown, which lies directly below the Little Milk Dipper of
Sagittarius.
STAR OF THE WEEK. BETA COM (Beta Comae Berenices). The naked-eye
sky is dominated by luminous stars, stars that are far brighter
than the Sun. Only a few stars like the Sun and fainter sneak
through. They simply do not have enough radiative power to be
visible unless they are quite close to us. ("Selection effects"
like this one, in which Nature shows us "what she wishes," pervade
science: the population of microbes far exceeds that of elephants,
yet only the elephants are visible without some kind of aid.) It
is then quite surprising to find the luminary of a constellation to
be a near-solar clone. Fourth magnitude (4.26) Beta Com, with no
proper name at all, just barely beats out Diadem (Alpha Comae
Berenices) for the honor (such as it is) of being the brightest
star within the faint but glorious constellation Coma Berenices
(Berenices Hair). Though within the formal constellation
boundaries, Beta Com is not a part of the star cluster that makes
the constellation's heart, its distance of only 30 light years
placing it 1/9 as far as the cluster and half as far as Diadem. At
6000 Kelvin, this class G (G0, alternatively classed as F9.5) star
is only slightly warmer than our class G2 (5780 Kelvin) Sun. A
hydrogen-fusing dwarf like the Sun, Beta Com is only 37 percent
more luminous than is the Sun and but 10 percent larger, the result
of 10 percent greater mass. There is some suggestion that the
star might have a close companion (detectable only via
spectrograph), though such a neighbor is unconfirmed and probably
unlikely. How sunlike is Beta Com? It is a bit metal-rich,
containing perhaps 7 percent more iron (relative to dominant
hydrogen) than the Sun. No planets have yet been spectroscopically
detected as they have for several similar stars. A search for a
residual dusty disk (one left over from planet formation) around
the star has also turned up nothing. Yet the spectrum reveals very
sunlike magnetic activity, implying starspots, flares, and all the
other trappings of the solar magnetic engine. Indeed, with a
rotation period only half that of the Sun, Beta Com is probably
more active than the Sun (rotation and up and down convection in
the outer stellar layers producing the magnetism). Like the Sun,
Beta Com also has a long-term magnetic activity cycle of 16.6
years, about 50 percent longer than the famed 11-year solar cycle.
(It may also have a secondary 9.6 year cycle.) If you want to see
something of what we would look like at a distance, look to Coma
Berenice's brightest star.
****************************************************************
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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