Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, July 20, 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 week begins with the Moon in its new phase, from which it will
begin to wax through crescent toward its first quarter, that phase
to be reached on Friday, the 27th. Enjoy the sight as the
partially-lit lunar disk climbs nightly from above the evening
horizon, the crescent outlined to the east with earthlight. You
might make the first sighting the night of Saturday, the 21st. By
the following night, the crescent will become easily visible.
Almost exactly a day after new Moon, the Moon passes its apogee
point, where it is closest the to Earth.
Mars, moving ever-so-slowly eastward against the stellar background
stands almost exactly on the formal border between Scorpius (to the
west) and Ophiuchus (to the east). Almost directly east of first
magnitude Antares in Scorpius and almost due south of second
magnitude Sabik in Ophiuchus, and quickly picking up angular speed,
the red planet will cross into Sagittarius on September 1. Very
near its southerly limit of 27 degrees south of the equator, Mars
is currently invisible from the Arctic, indeed north of 63 degrees
north latitude. Venus, on the other hand, shining in brilliant
splendor in morning skies to the east of the Hyades star cluster in
Taurus, is quite far north, and is not now visible over most of
Antarctica. Saturn, having just passed conjunction with Venus, now
stands slightly to the west of her and even closer to the Hyades.
Look down and to the left of Venus to find Jupiter, which has just
crossed over into Gemini and rises about as morning twilight
begins. Jupiter, nearly opposite Mars, is about as far north as it
will become, 23 degrees north of the equator.
Mars in the evening; Venus, Saturn, and Jupiter in the morning; at
midnight a full view of the constellations of northern summer.
Vega, in Lyra, shines nearly overhead from mid-northern latitudes.
With Deneb, a bit to the east, and Altair to the south, it forms
the Summer Triangle. Low to the south, below the celestial
equator, is the "Teapot" of Sagittarius. Having crossed the
meridian to the south about 10 PM daylight time, at midnight Mars
stands in the southwest rather than in the southeast.
Overflowing constellation boundaries, the Milky Way cascades
through Cygnus southward into Sagittarius and then into the
glorious deep southern hemisphere not visible from northern climes.
Consisting of the combined light of billions of faint stars, the
Milky Way is the disk of our flattened Galaxy. Some 26,000 light
years away, in the heart of the black blotches seen against the
Milky Way's background, lies the center of the Galaxy, thought to
be made of a two and a half million solar mass black hole with
gravity so fierce even light cannot escape.
STAR OF THE WEEK. SABIK (Eta Ophiuchi). At the dim end of second
magnitude (2.43), and (after Rasalhague, Alpha Ophiuchi) the second
brightest star in Ophiuchus, Sabik still received the lowly Eta
appellation from Bayer. The anomaly is the result of Bayer's
distributing the Greek letters not so much in order of brightness
in the constellation but in order of position: Alpha, Beta, and
Gamma lie at the northern end of the sprawling figure, while Delta
through Eta form the Serpent Bearer's lower "skirt," Sabik the
southern-most. The star's Arabic name, something of a mystery,
refers to one that "precedes" or "comes in first," and may perhaps
have to do with Sabik's position at the end of the stream of stars
at the bottom of the constellation (though if it were a race, it
would come in last as Ophiuchus moves toward the west). The star
is a close and rather unusual double that is very difficult for the
amateur to resolve. Two third magnitude class A stars (A2 at
magnitude 3.0 and A3 at magnitude 3.5) swing around each other in
mutual orbit every 88 years. The angular orbital size is only 1.3
seconds of arc, and the two are usually much closer than that. At
the star's distance of 84 light years, 1.3 seconds corresponds to
33.5 Astronomical Units, a bit farther than Neptune is from the
Sun. The most unusual aspect of the system is the very high
orbital eccentricity of 0.94, which means that the stars come as
close as 2 AU (0.5 AU farther than Mars is from the Sun) and then
only 44 years later are 65 AU (over half again farther than Pluto
is from the Sun) apart, the separation varying by a factor of 32.
The gravitational disturbances caused by such an orbit would make
planets impossible (and indeed there is no evidence for
circumstellar dust). Otherwise, the stars are rather ordinary.
The brighter has a temperature of around 8900 Kelvin, a luminosity
35 times that of the Sun, and a radius 2.5 times solar; the fainter
is 300 Kelvin cooler, and is 21 and 2.0 times brighter and bigger
than the Sun. The masses of the stars can be derived from their
luminosities, temperatures, and theory (which give 2.3 and 2.0
solar masses for the brighter and fainter respectively) or from the
gravitational solution of the orbit, which gives a sum of the
masses of 4.8 solar, 12 percent higher, the result of natural
errors in the observed orbit and in distance. There is some
evidence that one or both have enhanced metals, common among slowly
rotating A stars (only about 30 km per second), the result of
chemical diffusion in the stellar atmospheres.
****************************************************************
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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