Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, May 25, 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.
Skylights appears a day early this week.
The Moon passes through its first quarter this week on Tuesday, May
29. The night of Friday, the 25th, the slim crescent will (for
those in the Americas) stand smack in the middle of Gemini with
Castor and Pollux right above it as it sets. Two days before the
quarter, the Moon passes its perigee point, where it is closest to
the Earth.
The dark markings on the lunar surface, the "maria," or "lunar
seas," are huge lava-filled impact basins -- just very large
craters. Three of these circular features are readily visible at
first quarter. From upper left to lower right, see if you can make
out Mare Serenitatis (the "Sea of Serenity"), Mare Tranquillitatis
(the "Sea of Tranquillity"), and Mare Foecunditatis (the "Sea of
Fertility"). Toward the right edge of the Moon perhaps you can
also make out smaller Mare Crisium (the "Sea of Crises"). Apollo
11, carrying the first lunar voyagers, set down on Mare
Tranquillitatis on July 20, 1969.
Saturn is truly gone from sight as it passes conjunction with the
Sun this week, on Friday, the 25th. You can forget Jupiter too, at
least for now. Both planets, however, will make nice display
pieces in the morning sky by mid-summer. By odd coincidence (such
abound in astronomy, as there are so many events going on), the
planet Uranus enters retrograde motion in eastern Capricornus at
almost exactly the moment that the Moon passes first quarter. At
bright sixth magnitude, Uranus is just visible to the naked eye in
a dark site with no moonlight present. Neptune (a telescopic
planet), to the west of Uranus, began its retrograde movement
earlier in the month. Mars is moving backwards too though, since
it has just reversed its direction, at a not-very-fast pace. That
will pick up as the red planet approaches its very bright
opposition to the Sun on June 13. By then it will have moved from
its current residence in Sagittarius into Ophiuchus, the only
constellation not of the classical zodiac through which runs the
ecliptic (the apparent path of the Sun).
If you are far enough south, this is the moment for the Southern
Cross, which crosses the meridian in early evening around the time
that northern observers see Corvus the Crow to the exact south. In
between is the tail of Hydra, the Water Serpent, and the bright
stars of western Centaurus. The top two stars of Corvus point
leftward to Spica, in Virgo. If decently south of 40 degrees north
latitude, look about 35 degrees due south of Spica to see the fuzzy
ball made by the greatest star cluster in the Galaxy, the
magnificent globular cluster Omega Centauri.
Corrections: In last week's Star of the Week, Alkes, note
that the star is converting helium into carbon and oxygen
(not helium into hydrogen as stated!) and that the proper
motion is given 1000 times too high. Thanks to those who
caught the errors.
STAR OF THE WEEK. MIMOSA (Beta Crucis). Few are the first
magnitude stars, as their apparent brightness requires either great
luminosity, closeness, or both. Mimosa satisfies the first
requirement. Tied for apparent brightness at number 19 in the sky
with Deneb (apparent magnitude 1.25), among the stars of first
magnitude it ranks number 10 in luminosity. The second brightest
star of Crux, the Southern Cross, Mimosa is too far south to have
a traditional proper name, one assigned by the ancients. The
origin of its name, which comes from Latin and means "actor" (the
word also used in botany), is unknown. Mimosa is a magnificent
blue-white, very hot class B (B0.5) giant star with a temperature
that soars to 27,600 Kelvin. Such heat causes the star to radiate
most of its light in the invisible ultraviolet. To the eye, Mimosa
would appear 3000 times brighter than the Sun; if all the radiation
is taken into account, the luminosity climbs over 11 times higher
to 34,000 solar. From these figures we calculate a radius of 8.1
times that of the Sun, in good agreement with the value of 8.4
solar found from the small angular radius. Mimosa is clearly a
close double, but one whose components are too near to each other
to resolve separately and that take almost exactly 5 years to
orbit. The nature of the second star is unknown, but the agreement
between the two radii above suggests that it does not add much to
the total luminosity. If it does not, Mimosa has a mass of 14
times that of the Sun, and the pair is separated by around 8
astronomical units. Mimosa is also a multiply-periodic "Beta
Cephei" star (named after Alfirk) that varies between magnitudes
1.23 and 1.31 with periods of 5.68, 3.87, and 2.91 hours. Though
the star is classed a giant, it seems more to be nearing the end of
its hydrogen-fusing stage. Though young (the Galaxy's metal
content increases with time), no more than 10 or so million years
old, Mimosa has an iron content only about half solar, a common
occurrence among local young stars. Most likely, the star will --
after running through its death cycle, in which it will develop an
iron core -- blow up as a supernova.
****************************************************************
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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