Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 23, 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 new phase on Saturday, March 24th, and
for a couple days is invisible. The crescent will appear in
western evening twilight the night of Monday, the 26th (though
under ideal conditions the thin crescent will be visible after
sundown the night of Friday, the 25th). As the crescent waxes, it
will pass beneath Saturn the night of Wednesday the 28th and then
between Jupiter and Aldebaran the night of Thursday, the 29th, the
tilt of the lunar orbit taking the Moon somewhat to the south of
the two planets. Though Jupiter and Saturn are both moving well
into the evening western sky, both will be nicely visible through
April and into May as they make their way eastward against the
stars of Taurus.
Venus, however, says farewell. The planet will pass inferior
conjunction with the Sun, when it is between us and the Sun, on
Thursday, the 29th. "Between us and the Sun" is pushing the
definition a bit, however, as the tilt of the orbit causes the
planet to pass a full 8 degrees north of the Sun, and causes it
both to set after sunset and rise before sunrise. By early April,
Venus will be nicely visible in the northeastern dawn sky, and will
dominate the morning until near the end of the year.
At about the same time, on Tuesday the 27th, the asteroid Vesta
also passes conjunction with the Sun. Vesta's proper name is 4
Vesta, as it was the fourth asteroid discovered, by Heinrich Olbers
in 1807. The third largest asteroid, some 500 kilometers across
(15% the size of the Moon), Vesta is unusual in that it is the only
asteroid visible to the naked eye (though just barely).
Olbers is famed as the promulgator of "Olber's paradox," which can
be traced through Edmund Halley, perhaps even to Kepler. You can
make a profound cosmological observation any clear night. If the
Universe is static, unevolving, and infinite, any line of sight
should wind up at a star. Therefore the night sky should be at
least as bright as the surface of the Sun! Yet the night sky is
dark. The Universe can therefore not be static, unevolving, and
infinite all at the same time. The resolution of the paradox is
that the Universe is expanding and that it is evolving, that stars
do not live forever, but are born and die.
The new Moon of course gives us the chance to admire these stars.
By 8 PM or so, Orion is to the west of the meridian, and from mid-
northern latitudes, Gemini is close to overhead, Procyon in Canis
Minor nearly due south. Look a bit to the east of Gemini (between
Gemini and rising Leo) to see if you can spot the famed Beehive
cluster in dim Cancer.
STAR OF THE WEEK. NA'IR AL SAIF (Iota Orionis). Dangling from
Orion's Belt is the Hunter's mighty three-star sword, the trio
increasing in brightness toward the south. "Three-star sword" is
only a traditional appellation, however, as a view with any sort of
optical aid reveals nests of stars. The complex "middle star"
contains the famed Orion Nebula, which is lit by the tightly
compacted four-star "Trapezium" (Theta-1 Orionis), the quartet
dominated by the hot O (O6) star Theta-1 Orionis C (the Trapezium
at the top of a more-extended "Trapezium Cluster"). The bottom
"star" is dominated by the brightest of the set, bright third
magnitude (2.77) Na'ir al Saif, Arabic for "the Bright One in the
Sword," the name "Saif" (sword) wrongly transferred to Kappa
Orionis (Na'ir al Saif receiving the Iota designation from Bayer).
A class O (O9) giant, and one of the hottest and bluest stars that
make Orion's classical figure, Na'ir al Saif shines with a
temperature of 31,500 Kelvin from a quite-uncertain distance of
1300 light years, giving it a luminosity (corrected for ultraviolet
radiation and a bit of interstellar dust absorption) of 12,600
times that of the Sun. An O9 star should have luminosity at least
double that, suggesting that the true distance is closer to 2000
light years. The star is a complex multiple dominated by the 15-
solar-mass O star. At respective distances of 50 and 11 seconds of
arc lie an 11th magnitude class A or F dwarf and a 7th magnitude B
star, with true separations of at least 4400 and 20,000
Astronomical Units and orbital periods at least 75,000 and 700,000
years. The brilliant O star also has a far more interesting hot
class B1 spectroscopic companion that orbits in 29 days at a
separation of only half an AU (or so) in an unusually eccentric
path that takes it from 0.8 AU to a mere 0.11 AU. The collision of
their strong winds produces powerful X-rays. Na'ir al Saif and its
close companion help reveal the power of gravity and stellar
dynamics. Twenty-six degrees to the south lies fourth magnitude Mu
Columbae (in Columba, the Dove); 40 degrees to the north in Auriga
lies the variable star AE Aurigae. The two, with spectral classes
nearly identical to that of Na'ir al Saif, are hurtling away from
each other in opposite directions at 200 kilometers per second.
Called "runaway stars," they appear to have been shot out of Orion.
Recent calculations of the movements of the stars explain the two
runaways and Na'ir al Saif as well, showing that they were all
kicked out of the Trapezium Cluster. Some 2.5 million years ago,
the Trapezium region held a pair of tightly-knit double stars. In
a very close encounter between the two binaries, two of the four
stars were ejected, while the remaining two, instead of being
evicted, stayed more or less behind in highly eccentric embrace.
Na'ir al Saif still lingers near the Orion Nebula, marking the
place of the violent event. More violence is in store, as the star
is fated to explode 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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