Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 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.
We begin the week with the new Moon passing beneath a Sun now
moving through Aquarius on its way to Pisces and spring. By the
evening of Saturday the 24th, the Moon will be visible as a slim
crescent in evening western twilight, and will thence grow toward
first quarter, that phase to be reached next Friday, March 2.
The evening of Monday, the 26th, the Moon will appear well to the
south -- to the left -- of brilliant Venus. The passing angle
between Venus and the Moon is unusually large, 11 degrees. The
orbits of all the bodies of the Solar System are somewhat tilted
relative to Earth's, the Moon's by 5.2 degrees, Venus's by 3.4
degrees. Though the planets and Moon are all found close to the
ecliptic -- the apparent path of the Sun -- they therefore still
wander noticeably away from it. During this week, the Moon is well
south of the ecliptic, while Venus is on the north side, the effect
exaggerated by its proximity to Earth as it prepares to head toward
conjunction with the Sun. The champion wanderer, of course, is
highly-tilted dim Pluto, which now lies 10 degrees north of the
ecliptic in the constellation Ophiuchus.
The night of Thursday, March 1, the Moon will be positioned between
and below Saturn and Jupiter in Taurus, passing beneath Jupiter
after moonset in North America. Both planets are now moving in
direct motion to the east and pulling ever-farther apart. Saturn
is noticeably on the south side of the ecliptic, while Jupiter lies
very close to it.
The winter stars are now in full force, Orion and its companions to
the south in mid-evening. Included is the Winter Triangle of
Betelgeuse (up and to the left of Orion's three-star belt), Sirius
(the brightest star of the sky, down and to the left of
Betelgeuse), and Procyon (to the left of Betelgeuse). Oddly,
Procyon and Sirius, the luminaries of Orion's two hunting dogs,
both have faint tiny "white dwarf" companions, dead stars that have
shrunk themselves under gravity to the size of the Earth.
If you have had enough of northern winter, then look later in the
evening to the east and northeast to see the harbingers of Spring,
Leo and the Big Dipper in Ursa Major, climbing the sky, the Dipper
standing on its handle. As the Earth swings a degree per day
around the Sun, the stars seem to creep a degree per night toward
the west. Before long, for those in North America, the Dipper will
ride overhead on a warm May night, while people in the southern
hemisphere will be admiring a high view of the Southern Cross,
which is readily visible only below about 25 degrees north
latitude.
STAR OF THE WEEK. TEJAT (Mu Geminorum). The southwestern corner
of Gemini turns gently in a three-star tail that directs the eye to
the Summer Solstice, the position where the Sun will be on the
first day of northern summer. At the southeastern corner of the
long rectangle that makes the constellation and at the east end of
the stellar trio lies Tejat (soft "j"), an Arabic plural for a term
of unknown meaning and one that has been applied collectively to
all three of the stars. Though fourth brightest in the
constellation (after Pollux, Castor, and Alhena, Bayer
unaccountably assigned Tejat the Greek letter Mu. It is seemingly
closely paired with the middle star of the trio, Propus (Nu
Geminorum), which is also referred to as Tejat Prior, rendering Mu
Geminorum Tejat Posterior. The name has now been transferred
exclusively to Mu. Bright third magnitude, Tejat is noticeably
reddish in color, reflecting its cool 3650 Kelvin degree surface
and class M (M3) status. Lying 230 light years away (less than
half the distance of Propus), Tejat is a red giant that radiates
1540 times more energy than the Sun (after correction for a large
amount of infrared radiation). While not all that much compared
with some of the stars of the naked-eye sky, the low temperature
leads to a star of great proportions, one large and close enough
for an accurate measure of its angular size, 0.0135 seconds of arc.
>From its distance, Tejat therefore has a radius 104 times that of
the Sun, or 0.48 Astronomical Units, about half the size of the
Earth's orbit, which agrees nicely with that found from luminosity
and temperature. A ninth magnitude "companion" two minutes of arc
away (itself a double star) is most likely a line of sight
coincidence. Like many large giants and supergiants, Tejat is
slightly variable. Classed as a "giant irregular," its apparent
brightness erratically wanders around by about 25 percent, between
magnitudes 2.8 and 3.0. The star's greatest importance to
observers seems to be in calibration of the stellar temperature
scale.. Of greater interest is the star's evolutionary status.
Tejat's temperature and luminosity tell of a three solar mass star
(one that began life as a hot mid-class B dwarf) that has not only
given up hydrogen fusion, but also helium fusion, and now -- with
a dead carbon core -- is making a run to a much higher luminosity,
at which point it will vary like Mira and will eventually shuck its
outer envelope to become a massive white dwarf like Sirius B.
****************************************************************
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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