Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 2, 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 first quarter on Friday the 2nd, and will
spend the week waxing toward full, that phase reached next Friday
March 9, a day after it passes perigee, when it is closest to the
Earth for this orbital round. For those in the Americas, first
quarter will take place around the time twilight darkens the sky,
the Moon just to the east of Jupiter. In the third century BC, the
great Greek astronomer Aristarchus of Samos tried to estimate the
distance to the Sun by observing the angle between the Moon and Sun
at the time of its quarters. Only if the Sun is infinitely far
away will the angle be exactly 90 degrees. His value of solar
distance fell short by a factor of 20 (the Sun so far away that
naked-eye measure is not possible), but his idea that the Sun was
vastly farther than the Moon was correct, and vividly reveals the
knowledge and intellectual vitality of the times.
The Moon is positioned this week between the group of bright
evening planets -- Venus (brilliant to the west), Saturn, and
Jupiter (the latter two high to the south in Taurus) -- and lonely
Mars, which does not rise until nearly 1 AM. Venus makes special
news by beginning its retrograde, or westerly, motion against the
stars on Thursday the 7th. Since it passed greatest elongation
from the Sun last January 16, the Sun has been catching up with it
even though both have been moving east. The reversal in Venus's
direction means that the Sun will catch it very quickly now. Each
evening the planet will be lower in the sky, and it will disappear
by the end of the month. But do not despair, as it will as quickly
pop up in the morning sky. The planet has been exhibiting an
Arctic curiosity. The Sun is still south of the celestial equator,
whereas Venus is now well above it. From somewhat above the Arctic
Circle, Venus sets after the Sun, as it does at lower latitudes;
but it also rises shortly before the Sun, making it both a morning
and evening "star."
Mars, by itself, is not left out of the picture, however. On
Saturday the 4th, the red planet passes 4 degrees north of its
reddish namesake Antares in Scorpius ("Antares" meaning "like
Ares," "Ares" the Greek version of the god of war). The two have
similar colors, are both in the Zodiac, and while Mars is now about
half a magnitude brighter than the star (and brightening as well),
they still look quite similar and are easily mistaken for each
other.
Scorpius, beautifully placed within the Milky Way, rises roughly as
Orion and his pair of hunting dogs (Canis Major and Canis Minor)
set, and gives those in the northern hemisphere hope that summer is
not all that far off.
STAR OF THE WEEK. MULIPHEIN (Gamma Canis Majoris). The names
Wezen and Hadar (in Arabic form) were once applied to a pair of
stars. Though there are candidates, no one knows which pair. The
uncertainty was in older times expressed as an Arabic word that in
part referred to a pair of things that caused contention. The word
itself was then taken as the pair, much mangled to Muliphein
(sometimes seen as Muliphen), and then for no good reason given to
the little star that Bayer later tagged as Gamma of Canis Major
(while Wezen was given to our modern Delta, and Hadar to modern
Beta Centauri). Such is the logic of star names. The mid-fourth
magnitude (4.12) star itself, 400 light years away, is a class B
(B8) blue-white bright giant, similar in color to so many stars in
the Canis Major-Orion district. With a temperature of 13,600
Kelvin, it radiates 685 times more energy than the Sun and is 5
solar diameters across. If the star were as close as its
constellation-mate Sirius, it would shine in our sky almost as
brightly as Venus at her best. Muliphein's luminosity and
temperature tell of a 4.3 solar mass star that has recently ended
its hydrogen-fusing life and is now starting to evolve into a red
giant. Rotating slowly for a class B star (only 15 times the solar
rotation speed), it, like so many class B and A stars (including
Sirius), is chemically peculiar. Muliphein in particular is a
"mercury-manganese star" rather like Alpheratz in Andromeda.
Muliphein is 40 percent richer in iron and chromium (compared with
hydrogen) than is the Sun, but its mercury level is more than 2000
times solar as a result of physical lofting of certain elements by
radiation forces. The most curious thing about the star, however,
is its Gamma designation, as Muliphein is far from being the
constellation's third brightest, being beat out by several others,
including Omega! Bayer probably proceeded from top to bottom in
the constellation, as evidenced by his calling more-southerly
Adhara, the second brightest star, Epsilon. This sort of mis-
representation, however, has led some to think that Muliphein has
faded since Bayer's time (around 1600). Indeed there is a
reference that in 1670 the star disappeared and was not seen again
until 1693. Such statements have to be taken with suspicion,
however; moreover, there is no known mechanism that could cause
such an event. A dust cloud, for example, would also redden the
star, and its color is quite normal. Then again, the stars always
seem to hold surprises when we least expect them.
****************************************************************
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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