Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 9, 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 start the week with the full Moon, with the full illuminated
hemisphere of the Moon visible to us on Earth, which reveals the
great extent of the dark lunar maria. They are huge lava-filled
impact basins that make the face of the "man in the Moon," and are
three to four billion years old. They reveal the awesome violence
of the early Solar System, of the time when the planets were
sweeping up the debris of the smaller bodies of which they were
made. The Moon then spends the rest of the week waning through its
gibbous phase toward third quarter, which will be reached next
Friday the 16th.
The morning of Thursday, 15th, the Moon will appear just to the
west of Mars, the morning of Friday the 16th to the east of it.
The red planet is now passing far (about 10 degrees) to the south
of much more distant Pluto. While Mars invades Scorpius (and is
now to the northeast of its namesake Antares), Pluto, because of
its highly tilted orbit, lies among the stars of southern
Ophiuchus. Mars now rises shortly before 1 AM. Staying up until
dawn may afford a view of Mercury, which reaches greatest western
elongation, a whopping (for Mercury) 28 degrees from the Sun.
However, the northern hemisphere morning ecliptic this time of year
lies relatively flat against the horizon, so Mercury will still be
difficult to glimpse in eastern morning twilight. The view is far
better from the southern hemisphere. The little planet will pass
only a tenth of a degree north of Uranus the morning of Saturday
the 10th.
The planetary view in the evening is much better. While Venus now
descends the evening sky, it is still glorious in western twilight.
Farther east in Taurus, Jupiter and Saturn linger in the sky
through nearly the entire evening, the pair finally setting shortly
before midnight.
Taurus, with its marvelous Pleiades -- Seven Sisters -- star
cluster is the southern figure of a triangle of bright
constellations. To the northwest is Perseus, whose central region
is also made of a star cluster. To the northeast is Auriga, the
Charioteer, to which Taurus is actually connected through its
northern "horn" (one star part of both constellations) and which
contains the sky's sixth brightest (and most northerly first
magnitude) star Capella. All three are set into the Milky Way,
that fainter part of the Milky circle that is formed by the outer
part of the Galaxy. Perseus is filled with numerous bright hot
high mass stars from episodes of recent star formation, while
Taurus and Auriga are filled with thick dark clouds of dust in
which stars are now vigorously being born.
STAR OF THE WEEK. PLEIONE (28 Tauri). Mother to Taurus's
Pleiades, the famed Seven Sisters, at mid-fifth magnitude (5.09)
Pleione (Flamsteed number 28) ranks seventh among the named stars
of the cluster, just marginally behind the Pleiades' mythical
father, the god Atlas. All nine are hot class B stars, their blue-
white similarity giving the cluster so much of its sparkle.
Together with Sterope, Pleione is the coolest (class B8) of them,
its temperature 12,000 Kelvin. It and Sterope are also the only
dwarfs, stars that are fully fusing hydrogen in their cores and
have not yet begun to evolve (the others either subgiants like
Merope or giants like Alcyone). From its distance of 385 light
years, Pleione shines with a luminosity 190 times that of the Sun,
its radius 3.2 solar, its mass (from the luminosity and
temperature) 3.4 solar. Pleione's glory lies in its spectrum, its
array of colors. Along with Gamma Cassiopeia, Pleione is one of
the classic "Be" stars of the sky. The "e" stands for "emission,"
and refers to emissions of hydrogen that appear at specific
wavelengths or colors (particularly one in the red part of the
spectrum, hydrogen-alpha). A Be star's emissions come from a
surrounding ring of gas that is somehow (though no one is quite
sure) related to the star's great rotation speed, Pleione spinning
at least as fast as 329 kilometers per second at the equator, 165
times faster than the Sun, giving it a rotation period of under
half a day. The emissions of Be stars are split by the Doppler
effect as a result of one part of the ring rotating toward us, the
other receding. In the extreme "shell star" case, the ring also
produces absorptions from both hydrogen and from a variety of
elements caused by the ring directly blocking starlight. The
difference in Be star styles was once thought to be a matter of
orientation, but Pleione puts the lie to the theory by switching
among all three phases, normal B star, Be star, Be shell star, the
changes taking place at intervals of 17 and 34 years. These
changes are related to brightness variations (which gave Pleione
the variable star name BU Tauri). As the star enters the shell
phase it fades by several tenths of a magnitude, the latest episode
occurring in 1970. The switches may be related to the effect of a
binary companion (about which nothing is known) that orbits
eccentrically with a 35 year period and averages 28 astronomical
units from Pleione proper: but no one really knows.
****************************************************************
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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