[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 28, 2005.
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.

See "The StarGazer" at a planetarium near you: visit
     http://www.astro.uiuc.edu/~kaler/sg.html

Interested in Astronomy and Astro Education?  Join the Astronomical
Society of the Pacific (an international organization) to get the
outstanding astronomy magazine Mercury and a variety of other
benefits.  Call 1-415-337-1100, then press 1.

"Vault of the Heavens: Exploring the Solar System's Place in the
Universe," an accessible astronomy course on audio CD with 100 page
study guide narrated and written by Jim Kaler, is now available at
Barnes and Noble in stores and on line.

Having rolled through full last Tuesday, January 25, our Moon
begins the week deep into its waning gibbous phase and heading
toward its third quarter, which it will pass the night of Tuesday,
February 1, about the time it is seen to rise in North America.
The remainder of the week sees it as a waning crescent.  As it
descends through Virgo, Libra, and then Scorpius, the Moon will be
seen to pass just south of Jupiter the morning of Monday, January
31, then will position itself to the northwest of Antares the
morning of Thursday the 3rd and to the southeast of the star the
following morning, Friday the 4th.  Jupiter and Antares will
actually be occulted by the Moon, though not for residents of North
America (Jupiter in the south-central Pacific, Antares in parts of
Europe and Asia).

Speaking of Jupiter, it is the "big guy's" week, as the giant
planet, which dominates the Solar System (except, obviously, for
the Sun), begins its retrograde motion (to the west against the
stellar background) on Wednesday the 2nd as the Earth prepares to
pass between it and the Sun.  Jupiter's proximity to Spica in Virgo
(the planet just to the northwest of the star) will make the
movement easy to follow: note how the two will separate farther
from each other, the planet now rising about an hour before
midnight.  While admiring morning's Jupiter, look to the southeast
before dawn for Mars, which has moved easterly to a position
northwest of Sagittarius.  Neptune invisibly makes news too as it
passes conjunction with the Sun on Thursday the 3rd.  The early
evening still belongs to Saturn, which is well up in the northeast
in Gemini at nightfall, the planet crossing the meridian to the
south about as Jupiter rises in the southeast.

Finally, keep your binoculared eye out for Comet Machholz as it
moves north of the bright stars of Perseus.
The eye loves to pick out streams of things, and stars are no
exception.  Around the north celestial pole winds a long string of
stars that represent Draco, the Dragon.  In the Spring, the longest
constellation in the sky, Hydra, the Water Serpent, slithers its
way across the celestial equator toward the southeast.  In winter,
we have Eridanus, the River, which flows from the star Cursa (just
northwest of Rigel in Orion) to the west before turning and flowing
to the south, where it ends in brilliant Achernar.  Having few
bright stars, the constellation is often rather lost against the
brilliance of the other winter constellations, but is well worth a
look.  As do many stars, Cursa serves two constellations.  Though
really belonging to the River, it is also in myth "Orion's
footstool," upon which the Hunter rests his weary left foot.

Happy Birthday to the Star of the Week, now seven years old.

STAR OF THE WEEK: OMEGA ERI (Omega Eridiani).  One would think all
reasonably bright stars would be understood.  Not so.  Here's one
with an enduring mystery, one that stares back at us from a
prominent position just south of the celestial equator.  Little
attention is paid to Omega Eridani, the last-Greek-lettered star in
Eridanus, the River.  Just west of Cursa (Beta Eri), fourth
magnitude (4.39) Omega is not usually even included in the "dotted-
line outline" of the constellation.  Even its class is problematic.
The Yale "Bright Star Catalogue" persistently lists it as having a
composite class A-F giant-star spectrum (F4 and A6), as if two
giants were in orbit around each other, which given that giant
status is fairly fleeting, would be unusual.  More reasonably the
star is also classed as a single class A (A9) subgiant, implying
that it has ceased core hydrogen fusion and that it is on its way
to becoming an actual giant.  Rotating very quickly (at least 180
kilometers per second at the equator), the spectrum lines from
which the star is classified are broad and washed out, which is
perhaps responsible for the classification anomaly.  (The "Doppler
effect" causes shifts in the wavelengths of light as a result of
motion toward or away from the observer.)  Assume a single star.
Shining from a distance of 227 light years, with an estimated
temperature of 7500 Kelvin (consistent with the lack of interest in
the star, it has never been measured), Omega Eri has a respectable
luminosity 65 times that of the Sun, which gives it a radius 5
times solar and a rotation period of less than 1.4 days.
Omega then fits expectations of an A9 subgiant with a mass of 2.5
solar.  So far so good.  But Omega really IS a binary, or at least
so it seems.  Doppler shifts of the spectral absorptions tell of a
companion with a period of 3057 days (8.4 years).  Nothing is known
of it except that the mass should be at least 3.5 times that of the
Sun.  Yet the "companion" is not visible.  It is too massive for a
dead white dwarf as such stars are limited to under 1.4 solar
masses.  Decades ago, the star was a candidate for having an
orbiting neutron star or black hole!  Yet there is no other
evidence, including the expected X-ray emission that might come
from stellar interaction.  It could be that the washed out spectrum
is just fooling us too and that the star is really single.  And
since no one has made much attempt to find out (Omega Eri subject
to almost no current research), this modest star, one made easy to
find by its proximity to Orion's Rigel and Cursa, remains an
enigma.


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Jim Kaler
Professor Emeritus of Astronomy   Phone: (217) 333-9382
University of Illinois            Fax: (217) 244-7638
Department of Astronomy           email: kaler@xxxxxxxxxxxxxx
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)
      The StarGazer (a new planetarium show)
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