[NEIU Physics Students] Skylights

[Gregory Anderson]

Date: Fri, 11 Apr 2008 07:44:50 -0500

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 11, 2008. 
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.

Support science literacy by joining the Astronomical
Society of the Pacific, an international organization that is among
the world's premier providers of astro education.  Get on-line
"Mercury" and a variety of other benefits.  Call (415) 337-1100 or
go to http://www.astrosociety.org.  

Attend the 120th annual meeting of the Astronomical Society of the
Pacific May 31 to June 3, 2008 in St. Louis, MO, cosponsored with,
and part of, the Summer Meeting of the American Astronomical
Society.  For more information, go to the meeting site at
http://www.astrosociety.org/2008meeting. 

Enjoy "Science Educators Under the Stars: Amateur Astronomers
Engaged in Education and Public Outreach.  Available for $10 from
the Astronomical Society of the Pacific at
http://www.astrosociety.org/aspbook.html.  All proceeds benefit the
ASP's astronomy and space science education mission.

The Moon starts off the week nearly at first quarter, the actual
phase reached on Saturday, April 12, about the time of Moonrise in
North America.  It then waxes in the gibbous as it heads towards
full phase next week, on Sunday the 20th.  

Our companion than takes on a pair of planets and a star.  The
night of Friday the 11th, the Moon will make a fine pairing with
Mars, passing less than a degree to the north of the red planet. 
The Moon will actually occult Mars as seen from northeastern
Canada, Greenland, Iceland and other points far north.  Next up is
Regulus, which the Moon passes to the south of during the day on
Tuesday the 15th.  Four hours later the Moon then passes to the
south of Saturn.  The Moon will therefore be to the west of the
star and planet the night of Monday the 14th, then to the east of
them the following evening.  

Only half again as far from the Sun as Earth, Mars falls only
slowly behind us, which will keep the planet in the evening sky
until next autumn, the setting time (now around 2:30 AM Daylight
Time) not changing much from one night, or even week, to the next. 
As the sky darkens, find Mars high in the west in central Gemini to
the southwest of Castor and Pollux.  We now, however, see something
of a transition, as at mid-northern latitudes, Mars sets in the
northwest just as Jupiter rises in the southeast, the giant planet
then dominating the sky until bright dawn takes it away.  As
witnessed by the above lunar passages, Saturn remains securely in
Leo to the east of Regulus.  The ringed planet can be found high to
the south, transiting just after the end of evening twilight, and
not setting until nearly the commencement of dawn.  As a bit of a
coda to the planetary sky, Mercury passes the Sun in superior
conjunction (the other side of the Sun) on Wednesday the 16th as it
prepares for a rather nice appearance in mid-May.

Almost due south of Canis Major's Sirius, Canopus, the sky's second
brightest star, is visible only from the southern US and points
south.  It's the luminary of Carina, the keel of the Ship of the
Argonauts, which extends to within 20 degrees of the South
Celestial Pole and which contains one of the most massive stars of
the Galaxy, Eta Carinae.  Between Canopus and Sirius lie the stars
of Puppis, the Stern of the Ship, which wraps up and to the east
around the Great Dog.  If you are too far north to see Canopus, you
might imagine it gliding across the southern horizon.

STAR OF THE WEEK: CHI CAR (Chi Carinae).  Third magnitude (but not
by much, 3.47) Chi Carinae, which lies immediately to the east of
Canopus (the sky's second brightest star) and just to the southwest
of magnificent Gamma Velorum (meaning you have to be south of 38
degrees south latitude to see it), is just too obscure within its
fine setting to carry any sort of proper name.  But its history
lends itself to making one up: "Stella Rejecta," the rejected star,
not once, not twice, not thrice (a word you don't get to use very
often), but four times.  This nicely-blue class B (B3) subgiant
shines at us from a decently large distance of 387 light years
through relatively clear space, as it is dimmed by only about a
tenth of a magnitude by interstellar dust.  The temperature of
18,000 Kelvin, appropriate for a B3 star, is, however, still not
well-defined, other values up to 800 Kelvin hotter and cooler. 
After allowing for a lot of ultraviolet light, we then find that
Chi Car shines 2375 times more brightly than does the Sun, leading
to a radius of 5 times solar and a mass of 6.7 times that of the
Sun.  An ill-determined equatorial rotation speed of 78 kilometers
per second gives a short rotation period of under 3.2 days (as
opposed to the 25-day rotation period of the Sun).  So where did
our new name for it come from?  Why might Chi Car feel (if it could
feel) so rejected?  To begin, it is classed as a subgiant, one that
has given up core hydrogen fusion and is preparing to become a real
giant star.  Luminosity, temperature, and theory, though, toss it
out of the subgiant realm and instead show it to be a hydrogen-
fusing dwarf, though one entering the late stages of dwarfhood
(which will end when the star hits an age of 45 million years). 
That's not so bad, as many stars are actually not quite what they
are classed as based on their spectra alone.  It's the next three
that make the name.  First, Chi is classed as B3p, the "p" for
"peculiar," as it was thought to be silicon-rich (as a result of
elemental diffusion), even helium-poor.  Nope.  Detailed abundance
analysis shows the chemistry to be quite normal.  Second, Chi was
for a long time thought to be a Beta Cephei variable (one that
varies minutely over multiple short periods), Chi by 0.015 
magnitudes in 2.4 hours.  No again, not a variable at all. 
Finally, the star is listed as belonging to the vast Scorpius-
Centaurus association of hot class O and B stars (and which has
numerous subdivisions of stars born more or less at the same time). 
But it doesn't.  Instead, without even a binary companion to keep
it company, it's all alone.   
 

****************************************************************
Jim Kaler
Professor Emeritus of Astronomy   Phone: (217) 333-9382
University of Illinois            Fax: (217) 244-7638
Department of Astronomy           email: kaler at astro.uiuc.edu
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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