[NEIU Physics Students] [Fwd: Skylights]

[Gregory Anderson]

-------- Forwarded Message --------
From: Jim Kaler <kaler at ASTRO.UIUC.EDU>
Reply-To: Jim Kaler <kaler at ASTRO.UIUC.EDU>
To: SCIENCE at LISTS.CSI.CPS.K12.IL.US
Subject: Skylights
Date: Fri, 11 Jul 2008 07:49:41 -0500

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, July 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.  

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 brightens this week from waxing gibbous through full, that
phase reached as our week comes to an end the morning of Friday,
July 18.  Look the evening of Thursday the 17th to see the not-
quite-there-yet-full-Moon rising just barely before sunset.  The
waxing gibbous passes apogee, where it is farthest from the Earth,
the night of Sunday the 13th.  With the Sun now descending the
ecliptic within the confines of Gemini, this full Moon (the
"Thunder Moon," the "Hay Moon") will be just a bit higher in the
sky at midnight than it was last month.

As the Moon waxes, it passes barely south of Antares in Scorpius
the morning of Monday the 14th.  When the two rise the evening of
Tuesday the 13th, the Moon will therefore be just to the west of
the star, while the night of Monday the 14th, the situation will be
reversed.  Almost exactly three days after the Antares encounter,
the brightened Moon takes on much brighter Jupiter, passing three
degrees to the south of the planet the morning of Thursday the
17th, less than a full day before the full phase.  As the Moon
climbs the sky the night of Wednesday the 16th, it will appear to
the west of the planet, while again the night of Thursday the 17th,
it will have flipped to the east of Jupiter.    

Mars -- visible in evening twilight -- begins the week just to the
south of somewhat brighter Saturn, both lying in Leo to the east of
Regulus.  Moving rapidly, Mars will quickly shift to the east of
the other two, much more distant Saturn (4.5 times farther away
than Mars) moving easterly at a much slower pace.  By the end of
the week, the red planet will be nearly four degrees to the east of
the ringed one.  

As Mars and Saturn descend the twilight sky, both gone from view by
10:30 PM or so (Daylight), Jupiter, which now rises before sunset,
makes its glorious appearance in the far southeast still to the
northeast of classical Sagittarius.  While the bright Moon blocks
most of the constellation from view, Jupiter shines on.  And
whatever happened to Venus?  Having passed superior conjunction
with the Sun early last June, the planet is slowly working its way
up the western sky, but is still too close to the Sun and horizon
to be seen.  Give it another couple months.

Constellations differ greatly in size.  To the south of (and more
or less parallel to) the Big Dipper's handle is a pair of
neighborly stars that make most of Canes Venatici, the Hunting
Dogs, while north of Scorpius lies Serpens, whose two parts bracket
giant Ophiuchus and take close to three hours just to rise and set.

STAR OF THE WEEK: KAPPA SER (Kappa Serpentis).  Serpens, the
Serpent, wraps around Ophiuchus, the Serpent Bearer.  Divided in
two, the Serpent's head (Serpens Caput) lies to the west of the
Bearer, while the tail (Serpens Cauda) falls the east, the Greek
letters also divided twixt the two parts.  In the middle of the
snake's rather squared-off head lies a fourth magnitude (4.09)
reddish "eye," the class M (M0.5) red giant Kappa Serpentis. 
Temperature is a problem.  Angular diameter measures combined with
a distance of 348 light years give a physical diameter of 71 times
that of the Sun and a calculated temperature of 3575 Kelvin,
notably lower than expected for the class.  From apparent
brightness, distance, and this temperature (needed to assess the
large amount of invisible infrared radiation), we find a luminosity
of 1390 times that of the Sun and a much larger radius of 98 times
solar.  Using a more typical temperature of 3800 Kelvin for an M0.5
giant gives a lower luminosity of 820 solar and a more satisfactory
radius of 66 times solar.  With the lower temperature, the mass
comes in at 2.5 times that of the Sun, while it is reduced to about
double solar with the higher value.  In either case, Kappa
Serpentis stands out as a red giant at a peak luminosity at which
it is about to fire up its core helium to fuse to carbon and oxygen
(if it has not done so already), after which it will shrink and dim
by some fifty percent while the helium merrily burns along.  Such
stars can be unstable, but there is no firm evidence for
variability.  One study suggests a very uncertain five percent
variation over a 30 day period.  Aside from its important lesson in
stellar evolution, Kappa Ser (which has no known companions) is
mostly used as a rather staid calibrator for those measuring
angular diameters and magnetic activity.

 


****************************************************************
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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