[NEIU Physics Students] Skylights

[Gregory Anderson]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, August 4, 2006. 
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

Support science literacy by joining the Astronomical Society of the
Pacific, among the world's premier providers of astro education and
recognized by the Independent Charities of America as one of the
top charitable organizations in the US.  Get the outstanding
astronomy magazine Mercury and a variety of other benefits.  Call
1-415-337-1100, then press 1.

"Astronomy: Stars, Galaxies, and the Universe," an audio course on
CD with 100 page study guide narrated and written by Jim Kaler, is
now available from Recorded Books.

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

Skylights now resumes its normal weekly schedule.  Thanks for your
interest and patience.

The week begins with the Moon in its waxing gibbous phase as it
prepares to pass full the morning of Wednesday, August 9, around
the time of moonset in North America.  It then begins to diminish
in the waning gibbous.  The night of Friday the 4th, the bright
lunar disk will shine just to the east of Antares in Scorpius. 
Only an hour past full, the Moon passes south of Neptune, while at
the end of the week, the night of Thursday the 10th, it meets up
with Uranus.  Just over a day past full, the Moon passes perigee,
where and when it is closest to Earth, giving us slightly more than
average light for the phase.

Saturn passes conjunction with the Sun on Monday the 7th.  With
Mars practically invisible in bright western twilight, the evening
belongs to Jupiter, which shines gloriously out of Libra well to
the east of Spica, the bright planet now setting just before
midnight Daylight Time.  The morning hosts a similar, if not
better, show, with both Venus and Mercury gracing the dawn sky. 
Mercury reaches greatest western elongation from the Sun on Sunday
the 6th, giving it its best visibility.  If you have never seen it
(or even if you have), find it the morning of Wednesday the 9th
just below brilliant Venus, the two almost, but not quite, in
conjunction.  That same morning, Venus shines down and to the right
of (and on a line through) Gemini's Castor and Pollux.

August is "meteor month," as it hosts the best known of all
showers, the Perseids (which seem to emanate from the constellation
Perseus).  Though they will be at their best the morning of
Saturday the 13th, they take some time to build up, so some may be
seen toward the end of the week.  Unfortunately, the event will be
much dampened by a very bright gibbous Moon, which will wipe out
all the fainter ones.

Find Arcturus over to the west.  Then look east through the
graceful curve that makes Corona Borealis to one of the sky's great
heros, Hercules, which is at his best in early August evenings. 
Though his stars are not all that bright, they make a memorable
pattern that at the northern end includes the informal "Keystone,"
which provides a guide to that grand globular cluster, Messier 13.

STAR OF THE WEEK: 95 HER (95 Herculis).  Not just a double star, 95
Herculis (in far eastern Hercules and known best by its Flamsteed
Number) is a glorious double that allows a two-for-one special. 
It's also quite unusual in that, rather like Capella, it consists
of a pair of fifth magnitude (4.96 for 95 Herculis A, 5.18 for 95
Her B) giants separated by 6.3 seconds of arc that combine into
what appears as a single fourth magnitude (4.31) star at a healthy
distance of 470 light years.  Though both are evolving stars that
have given up core hydrogen fusion, they are nevertheless in
different stages of life, rendering them slightly different colors,
95 Her A being a relatively uncommon white class A (A5) giant, 95
Her B a class G (G8) yellow-white giant.  Their proximity and
subtle brightness difference, however, greatly magnifies the color
contrast to the eye, which resulted in vivid early descriptions,
Admiral Smythe calling them "apple-green and cherry-red" (one of
the reasons people love looking at double stars).  Some astronomers
even thought the colors changed with time.  They do of course, but
not in any human lifetime.  With a temperature (estimated from
spectral class) of 8000 Kelvin, the warmer and brighter star (95
Her A) shines with a total luminosity 167 times that of the Sun. 
While 95 Her B is somewhat dimmer to the eye, when its infrared
radiation (the result of its cooler 4900 Kelvin temperature) is
taken into account, it actually comes out as the more luminous star
(194 solar).  It is also the larger, 19.4 solar diameters as
opposed to 6.8 for 95 Her A.  As would be expected from its
spectral class and smaller size, 95 Her A is by far the faster
rotator, spinning at its equator with a speed of at least 233
kilometers per second, while 95 Her B ponderously spins at
(minimum) 5.7 km/s, giving them respective highly contrasting
rotation periods of less than 1.5 and 170 days (the latter quite
expected for a large class G giant).  Masses are similar, 2.8 solar
for 95 Her A and 3.2 solar for 95 Her B, as expected, since higher
mass stars evolve earlier and 95 Her B is the more advanced (the
pair about half a billion years old).  While 95 Her A has recently
given up hydrogen fusion and is in a transition state in which it
is rapidly (on an astronomical time scale) expanding and cooling,
95 Her B has either just completed its transition or is happily now
fusing its core helium into carbon and oxygen.  The metal content
of 95 B is a bit low, 60 percent that of the Sun, while that of 95
A is not known.  Together, they move about twice as fast relative
to the Sun than average.  While no orbital motion has been
detected, the separation between the two has stayed the same for
the past 180 or so years, showing that they are travelling through
space together.  The consistency of their computed evolutionary
states also shows them to be at the same distance and that they are
clearly a real binary pair.  With a separation of at least 900
Astronomical Units, they would (given their masses) take at least
11,000 years to make a full orbit around each other, so it is no
surprise that we have not seen relative orbital movement.  If you
can, find a telescope and admire them.  Do you see apples and
cherries too?  (Thanks to Jenny Hall, who suggested this star.)


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