[no subject]

With its highly tilted orbit, Pluto (faint, requiring a decent
telescope to see) is well off the ecliptic and is now in the
morning sky in the constellation Serpens (the eastern part, Serpens
Cauda).  Averaging 40 Astronomical Units away, for a brief 20 years
of its 248-year orbit, it actually comes closer to us than Neptune
(30 AU away, the AU the average distance between Earth and Sun). 
Pluto is also gravitationally controlled by Neptune: averaged over
a long period of time, Pluto goes around the Sun twice for every
time Neptune goes around thrice.  Planet or not?  Pluto is really
the largest body of the Kuiper Belt of small bodies that lies
outside the orbit of Neptune (which supplies us with short-period
comets).  The stuff was too thinly spaced to assemble into a larger
planet like Neptune or Uranus.  There is no reason, however, why
Pluto cannot be both a Kuiper Belt object and a planet at the same
time.

Not of course to forget the others, very bright Jupiter (which now
rises around 9:30 PM) and Mars, which rises in the southeast around
4 AM, an hour after Jupiter crosses the meridian.

Admire Orion, now crossing the meridian to the south around 8 PM. 
It is the top of a stack of constellations to the south of it that
begins with Lepus (the Hare) and continues with the triangle that
makes Columba (the Dove).  Farther south and visible only from
southern climes is Pictor (the Easel).  Plunging farther down we
find Dorado (the Swordfish), Mensa (the Table), finally the sky's
south pole, which is surrounded by Octans (the Octant).

STAR OF THE WEEK.  BETA DOR (Beta Doradus).  Among the most
important of all stars are the Cepheid variables, named after the
prototype, Delta Cephei.  Many of them dot the naked-eye starry
sky, their number including Eta Aquilae, Zeta Geminorum (Mekbuda),
even Polaris, the brightest of them (though Polaris's variations
are too small to be witnessed by eye).  Here is another bright one,
fourth magnitude (averaging 3.76) Beta Doradus, the second
brightest star (after Alpha) in the modern southern constellation
Dorado, the Swordfish.  At its peak, Beta Dor actually reaches into
third magnitude, 3.46, and then falls to 4.08, oscillating between
the two over a slightly variable 9.942... day period.  Cepheids are
evolved supergiants, and this one is no exception, changing during
its variation cycle from class F6 to about G5, which shows that the
temperature changes as well (the average about 6000 Kelvin).  From
the distance of 1040 light years, the star shines with a mean
luminosity of 3000 Suns, which implies a radius of 50 times solar
and a mass 6.5 solar.  Direct measure of angular diameter through
interferometry, however, suggests a larger star, one closer to 65
times bigger than our Sun.  Cepheids pulsate, alternately growing
larger then smaller, hotter then cooler (which may explain part of
the difference), as a deep layer within them traps and releases
heat.  In an unstable state, they expand, shoot past equilibrium,
fall back under the force of gravity, then over-compress and expand
again.  The effect involves only the outer layers -- the core has
no idea of what is going on.  Though we know they long ago gave up
hydrogen fusion in their cores, they can be in any of a variety of
conditions.  Beta Dor may still have a contracting helium core, or
it could be fusing helium into hydrogen; it could even have given
up core helium fusion.  No matter which, the star is only about 60
million years old: higher mass stars age fast.  Cepheids follow a
strict "period-luminosity" relation.  The more luminous they are --
in total power output -- the longer the pulsation period (which
makes sense, since brighter stars are the larger, and they just
take longer to go through the cycle).  The pulsation period of any
random Cepheid thus gives its absolute brightness, while comparison
with apparent brightness gives the distance.  Find a Cepheid in
another galaxy, and you know that galaxy's distance.  They are the
key to finding the structure of the Universe and the nature of its
expansion.  As expected, Beta Dor fits the relation almost
perfectly.



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