[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 30 2007. 
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, 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
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.

NEW! The Cambridge Encyclopedia of Stars.

'Tis the week of the full Moon.  Preceded by a few days of the
waxing gibbous, full phase is reached during the day on Monday,
April 2, so it will rise just past full that night, and therefore
just after sundown.  The remainder of the week sees it waning in
the gibbous phase toward third quarter, which does not take place
until next week.  The traditional names of the April full Moon --
Grass Moon, Egg Moon, Planter's Moon -- reflect the warming of days
under the spring Sun.  As the Sun climbs higher to the north, the
full Moon drops lower to the south, this one taking place in Virgo. 
Watch for it the night of Monday the 2nd just to the west of Spica. 
Less than a day after full phase, the Moon passes apogee, where it
is farthest from the Earth.

While Venus climbs ever higher into each western evening sky,
Saturn shifts more and more in the opposite direction.  Not setting
until 10:30 Daylight Time, the second planet from the Sun quite
rules early-to-mid-evening.  Almost an hour before Venus sets, you
can find the ringed planet to the south between Leo and Cancer and
to the west of Regulus.  Jupiter now makes two transitions, first
by beginning to rise before local midnight (1 AM Daylight) as it
formally moves into the evening sky; second by ceasing its normal
easterly motion against the stars on Thursday the 5th and beginning
this year's retrograde (westerly) motion as the Earth prepares to
swing between it and the Sun.  Antares, to the southwest of the
giant planet, will provide a fine marker against which to see
Jupiter's increasing motion, much as Regulus now does for Saturn. 
A few days earlier, on Friday the 30th, Pluto, in extreme
northwestern Sagittarius and not that far from Jupiter (at least in
angle, but almost six times farther away), enters retrograde as
well.  To finish things off, Mercury (in bright morning twilight)
passes conjunction with Uranus on Sunday the 1st, while the rising
of Mars still tracks the cracking of dawn.

Three pairs of constellations come in major and minor forms.  The
best known are ancient Canis Major/Minor (Big and Small Dogs) and
Ursa Major/Minor (Big and Small Bears).  These are so prominent
that we little note Leo and Leo Minor, the Big and Small Lions, the
little one faintly riding the back of the big one.  Look for it
roughly between Leo the pairs of stars that make the feet of Ursa
Major.

STAR OF THE WEEK: BETA LMI (Beta Leonis Minoris).  Most
constellations follow the broad general rule that the stars are
given Greek letters more or less in order of brightness, though the
exceptions are legion.  In a surprising number of cases, "Alpha,"
for example, does not lead the pack.  Among the odder instances is
that of Leo Minor, the Smaller Lion.  It not only has no Alpha
star, but only one with a Greek letter assigned to it, Beta (and
precious few Flamsteed numbers), the probable result of both
faintness and shifting constellation boundaries.  And Beta LMi is
not even the luminary, that honor going to third magnitude 46
Leonis Minoris, which oddly carries a proper name, Praecipua.  At
least Beta LMi is consistent in being the constellation's second
brightest star.  Its glory lies not in its brilliance, but in its
duplicity.  It's a fine double, though one whose components are so
close (a few tenths of a second of arc) that they are inseparable
through the small telescope and require sophisticated
interferometry.  The fourth magnitude (4.40) primary star, Beta LMi
A, is a class G (G8) core-helium-fusing giant.  With a temperature
of 5075 Kelvin, it shines with the light of 36 Suns, leading to a
rather small radius for a giant of 7.8 solar and a modest mass of
around twice solar or perhaps a bit less.  The lesser sixth
magnitude (6.12) companion (Beta LMi B) is a class F (F8) hydrogen-
fusing dwarf with a temperature estimated at around 6200 Kelvin, a
luminosity just 5.8 solar, a radius double solar, and a mass only
35 percent greater than that of our own star (showing how sensitive
dwarf luminosity is to mass).   The pair orbit each other every
38.62 years at an average distance of 16.25 Astronomical Units, a
high eccentricity taking them as far apart as 27 AU and as close as
5.4 AU.  While planets are known among binary stars, these are
probably too close together to have allowed them (and indeed none
is known).  If there were one orbiting Beta B, Beta A would not be
very overwhelming, and would on average shine only 15 percent as
brightly as the Sun does in our sky and appear at only half our
Sun's angular size.  Like many other binary systems, the pair
beautifully illustrates stellar evolution in action, the more
massive of the pair (which began life as a mid-class-A dwarf) being
the first to evolve to gianthood.  The secondary will eventually
follow, quite likely leading to a double white dwarf.


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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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