[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 26, 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
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"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.

We begin the week with the Moon just past its first quarter and in
the waxing gibbous state, which grows until full Moon is reached on
the night of Thursday, February 1, around midnight in North
America, allowing you to see it full-on at its highest point in its
daily arc across the sky as it anticipates Groundhog Day on Friday,
February 2.  On the night of full phase, Thursday the 1st, the Moon
will lie just to the west of Saturn, the Moon, Saturn, and Leo's
Regulus all in a fine row.  Watch them as they rise in early
evening.

The western evening twilight sky now holds both the inferior
planets (so called because they are closer to the Sun than Earth,
in old fashioned terms being "below" the Earth).  Look for Venus in
the southwest after sundown.  Because of the planet's brilliance,
you won't have to look too hard.  Then before Venus gets too low,
look several degrees down and a bit to the right to find Mercury,
which is heading toward greatest eastern elongation on February 7. 
About half an hour before Venus sets (just after twilight ends
around 6:45 PM), Saturn rises almost due east followed by Regulus,
and is then with us all night long, crossing the meridian to the
south about 1 AM.  Two and a half hours later, around 3:30 AM,
Jupiter then joins the ringed planet in the sky.  Lording over the
southeastern sky, Jupiter does not finally fade away until bright
twilight takes over.  Look for Antares to the southwest of the
bright planet.  Mars, whose rising still tracks the onset of dawn,
remains tough to find.

The most northerly constellations of the Zodiac are now on fine
display.  Look for Taurus to cross the meridian high to the south
around 8 to 9 PM, with Gemini following an hour or two later.  The
Summer Solstice lies between the two constellations, classically in
Gemini, technically just over the artificial border (drawn up in
the 1920s) into Taurus.  Each has a first magnitude star to admire,
Taurus holding Aldebaran (seemingly surrounded by the more distant
Hyades cluster), Gemini holding Pollux.  Gemini also contains the
brightest second magnitude star (sextuple Castor), while Taurus has
Elnath, the Bull's northern horn and number five in the second-
magnitude category.  Elnath is properly the Beta star, while
Castor, dimmer than Pollux, was made Alpha Gem.  Elnath connects
Taurus with more northerly Auriga, and also carries the no-longer-
used name Gamma Aurigae.

STAR OF THE WEEK: NU GEM (Nu Geminorum).  At fourth magnitude
(4.15) bright enough sometimes to be considered part of the
"connect the dots" pattern of its constellation, Gemini, one would
think Nu Geminorum (of no proper name) would be rather well
understood.  It isn't.  It's a double, probably triple, maybe
multiple star that has yet to be fully sorted out.  Even the names
of its components are hard to get straight.  However many there
are.  At a healthy distance of 500 light years, Nu Gem is also a
"Be" star (class B6), one that exhibits emissions in its spectrum
that come from a surrounding disk (like Gamma Cassiopeiae, Zeta
Tauri, Delta Scorpii, and a host of others).  But we don't even
know which of the members of the system is the culprit.  The bright
member, seemingly a class B6 giant, is a very close double (with
the B6 giant dominant) with a likely (but uncertain) period of
53.72 days.  These two are orbited by a third member that takes
(best guess) about 13 years to make a circuit.  These are
collectively all called Nu Gem A.  Farther out, nearly two minutes
of arc away, is eighth magnitude class B8 Nu Gem B, which is too
faint for such a classification.  Either the star is just a line-
of-sight coincidence or the class is wrong.  Or both.  The inner
pair then has Nu Gem Ab orbiting the tight double Aa1 and Aa2. 
Different people, however, use different nomenclature, leading to
considerable confusion.  Nu Gem Aa and Ab are very difficult to
separate, being at most only one or two tenths of a second of arc
apart.  Best estimate is that Aa1 (the bright B6 star) shines with
the light of about 840 Suns (Aa2 is probably inconsequential),
while Ab puts out maybe 330.  These, with a temperature for Aa1 of
14,000 Kelvin, gives us respective masses for Aa1 and Ab of 4.5 and
4.0 solar (or as high, depending on the exact state of evolution,
of 4.8 and 4.2).  Bright Aa1 is then seen not to be a real giant,
but more likely a subgiant that has just recently given up its core
hydrogen fusion (or will shortly).  The 54-day orbital period gives
an average orbital separation between Aa1 and Aa2 of about half an
Astronomical Unit.  The orbit of Ab around Aa is much wider, around
10 AU, but with a very high eccentricity that takes the stars from
as far as 20 AU to as close as 1.5.  Nu Gem Aa1 spins with an
equatorial speed of at least 220 kilometers per second, typical for
the class, but low for a Be star.  The emissions, however, do not
follow the orbit of the Aa1-Aa2 pair.  Odds are that the emissions
are actually coming from Ab.  Nobody really knows.  If Nu Gem B is
really a member, it is at least 17,000 AU away from the inner trio
and takes at least 1500 years to orbit.  Four other listed members,
faint stars between one and two minutes of arc out oddly called Nu
Gem P, Q, R, and S (suggesting that a lot of others were once
considered), probably again just lie along line of sight.  Little
about Nu Gem is certain.  (Much of this description was taken from
a summary by Th. Rivinius, S. Stefl, and D. Baade in "Astronomy and
Astrophysics."  Thanks to Bill Hartkopf for discussion.)




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