Skylights

[Jim Kaler]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, March 7, 2003.
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

As Spring approaches and the Sun climbs higher in the sky, we see
a rather busy celestial week.  The growing Moon spans first
quarter, crescent during the early part of the week, gibbous the
latter, the quarter reached the night of Monday the 10th, just
about the time of Moonset in North America.  With the Sun closing
in on the Vernal Equinox, the first quarter will be just shy of the
Summer Solstice, and will be nestled within the stars of Taurus,
the equally bright stars of Auriga to the north of it, Saturn just
to the east as the Moon prepares to pass to the north of it.  The
previous night, on Sunday the 9th, look for the crescent between
Taurus's two great clusters, the Hyades and the Pleiades.  Then
watch as the Moon heads towards great Jupiter, which it will pass
the night of Friday the 14th.  

While the Moon and the two giant planets dominate the evening sky,
Saturn already past the meridian to the south at sunset, Jupiter
crossing it around 9:30 PM, the morning sky is still taken up with
brilliant Venus.  Though the second planet from the Sun is getting
lower in the southeastern dawn sky, the planet rising just as
twilight begins, it is still very obvious.  On Wednesday the 12th,
Venus will make a very close conjunction with Neptune, the
brightest and closest of all planets passing a mere 12 minutes of
arc to the north of the (discounting much fainter Pluto) farthest
and dimmest.  Venus will then be 1.1 Astronomical Units (distance
between Earth and Sun) away, while Neptune will lie almost 30 times
farther.  While admiring Venus, look for reddish Mars well to the
west of Venus.  About as far south as it can get, Mars is moving
against the stars of Sagittarius, close to the Winter Solstice and
just to the northeast of Sagittarius's great Lagoon Nebula, the
sight easily visible in binoculars, as is Mars's night-to-night
motion.

The brightest star in the sky and, at a distance of only 8.6 light
years one of the closest, now passes the meridian to the south
around 8 PM.  Sirius is the luminary of Canis Major, the Larger
Dog, which looks for all the world like a hound standing on his
hind legs.  Down and to the left -- to the southeast -- lies one of
the great figures of the ancient sky, the sprawling constellation
Argo Navis, which memorializes the ship of Jason and the Argonauts. 
The figure is so large that it has been "broken" into three still-
large constellations, Vela the Sails, Puppis the Stern, and Carina
the Hull.  Puppis lies adjacent to Canis Major: look for its bright
stars due south and southwest of the Dog.  Carina, which contains
the second brightest star of the sky, Canopus, is below the horizon
for mid-northern latitudes, while Vela lies to the east rather
between the two.

STAR OF THE WEEK.  ASMIDISKE (Xi Puppis).  This star, easy to find
in Puppis (the Stern of Argo) just to the east of the bright
triangle that makes the lower portion of Canis Major, epitomizes
the confusion that attends to star names.  Given Greek letter Xi by
Bayer, its proper name Asmidiske was both mis-spelled and
improperly stolen from "Aspidiske" (from Greek, meaning "little
shield"), which belongs to Iota Carinae (of Carina, Argo's Hull). 
Asmidiske makes up for this minor insult by being a magnificent,
rather rare, kind of supergiant of sunlike color.  Originally
classed as a "warm G" (G3) lesser supergiant, it is now considered
a cooler G (G6, temperature 4990 Kelvin) brighter supergiant.  The
star is so far away that its distance is uncertain, but it lies in
the neighborhood of 1350 light years, making it 8300 times more
luminous than the Sun, from which we calculate a radius 120 times
solar, or 0.57 Astronomical Units (which would take the star
between the orbits of Mercury and Venus.  Asmidiske is rather
rapidly evolving, though it is not possible to tell just how.  It
may be a 10 solar mass star with a dead helium core that is growing
and cooling a its surface to become a red supergiant, or it may be
an 8 solar mass core-helium burner that has already been a red
supergiant and is "looping back" to higher temperatures.  Even
then, we cannot tell whether it is in a (surface) warming or
cooling phase.  Whatever its internal condition, it is just to the
cool side of being a Cepheid variable, a pulsating star like Delta
Cephei or Eta Aquilae.  It may already have been one, or it may (on
an astronomical time scale) shortly become one.  Given the star's
great luminosity, the pulsation period will be (or was) rather
long, around 20 days.  The star is reputed to be "super-metal-
rich," containing 60 percent more metals (relative to dominant
hydrogen) than the Sun.  There is also some evidence that Asmidiske
is accompanied by a close-in companion that lies about 2
Astronomical Units away and orbits with a period of a year. 
Farther out, about 5 seconds of arc away, is a thirteenth magnitude
star that is very much like the Sun and that orbits at least 2000
AU away and takes at least 26,000 years to make a full circuit
around the inner pair.  From Asmidiske proper, the sunlike
companion would be about as bright as a gibbous Moon, while from
the sunlike companion, Asmidiske would shine with the light of 1000
full moons, the close inner companion 3 minutes of arc away at
best.


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Jim Kaler
Professor 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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