[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 4, 2005.
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

Interested in Astronomy and Astro Education?  Join the Astronomical
Society of the Pacific (an international organization) to get the
outstanding astronomy magazine Mercury and a variety of other
benefits.  Call 1-415-337-1100, then press 1.

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

A belated Happy Groundhog Day (February 2) to everyone.  An
astronomical "holiday," Groundhog Day is one of the four "cross-
quarter days" that split the differences between the passages of
the Sun over the equinoxes and the solstices.  We are halfway to
spring (in the northern hemisphere).

We start the week with the waning crescent Moon in the morning sky,
as it goes toward its new phase, with the Moon between us and the
Sun, on Tuesday, February 8.  By the night of Wednesday the 9th, if
you are lucky you might get to see the very thin waxing crescent in
the southwestern evening twilight sky.  By the night of Thursday
the 10th, you will not have to look as such, as the Moon -- in
twilight -- will be pretty obvious.  Then, and over the following
few nights, it will present a beautiful sight, as its nighttime
side will be aglow with Earthlight, with light reflected from (from
the point of view of the Moon) the nearly-full Earth.  Almost
exactly one day before the Moon passes new, on Monday the 7th, it
runs through perigee, which will make for nicely high (and
especially low) tides at the coasts (not an issue in the Midwest).

The morning of Friday the 4th the waning crescent was just to the
east of Antares in Scorpius.  The following morning, that of
Saturday the 5th, the even thinner Moon will pass four degrees
south of Mars, which is now to the northwest of Sagittarius.  Mars,
however, pales next to Jupiter, which dominates the morning sky,
the planet now in retrograde and up and to the right of Spica in
Virgo.  Ever so slowly the giant planet is moving into the evening
sky.  Now rising around 10:30 PM, it is rather well to the west of
the meridian as dawn lights the morning.  Jupiter and its brother
planet Saturn -- the two of similar size and construction -- are
working in a sort of synchrony.  Saturn now rises well before
sunset and is nicely up in the northeast as twilight darkens the
sky, and for the next month or so, it crosses the meridian just as
Jupiter rises.

Watch Auriga, the ancient Charioteer, cross the sky nearly overhead
in mid-northern latitudes.  The constellation, halfway around the
pole between the Big Dipper and Cassiopeia, is one of the most
prominent of northern constellations, its luminary Capella the most
northerly of first magnitude stars.  Plunge southward and you pass
through Taurus (with bright Aldebaran), Orion (with Betelgeuse and
Rigel), Lepus (the Hare), and Columba (the Dove).  Auriga's
positional counterparts in the deep southern hemisphere, far to the
south of Orion, are (roughly) Pictor, the Easel, and Dorado, the
Swordfish, modern constellations that are much too far south to be
seen from ancient northern lands.  Dorado contains one of the most
remarkable objects of the sky, the Large Magellanic Cloud, a nearby
satellite galaxy 150,000 light years away.

STAR OF THE WEEK: ALPHA DOR (Alpha Doradus).  At mid-third
magnitude (3.27), the luminary of Dorado, the Swordfish, is fairly
prominent, well outshinig most of its deep southern surroundings. 
It does, however, fool the eye, as its modest brightness is the
result of it being two stars close together that even through a
fine optical telescope appear as one.  The dominating fourth
magnitude (3.64) brighter component is a class A (A0) giant.  Close
examination with sophisticated instrumentation reveals a fifth
magnitude (4.55) companion that is listed as a class B9 subgiant,
the pair 175 light years away.  Their separation of only a couple
tenths of a second of arc render the individuals difficult to
study.  Alpha Dor A has a measured temperature of 12,200 Kelvin
(and maybe more), anomalously high for class A0 star, which we
adopt for Alpha Dor B as well, as it is more in line with that
expected for a B9 star.  Quite possibly, the light from the primary
(A0) star has been contaminated by its companion, leading to an
erroneous temperature.  Adopting that temperature for both stars
(to allow for ultraviolet light) leads to respective luminosities
of 157 and 68 times that of the Sun, radii of 2.8 and 1.9 solar,
and masses of 3.0 and 2.7 solar.  The primary (Alpha Dor A) has
begun to evolve and has probably given up core hydrogen fusion
(hence its giant classification), while the secondary (Alpha Dor B)
is really a hydrogen-fusing dwarf rather and a true subgiant. 
Close as they are, the orbit is rather well known and is a bit odd. 
Averaging 9.7 Astronomical Units apart (about the distance of
Saturn from the Sun), they orbit each other in 12.1 years (far
shorter than Saturn's period as a result of much greater masses and
gravitational fields).  These orbital parameters give a mass-sum of
6.2 times solar, a bit greater than the 5.7 solar derived from the
luminosity, temperature, and theory, but given the uncertainties
the agreement is rather good.  The orbital eccentricity is
remarkably high, however, the stars separating by as much as 17.5
AU (near Uranus's distance from the Sun) and then screaming in to
as close as 1.9 AU (just 25 percent farther than Mars is from the
Sun), the last close approach taking place in 1998.  The measured
rotation speed of the primary of at least 70 kilometers per second
leads to a rotation period of under two days.  Of greatest
interest, Alpha Dor A's spectrum is "peculiar" in that it is
especially rich in silicon, the result of separation -- migration
really -- of chemical elements in a relatively quiet stellar
atmosphere, some settling downward because of gravity, others
lofted upward by radiation pressure and effects of magnetic fields. 
The silicon seems to be concentrated into a magnetic spot, which
tells of a real rotation period of 2.95 days, which shows that the
above rotation speed cannot be correct, probably as a result of
contaminating light from Alpha Dor B.



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