[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 11, 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.

The Moon grows through its waxing crescent phase the first part of
the week, and reaches its first quarter -- where it is 90 degrees
to the east of the Sun -- on the evening of Tuesday the 15th just
as it crosses the meridian to the south, allowing you to see an
exact first quarter about as high in the sky as possible.  Even
better, the first quarter will be passing close to the Pleiades
cluster (the Seven Sisters) in Taurus, up and to the right of the
Hyades, which (with the nearby star Aldebaran) make up the head of
Taurus, the Bull.

Everything (two things anyway) seems to happen on Monday, the 14th. 
The problem is that the events are both invisible.  First, Mercury
is in superior conjunction with the Sun, when it is on the opposite
side of the Sun from us.  It will shortly thereafter pop up in the
evening sky.  (Look for it in mid-March.)  Then, with Neptune
having gone though conjunction with the Sun last February 3, it is
now in the morning sky and passes conjunction with Venus.  A mere
1 degree will separate them, but since they are in bright twilight,
observation is impossible.

We do, however, have three bright planets to admire.  Saturn, well
up in the east in Gemini at the end of twilight, crosses the
meridian to the south around 10 PM, almost exactly as brilliant
Jupiter rises.  Majestically crossing the sky northwest of Spica in
Virgo, Jupiter transits the meridian at 3:30 AM, half an hour
before Mars rises.  Though bright (first magnitude), Mars, just to
the north of Sagittarius and about as far south as it can get, is
still a bit difficult to catch, especially if you wait until the
sky begins to brighten.

In mid-evening, two northern constellations vie for attention, both
about the same distance from the celestial equator, and from 40 to
50 degrees north latitude passing nearly overhead.  From more
southern skies, they appear to be a bit to the north of the zenith. 
To the west is Auriga, the Charioteer with bright Capella (the
"she-goat"), flanked by the triangle of fainter stars that makes
her "kids."  To the east is Perseus.  While having no first
magnitude stars, Perseus has a marvelous collection of star-streams
that center around Mirfak (at the core of a wide cluster) and
Algol, the most famed eclipsing double star of the sky.  Making a
great triangle with them is Taurus, the Bull, which lies to the
south of a line between the two.

STAR OF THE WEEK: TAU PER (Tau Persei).  At mid-fourth magnitude
(3.95), one hardly notices Tau Per among the bright and famed stars
of Perseus, dominated as it is by the Alpha Persei cluster
(featuring Mirfak) and Algol, which is among the most famed stars
of the sky and epitomizes eclipsing doubles.  Tau Per, however, has
its own fascinating -- and mostly unsung -- story to tell.  The
spectrum clearly shows the signatures of two stars, a dominant
class G (G4) giant coupled with a significantly fainter class A
(A4) dwarf.  Doppler shifts reveal that each orbit the other with
a period of 4.419 years, implying reasonably large separation. 
Extremely short-exposure images (to avoid atmospheric turbulence --
"twinkling") resolved the pair and allowed the separation and an
orbit to be determined, the two only a few hundredths of a second
of arc apart.  The odds that the orbital plane would be aligned in
the line of sight such that the stars eclipse each other seemed
impossibly high.  Algol after all orbits in a mere 2.9 days, with
the stars separated by only 0.05 Astronomical Units (AU).  But sure
enough they do.  In late 1984, astronomers discovered a small dip
in brightness as the smaller A dwarf moved partially in back of the
G giant for just two days.  A campaign was then mounted to observe
the January 1989 eclipse, which involved observatories around the
world such that the star could be continuously monitored throughout
its short eclipse cycle.  The result was the establishment of light
curves in three colors, the visual brightness showing a dip of
about five percent.  Analysis of the combination of direct,
spectroscopic, and eclipse observations allowed full establishment
of orbital and stellar properties, as well as a distance of 272
light years (more accurate than the parallax that gave 248 light
years).  While the average separation is 4.2 AU, a very high and
unusual eccentricity takes the stars from 7.2 AU to 1.13 AU.  Tau
Per A (the G giant) has a mass of 2.4 Suns and a radius of 14
solar, while Tau Per B respectively comes in at 1.8 solar masses
and 2.2 solar radii.  The measured respective temperatures of 5160
and 8970 Kelvin give luminosities of 150 and 26 solar, but larger
masses of 3.3 and 2.1 solar (no surprise given the various
uncertainties).  The dwarf is still fusing hydrogen in its core,
while the giant -- which started life as class B star (hotter than
the dwarf) -- has given it up and is most likely fusing helium into
carbon.  The secondary eclipse, wherein the A dwarf clips the edge
of the G giant, is too subtle to be observed.  While the dwarf
spins relatively quickly, at 50 km/sec at the equator (for a
rotation period of 1.8 days), the giant -- as expected -- spins
ponderously with about the same speed as the Sun for a rotation
period of at least 165 days.  A nearby faint seeming companion, Tau
Per C, itself double, is probably just a line-of sight coincidence. 
All in all, the star, unlike so many, presents us with an amazing
amount of information.

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