[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, November 12, 2004.
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 week belongs to the waxing crescent Moon.  We begin on Friday,
November 12, with the new Moon and end the night of Thursday the
18th with the first quarter, the phase taking place around the time
of Moonset in North America.  The slim crescent will first be
visible in the southwest the evening of Saturday the 13th.  With
the Sun over halfway from the Autumnal Equinox to the Winter
Solstice, the crescent will be swinging low through the southern
constellations of the Zodiac such that the first quarter will be
seen near the Capricornus-Aquarius border.  Waxing along,
Earthlight on the nighttime side gradually fading away, the Moon
will pass Mercury the night of Saturday the 13th, though the little
planet will be quite difficult to see in bright twilight.  The Moon
actually occults, or passes in front of, Mercury, but only from the
ice cap of Antarctica.  On the night of Wednesday the 17th it
passes well to the south of Neptune in Capricornus.  Just two days
after new Moon, our satellite will pass perigee, where it is
closest to the Earth in its monthly round.

Aside from elusive Mercury, the ancient planets (those known since
ancient times) have but one evening representative, Saturn, which
is now rising in eastern Gemini around 9 PM.  A telescope at 40
power or above reveals the planet's magnificent ring system.  All
the big planets have rings (disks made of finely divided icy
debris), but none like Saturn (those of Jupiter, Uranus, and
Neptune dark and very hard to see).

Planetary glory really belongs to the morning sky, where Venus and
Jupiter rule together in their brilliance.  Following their
November 4 conjunction, Jupiter has steadily moved to the west of
Venus, and now rises about 3 AM, roughly an hour before Venus.  A
telescope quickly reveals up to four of Jupiter's moons (which can
actually be seen in binoculars) and Venus's gibbous shape.  On the
morning of Tuesday the 16th, Venus will pass four degrees to the
north of the first magnitude star Spica in Virgo.  An hour after
Venus rises, just before the beginning of twilight, up comes much
dimmer reddish Mars

Beautifully climbing the eastern sky at sundown are the classic
constellations of northern Autumn.  By late evening Cassiopeia --
noted by the upside-down "W" -- is nearly overhead, Andromeda to
the south, while the northeast is graced by the star streams that
make Perseus, the Hero and rescuer of Andromeda.  Following well
behind Perseus, lighting up the far northeast in late evening, is
Capella in Auriga, the most northerly of the set of first magnitude
stars.

STAR OF THE WEEK.  ATIK (Omicron Persei).  At the end of one of the
star-streams that make most of Perseus, just to the west of bright
Zeta Persei, lies seemingly modest Atik, Omicron Persei, the name
apparently referring to a "shoulder" of the Pleiades, which lies
just to the south of the Hero's stars.  Atik's fourth magnitude
(3.83, really not all that faint) status is the result of
considerable distance, direct measure giving 1475 light years (with
a large uncertainty).  The faintness is also caused by considerable
dimming by interstellar dust in the Milky Way; a clear path would
render Atik about a magnitude brighter.  Atik's chief attribute is
its duplicity.  It is a spectroscopic binary (one detected through
Doppler shifts in a composite spectrum) with a very short period of
4.419171 days in which a hot class B (B1.5) giant with a
temperature of 22,000 Kelvin mutually orbits another B star (this
one a B3 dwarf 2.5 magnitudes fainter with a temperature of
18,600).  Mutual tides distort the components into ellipsoidal
shapes, which make the binary appear to vary over the orbital
period by a few hundredths of a magnitude.  The giant is evolving
with a dead or near-dead helium core, while the dwarf is a common
hydrogen-fuser.  The combined luminosity of the pair (with
considerable allowance for ultraviolet light) comes in at 82,000
solar, which given that the giant is 10 times the luminosity of the
dwarf, leads to respective luminosities of 75,000 and 7500 solar.
Combination of data yield masses of 17 and 8 solar.  Each star
rotates with an equatorial velocity greater than 85 kilometers per
second.  However, all is not well, since orbital analysis gives a
bit of a different picture, with respective luminosities of "only"
12,300 and 2000 solar, lower masses of 10 and 7 solar, and radii of
7.6 and 4.0 solar (which with rotation speed gives rotation periods
less than 4.5 and 2.4 days).  The problem is most likely one of
distance.  Atik lies at the edge of the ability the Hipparcos
satellite to produce good parallaxes.  Factoring in the errors
could make the star as close as 1000 light years, which would much
narrow (but still not close) the gap between the two
determinations.  Atik is a source of X-rays that suggest gasses at
two temperatures.  A hot gas at 3 million Kelvin is probably
produced when the winds of the two stars collide, while a much
hotter temperature of 16 million may be from some kind of hot
corona (odd, since the stars should not have the required magnetic
fields).  The giant is near the lower mass limit for which stars
explode, while the dwarf will become a massive white dwarf like
Sirius B.  Nearby, only a second of arc away, is an eight magnitude
possible companion, about which nothing is known.  Atik's
membership in the Perseus OB2 association of hot O and B stars
(which is associated with the cluster IC 348 and which houses both
Zeta and Xi Per) has long been disputed, the consensus now being
that the star is not a member of the troup.



****************************************************************
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)
*****************************************************************

***********************************************************
For problems with ISTATALK-L, please contact
listadmn@xxxxxxxxxxxxxxxxxxxxxxxxxxxx for assistance.

To unsubscribe, send a message to listadmn@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
that states, "Please unsubscribe me from the ISTATALK-L listserv."
************************************************************