[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 31, 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.

We start with the last night of the year, of 2004; welcome to 2005.
To all a Happy New Year to live life and to enjoy the sky.

During the early part of the week the Moon descends through its
waning gibbous phase to its third quarter, the phase reached around
noon on Monday January 3rd, about the time of moonset in North
America.  The Moon then takes on the stars and planets.  The
morning of Monday the 3rd finds the quarter Moon just to the west
of Jupiter (which rises just before midnight), which it passes in
daylight (even occulting it in parts of the eastern hemisphere) and
to the south of Porrima (Gamma Virginis), while the following
morning finds the Moon just to the east of the giant planet (and to
the north of Virgo's Spica).  Then it is Mars's turn, the Moon to
the west of the red planet -- and the star Antares in Scorpius --
the morning of Friday the 7th.

Clearly Mars is close to Antares, with which is can be confused
since the colors are similar (though Antares is much the brighter).
Indeed, "Antares" means "like Ares," Ares the Greek version of
Mars.  It's a treat to see the two together, Mars just to the north
of the star as the year begins.  Saturn, which is now a prime
evening object that rises in mid-twilight, also gets into the act,
passing seven degrees south of Pollux in Gemini on Thursday the 6th
(Saturn, Castor, and Pollux making a fine trio), Mars passing north
of Antares the following day.  The prize for "best relationship"
though goes to Venus and Mercury, which as New Year's morning dawns
are in tight configuration, Venus just a degree below much fainter
Mercury, both down and to the right of Mars and Antares.  The
twilight morning sky does not get much better.

Early January is host to one of the better meteor showers of the
year, the Quadrantids, named after the defunct constellation
Quadrans (which is located near the handle of the Big Dipper).  The
shower will peak the morning of Monday the 3rd near dawn, though
its typical rate of up to 100 meteors an hour will be quite
restrained by the bright Moon.

Even Earth involves itself in the New Year, as it passes perihelion
--its closest point to the Sun -- on New Year's Day, when it is 1.7
percent closer than average, at a distance of 91.4 million miles,
or 147 million kilometers.  Clearly, given the northern
hemisphere's winter chill, the distance to the Sun has nothing to
do with the seasons, which are caused by the 23.4 degree tilt of
the Earth's axis relative to the orbital perpendicular.

Nothing quite proclaims the New Year and winter like Orion, the
celestial representation of the ancient Greek Hunter.  Already
risen at the end of evening twilight, straddling the celestial
equator, Orion crosses the meridian to the south around 11 PM.
Among his best features are the first magnitude stars Betelgeuse
(at the upper left corner) and Rigel (at lower right), which
bracket the famed "Belt," three bright stars that the ancient Arabs
called the "String of Pearls."

STAR OF THE WEEK: THETA-1 ORI (Theta-1 Orionis).  Dropping nearly
straight down from Orion's brilliant three-star belt (from right to
left Mintaka, Alnilam, and Alnitak, or Delta, Epsilon, and Zeta
Orionis) is the Hunter's three-star "Sword," in the middle of which
is one of the great sights of the telescopic sky, the Orion Nebula.
Also known as Messier 42, the Nebula is an immensely complex cloud
of dusty gas 1400-1500 light years away and 20 light years across
(depending on just where you draw the ill-defined boundaries) that
is made to fluoresce by the hot stars of Theta-1 Orionis, which are
situated directly in front of it.  (The Orion Nebula is a "blister"
on the face of the great Orion Molecular Cloud, which lies behind
the Hunter and hosts several sites of active star formation.
Though invisible to the eye, it glows brightly in the radio
spectrum).  Even a small telescope shows Theta-1 to be a quartet,
which carries a group name, the "Trapezium," from west to east
labelled Theta-1 A, B, C, and D.  Clumped within a span of 22
seconds of arc (10,000 Astronomical Units), all are hot class O and
B stars that together make an apparent "single star" of magnitude
5 (4.7).  Taken separately, from A through D, they are of magnitude
6.7, 8.0, 5.1, and 6.7 and blue spectral classes B1, B0, O6, and
B0.5.  All contribute to the energetic ultraviolet light that
energizes (ionizes, stripping electrons from atoms) the Orion
Nebula.  By far the leader of the pack is Theta-1 C, a great 40-
solar-mass star with a temperature of 40,000 Kelvin (making it the
hottest "naked eye" star, though the 4 are inseparable without
optical aid), a huge luminosity 210,000 times that of the Sun (85
percent of the Trapezium's total), and a 1000 kilometer/second wind
with 100,000 times the flow rate of the solar wind.  The power of
the star is such that it is evaporating dusty disks around nearby
new stars that in other settings might form planets.  The other
members of the Trapezium pale only in comparison with "C," all
containing over 10 solar masses.  A main interest lies in their
multiplicity.  Theta-1 A is an eclipsing double also known as V
1016 Ori.  Every 65 days, the star dips by a magnitude as a star
still in the process of formation just one Astronomical Unit away
passes in front of the bright component, the whole thing watched by
another companion 100 AU off.  Theta-1 D seems to have a companion
as well.  The champion in this contest, however, is Theta-1 B,
which has a companion 60 AU  away called "B1."  "B" itself is
another eclipser (known also as BM Ori) that drops by nearly a
magnitude every 6.5 days, the companion probably much like the Sun.
Since "B1" is also double, Theta-1 B is quadruple.  Adding them all
up (and including fainter Theta-1 E, which lies close by), the
Trapezium is a complex multiple of 11 stars!  And that is not
really the end, as the Trapezium is really the core of an
incredibly dense star cluster that seems to fill the background of
the Orion Nebula, all of it born less than a million years ago.
Most multiple stars are hierarchical, a distant star going around
a close double (like Theta-1 B), or two close doubles going around
each other (like Mizar or Epsilon Lyrae), which gives great
stability.  The Trapezium, on the other hand, is gravitationally
unstable, the stars all too close together.  As a result, one after
the other will be ejected from the group.  After only a few million
years, the leader of them all, Theta-1 C, will inevitably explode
as a great supernova, the others probably doing so as well, all
lighting the dusty gases of interstellar space, all providing shock
waves that will promote new star formation within the local
molecular clouds.  (Read more about Theta-1 Ori in Jim Kaler's
"Hundred Greatest Stars").



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