[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 9, 2007. 
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.

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"Astronomy: Stars, Galaxies, and the Universe," an audio course on
CD with 100 page study guide narrated and written by Jim Kaler, is
available from Recorded Books.

"Vault of the Heavens: Exploring the Solar System's Place in the
Universe," an audio course on audio CD with 100 page study guide
narrated and written by Jim Kaler, is available from Barnes and
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NEW! The Cambridge Encyclopedia of Stars.

We are in a long period in which the momentary phases of the Moon
rather match Skylight's Friday-to-Friday week.  The third ("last")
quarter of the Moon is to be passed on Saturday, February 10th,
which leaves us with the waning crescent all week, until the new
phase on Saturday the 17th.  Watch in the early morning hours as
the ever-thinning crescent rises later and later until it comes up
only in dawn.  The morning of Sunday the 11th, the Moon will reside
to the west of Antares (the luminary of Scorpius), with Jupiter
farther to the left.  The following morning, that of Monday the
12th, sees the Moon six degrees below the giant planet, the three
(Moon, Jupiter, Antares) making a delightful, constantly shifting,
sight.  On the following two mornings, the Moon plows through
Sagittarius, which is now starting to clear dawn.

While Venus dominates the very early evening, shining brilliantly
in southwestern twilight, the week really belongs to Saturn, which
goes through opposition to the Sun at the beginning of the week, on
Saturday the 10th.  At that time, it will rise at sunset, set at
sunrise, and cross the meridian to the south at local midnight,
giving us our best view of the year.  With the Earth between Saturn
and the Sun, the planet is now also in its fastest retrograde
motion, though so far away (8.2 times the distance between Earth
and Sun) that the movement is slow.  Use Leo's Regulus, just to the
east of Saturn, as a reference.  Not quite three hours after Saturn
transits, Jupiter rises to the east of Antares in Scorpius.  By
bright dawn, with Saturn setting, the giant planet is well up in
the south-southwest.  Mars, rising as dawn begins, is difficult, as
is Mercury, which you might glimpse down and the right of evening's
Venus.

Orion, who dominates the mid-evening sky, makes a wonderful guide
to so many other constellations.  Below the Hunter is Lepus, the
Hare, and below that, a pretty triangle that makes Columba, the
Dove.  Further down, though below the horizon for temperate
northerners, is Pictor, the Easel.  To the southeast barks Canis
Major, the Larger Dog with bright Sirius, while to the southwest
flows Eridanus, the celestial river.

STAR OF THE WEEK: NU ERI (Nu Eridani).  Some stars just stand out
in their classes.  Just west of Mu Eridani and the third stepping
stone in Eridanus, the River (third stone if you count fainter
Omega), fourth magnitude (3.93, just a hair brighter than Mu) Nu
Eridani is the champion of its kind.  Even by ordinary standards,
Nu stands out as a hot (22,000 Kelvin) blue class B (B2) "giant"
(but see below) that lies a respectable 590 light years away,
somewhat farther than Mu's 530 light years.  The distance is enough
that, even though the star is well off the plane of the Galaxy,
there is sufficient dust in the line of sight to dim it by 10
percent.  (Like Mu, Nu is part of the local tilted "Gould Belt" of
hot stars.)  A luminosity of 5200 times that of the Sun (which
includes a whopping amount of ultraviolet light) leads to a mass of
8.5 times solar, and clearly shows the star to be not a giant, but
a hydrogen-fusing dwarf about midway through its projected dwarf
life of 28 million years, when it will indeed give up internal H-
fusion and start to turn into a true giant.  Nu Eri's claim to Hall
of Fame status lies in its subtle variability (typical of stars in
its class) as an extraordinary "Beta Cephei" star, one that
chatters away by a few hundredths of a magnitude with multiple
periods all going on at the same time (caused by the valving of
heat below the stellar surface).  The largest oscillation makes the
star vary by just 0.04 magnitudes (4 percent) over a period of 4.16
hours.  The clear record holder, Nu is observed to have 11 more
oscillation periods, the shortest being 3.03 hours (0.0012
magnitudes), the longest 4.27 hours (0.025 magnitudes).  At the
same time, this unique star has two more much longer oscillations
of a few thousandths of a magnitude over 2.31 and 1.63 days, making
not just a rather amazing Beta Cep star but also a "slowly
pulsating B" (SPB) star like Mu Eri and the prototype of the class,
53 Persei.  The multiple periods all interact to produce numerous
"beat" periods of the kind you hear when two just-out-of-tune
guitar strings play against each other.  All this action is watched
by a distant, though probable, 13th magnitude companion 51 seconds
of away.  This (most likely) K5 dwarf orbits with a radius of at
least 9200 Astronomical Units over a period of at least 300,000
years.  Study of Nu's oscillations revealed that both the stars
used for comparison, Mu and Xi Eri, are also variable.  While Mu
and Nu Eri are not real a pair, they are not all that far apart,
just 60 light years.  From each, the other would shine at the minus
first magnitude, not much different than Canopus, or even Sirius,
shines in our sky.  And that is nothing compared with Rigel.  Just
under 200 light years away,  Rigel would brighten Nu Eri's sky at
magnitude -3, about like our Jupiter at opposition to the Sun.  At
8.5 solar masses, Nu Eri is right near the edge of being a
supernova candidate.  If it does not blow up, it will become a
massive white dwarf, maybe one with a rare neon-oxygen interior. 
(Oscillation data from M. Jerzykiewicz et al. in "Astronomy and
Astrophysics.")


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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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