[ISTATALK-L] Skylights

[Jim Kaler <kaler@xxxxxxxxxxxxxx>]

Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 13, 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.

Support science literacy by joining the Astronomical Society of the
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1-415-337-1100, then press 1.

NEW! The Cambridge Encyclopedia of Stars.

This is the week of the crescents, crescent Moons that is, waning
in the morning sky the early part of the week, waxing in the
evening during the latter part, the sequence split by the passage
of new Moon on Tuesday, April 17, about the time of sunrise in
North America.  Just six hours before new Moon, the Moon will pass
perigee, where it is closest to the Earth, the increased
gravitational tug resulting in especially high and low tides at the
coasts.

The morning of Saturday the 14th finds the waning crescent just to
the east (to the left of and a bit below) Mars.  It thus acts as a
nice guide to the currently elusive red planet, whose rising still
closely tracks the onset of dawn.  The morning of Monday the 16th
affords the last chance to see an ultrathin crescent glowing in
bright twilight.

Flipping quickly to the evening sky, the first glimpse of the
waxing crescent can be had the evening of Wednesday, the 18th,
again in twilight.  The evening of Thursday the 19th be sure to
take a look at an especially nice convening of the crescent and
brilliant Venus, the Moon somewhat down and to the right of the
planet, not like directions are needed to find either of them.  The
sight is enhanced by the Pleiades star cluster, which will be down
and to the right of the Moon, the three -- Venus, Moon, Pleiades --
all in a row slanting down and to the right.

Setting ever later, our "evening star" does not now go down until
around 11 PM Daylight Time.  Still at the Leo-Cancer border to the
west of Regulus, Saturn, crossing the meridian in mid-twilight,
shifts steadily toward the Sun.  It's a special week for the ringed
planet, as on Thursday the 19th it ceases retrograde motion and
finally begins heading to the east against the stars and back
toward classical Leo.  Just a hour after Venus sets, around
midnight Daylight Time, Jupiter rises still to the northeast of
Antares in Scorpius, the rising of the Scorpion now telling us that
summer is really on the way.

If it's spring, we look to Leo, which in mid-evening stands proudly
high to the south.  At its front lies the famed "Sickle" with
Regulus on it's south-reaching handle.  Toward the back, at the
Lion's tail, is Denebola.  The star is the western anchor of the
large "Great Diamond," whose other stars are Cor Caroli (to the
north in Canes Venatici under the Big Dipper's handle), Arcturus in
Bootes to the east, and Virgo's Spica to the south.  More or less
centered within it is the lovely constellation and cluster Coma
Berenices.

STAR OF THE WEEK: RHO LEO (Rho Leonis).  Magnificent stars are oft
neglected, in part because of their surroundings, and here is a
prime example.  The constellation Leo is so well known, its figure
looking really quite like what it is named for, its bracketing
stars Regulus and Denebola so famed, that no one pays much
attention to fourth magnitude (3.85) Rho Leonis, which falls off
the main pattern to the southeast of Regulus.  To the contrary, as
a class B (B1) "lesser" supergiant, Rho is one of the hottest,
bluest, and most massive stars that you can readily see with the
naked eye.  But hold that for later.  From our perspective on
Earth, Rho is also a prime "ecliptic star."  Falling only eight
minutes of arc off the apparent solar path, it marks the position
of the Sun on August 29.  A line drawn from Regulus through Rho (on
the way to Spica) pretty much shows the ecliptic's locus.  Moving
just under a degree per day, it takes the Sun eight days to move
between the two stars.  Rho Leo is one of the few brighter Greek-
letter-named naked eye stars that are too far away for parallax
measure.  We can only estimate its distance on the basis of its
spectral class.  Class B1 lesser supergiants (in the trade it's
called a B1 Ib star, Roman "I" for "supergiant") commonly have
"absolute visual" magnitudes of -6 (the absolute magnitude being
the magnitude the star would have at a distance of 32.6 light
years, where it would far outshine Venus).  The difference between
the observed and absolute magnitudes then gives distance (just as
it does for a much better known star, Deneb).  Rho Leo is a bit
complicated by an uncertain 0.19 magnitudes of dimming thanks to
interstellar dust and the fact that it is a very close double, in
which fourth magnitude (4.4) Rho Leo A (the supergiant) is coupled
to fifth magnitude (4.8) Rho Leo B.  The result is a distance of
3650 light years (with a large uncertainty).  A temperature of
24,400 (typical of the class) yields a magnificent luminosity for
the primary star of 165,000 times that of the Sun!  Blue
supergiants like this one are really not all that large (the term
referring more to evolutionary status), Rho-A just 23 times larger
than the Sun.  A rather fast projected equatorial rotation of
velocity of 55 kilometers per second gives a rotation period under
21 days.  Like many supergiants, it is slightly variable, changing
in brightness erratically by about seven percent.  Luminosity and
temperature combine to give a mass estimate 23 times that of the
Sun, well above the limit (about 10 solar masses) at which stars
blow up as supernovae.  With an age of roughly 7 million years,
Rho-A has just given up core hydrogen fusion, and does not have
long to wait until the ultimate catastrophe.  Nothing much is known
about Rho B, not even its class, though being less than half a
magnitude fainter than Rho A, it's massive as well.  No orbit is
known, but from a separation estimate the stars probably take about
a year to go around each other.  The real importance of Rho Leonis
is that it has a very simple spectrum that provides a wonderful
background with which to study the intervening interstellar medium. 
It also provides a lesson in stellar "dynamics."  Most stars of
this class stick very close to their birthplaces in the Milky Way,
the plane of the Galaxy.  Quite far off the Milky Way, Rho has
somehow "run away" through some kind of gravitational action, and
seems to be a lesser version of the more famed "runaway" pair Mu
Columbae and AE Aurigae.


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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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