Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, June 1, 2001.
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.
The next Skylights will appear Sunday, June 10. The Moon passes
through its full phase this week, when it is opposite the Sun, on
Tuesday, June 5. It will on that day rise near sunset and set near
sunrise. Thereafter, it begins to thin through its waning gibbous
phase. The night of June 5 the Moon will appear up and to the
right of the planet Mars, while the following night it will have
moved to appear up and to the left of the red planet. As we
approach the beginning of summer in the northern hemisphere, when
the Sun will be as far north as it can get (and as high as it can
get for northerners), this full Moon will be the year's second-most
southerly, the "Rose Moon" rising in the southeast, setting in the
southwest.
The brightest and dimmest planets (as seen from Earth) make the
rest of the planetary news. Venus, very slightly dimming, reaches
its greatest elongation west, when it is 46 degrees to the west of
the Sun. This lovely planet, third only to the Sun and Moon in
apparent brightness, now rises in the east just ahead of morning
twilight. Even though the angle between it and the Sun now
decreases, however, Venus will continue to rise earlier, and until
mid-August into ever darker skies. Earliest Venus-rise will occur
around mid-July. At the same time, dim Pluto, not visible without
a good-sized telescope, is in opposition to the Sun the night of
Monday, June 4. When the Moon reaches its full phase, it will lie
roughly 10 degrees below the frigid outer planet, which some take
not to be a planet at all. In truth, Pluto appears to be some kind
of hybrid object that bridges the gap between the outer planets and
the building blocks (the comets) that created them. Apparently
there was just not enough raw material in these distant reaches of
the Solar System to make a respectable planet like Neptune or
Uranus.
The early evening presents us with the tail of the longest
constellation in the sky, Hydra , the Water Serpent, which wraps
itself a third of the way around the celestial sphere. Find
Corvus, the Crow, a small irregular box of stars that for
northerners appears rather low in the south around 9 PM. The top
two stars point leftward to Spica in Virgo, while the bottom two
point to otherwise un-named Gamma Hydrae, the next-to-the-last
bright star (such as it is) that lies in the celestial snake.
Snakes of some sort are quite popular, others being summer's
Serpens (the Serpent), which comes in two parts, the southern
hemisphere's Hydrus (another water snake), and, if you wish to
stretch the definition a bit, the northern hemisphere's Draco, the
Dragon, whose tail winds between the Dippers.
STAR OF THE WEEK. GIAUSAR (Lambda Draconis). The front bowl stars
of the Big Dipper are famed for pointing at Polaris in Ursa Major's
Little Dipper. What, however, of the stars along the way? The
path to Polaris is so familiar that we rarely stop to see the other
sights that lie along it. About a third of the way from Dubhe (the
Big Dipper's front bowl star) to Polaris (and a just a bit to the
east) lies Giausar, the tail star of Draco the Dragon, to which
Bayer assigned the Greek letter Lambda. The Arabic name of this
mid-fourth magnitude (3.84) star is confusing at best. At times
thought to refer to a "central one" much as does the Arabic name of
Orion, the word actually refers to the "nodes" of the lunar orbit,
the points at which the Moon crosses the ecliptic plane twice a
lunar month -- which makes little sense, since Draco contains the
ecliptic POLE, and is therefore quite distant from the ecliptic
itself. Rather clearly, the name was applied in error. The star
is about as neglected by research astronomers as it is by even
dedicated skywatchers, rather too bad as it has -- as a class M
(M0) red giant -- one of the rarer of naked-eye types. Over the
past 20 years it has been mentioned less than 40 times. It is
neglected by other stars too, as it has no known companions.
Giausar is one of the sky's cooler and larger stars. With a
temperature of 3525 Kelvin, it shines to us (if the estimate of
invisible infrared radiation is correct) from a distance of 335
light years with a luminosity 1870 times that of the Sun, which
leads to a radius of 0.55 astronomical units, half the size of
Earth's orbit. Large enough to have had its angular diameter
measured (at 0.0073 seconds of arc), direct measure of radius makes
it somewhat smaller, a "mere" 0.37 astronomical units, about the
size of Mercury's orbit. Even the star's general behavior is
obscure. Classified as a "semi-regular variable," there is some
indication that it changes brightness erratically by about a tenth
of the magnitude. Giausar appears to be on the "asymptotic giant
branch," in a portion of its evolution in which it is brightening
as a giant star for the second time. With a dead carbon core, the
star (of perhaps two solar masses) will most likely begin to
pulsate more vigorously as it prepares to shed its outer layers and
to turn itself into a white dwarf, as someday will the Sun.
****************************************************************
Jim Kaler
Professor of Astronomy Phone: (217) 333-9382
University of Illinois Fax: (217) 244-7638
Department of Astronomy email: [EMAIL PROTECTED]
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)
Astronomy! A Brief Edition (links and updates)
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