Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, October 5, 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.
Our Moon passes through its new phase this week, on Tuesday the
16th. As the crescent wanes and descends the early morning sky, it
will stand directly above brilliant planet the morning of Sunday
the 14th, and down and to the left of it and close to the horizon
in dawn the morning of Monday the 15th. The Moon will then make
its first appearance in the western evening sky as a slim waxing
crescent the night of Wednesday the 17th. At the time of both
crescents be sure to admire the Earthlight on the Moon, light
reflected from Earth that lights up the Moon's nighttime side.
Mars continues to hang out in the post-twilight southwestern sky,
as it will for the rest of the year, while Saturn and Jupiter
encroach ever more into the late evening before midnight, Saturn
now rising in Taurus shortly after 9 PM Daylight Time, Jupiter,
smack in the middle of Gemini, about two hours later. Still-bright
Mars is moving out of Sagittarius and into Capricornus (which it
will enter toward the end of the month), and will pass Neptune
early in November. The planetary sky, however, more belongs to
events you cannot see that involve the planet closest to the Sun
and the one (excepting tiny Pluto) farthest from the Sun: little
Mercury goes through inferior conjunction, when it is more or less
between us and the Sun, on Saturday the 13th, and Neptune, in
Capricornus, begins its retrograde motion on Wednesday the 17th.
The debate about Pluto's status as "planet" continues unabated.
Now well north of the ecliptic path in southern Ophiuchus and
nearing conjunction with the Sun, the small body -- about the size
of the Western United States -- has as much in common with a slew
of even smaller bodies in the "Kuiper Belt" of comets that extends
from just beyond Neptune's orbit to well outside Pluto's. Pluto is
the largest of them, and could be considered a transition object,
so in a way it is both, a planet and a Kuiper Belt object (a "KBO"
in the trade) at the same time, so everyone can feel satisfied.
Look down and to the right of Mars for a last admiration of
Sagittarius and its five-star "Little Milk Dipper," which as autumn
advances will slip into evening twilight. It and its summer cohort
are now being replaced by the full autumn sky, the Great Square of
Pegasus well up in early evening and crossing the meridian to the
south around 11 PM, by which time we easily see Taurus and its two
clusters: the Hyades (which make the Bull's Head) and the charming
"Seven-Sisters," the Pleiades, which at first look like a fuzzy
little ball until closer scrutiny resolves them into a small host
of stars, the object brilliant in binoculars.
STAR OF THE WEEK. KAUS MEDIA (Delta Sagittarii). Sagittarius is
known more for its two "asterisms" (informal constellations), the
"Little Milk Dipper" and the "Teapot," than it is for the figure it
was meant to represent, a Centaur Archer. And what is an archer
without a bow? Sagittarius has a fine one, not just a bow, but an
arrow along with it that points into the heart of the Milky Way.
At the bow's northern end lies Kaus Borealis, at the southern Kaus
Australis, and in the middle and marking the eastern end of the
arrow Kaus Media, the names an odd mixture of languages, Kaus
coming from an Arabic word meaning "bow," while the other three
words respectively mean, from Latin, "northern," "southern," and
"middle." At the bright end of third magnitude (2.70), Kaus Media
ranks second brightest in the bow (after Kaus Australis) and fourth
in the constellation (fitting for Bayer's Delta star), behind Sigma
(Nunki), Epsilon (Kaus Australis), and Zeta (Ascella); it's hard to
know what Bayer had in mind! Kaus Media is yet one more class K
(K3) giant star, but one a bit on the bright side. Its distance of
305 light years and an uncertain temperature of 4300 Kelvin (needed
to account for infrared radiation) lead to a luminosity 1180 times
that of the Sun and a radius 62 times solar, the star three-
quarters the size of Mercury's orbit. Oddly, the temperature has
never actually been measured, and can only be inferred from the K3
spectral class. The amount of dimming by interstellar dust is also
uncertain, and while it cannot be much, the star may actually be up
to 30 percent brighter and 15 percent larger. With a relatively
high mass 5 times that of the Sun, Kaus Media is mostly likely
fusing helium into carbon in its core. The star is an example of
how little we can know about companions, especially within the rich
star fields of the Milky Way. Catalogues list three faint
neighbors, 14th, 15th, and 13th magnitude stars at separations of
26, 40, and 58 seconds of arc. If these are true companions, they
are class K and M dwarfs, are at real distances of at least 2400 to
5400 Astronomical Units (Earth-Sun distances) from Kaus Media
proper, and take at least from 53,000 to 180,000 years to orbit.
The little stars might also be line of sight coincidences. Spectra
display some evidence for a possible close companion as well, which
might be related to the star's uncertain reputation as a weak
"barium star," one that has been contaminated with heavy elements
by an evolving orbiting neighbor. Clearly the star needs more
attention than it gets.
****************************************************************
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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