Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, May 18, 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
22nd. Watch as the waning morning crescent, seen in eastern dawn,
thins early in the week and then appears on the other side of the
Sun to the west the evening of Thursday, the 24th. The morning of
Saturday the 19th, the Moon will appear a few degrees to the south
of brilliant Venus.
Saturn is now gone, lost to evening twilight, and Jupiter, just a
little behind, is quite difficult to find, as it now sets just a
bit over an hour after the Sun. The evening sky does present us
with a fine apparition of Mercury, however. Only a day before the
Moon passes new (on Monday the 21st), the little planet, the one
closest to the Sun, passes its greatest eastern elongation, when it
is 22 degrees to the east of the Sun and maximally visible. For
those in North America, the Moon will provide a fine guide, as the
lunar crescent will be just a few degrees to the left of the
planet the night of Thursday, the 24th. Look in bright twilight
and follow the Moon as it sets and the sky darkens to see Mercury
emerge from the fading glow. The planet remains mysterious. Of
the nine planets, only Pluto is smaller. Not quite 40 percent the
size of Earth, Mercury has the largest iron core relative to its
size of any of them. Dangerously close to the Sun, it has been
visited by but one spacecraft (Mariner 10 passing it 3 times in
1974 and 1975), and only about half has been imaged. So close in
angle to the Sun that it is visible only in the daytime or in
twilight, surface features are nearly impossible to see from Earth.
Once Mercury sets, the evening sky awaits the rising of Mars, which
climbs above the southwestern horizon around 10:30 PM Daylight
Time. Now moving retrograde in far western Sagittarius, the red
planet, fourth from the Sun, and the last of the "terrestrial
planets" (those constructed like Earth), will be rising in evening
twilight by the end of the month.
In the early evening, for those in mid-northern latitudes, look out
perpendicular to the plane of our Galaxy. The Milky Way is about
as absent as it can get, and lies around the horizon where it is
invisible. Our view is unobstructed by the dust in the Galactic
plane, allowing us to see outward as far as our instruments will
carry us, to distant galaxies billions of light years away. From
the southern hemisphere, however, the Milky Circle is high and
spectacular as it passes through Centaurus and Crux, the Southern
Cross.
STAR OF THE WEEK. ALKES (Alpha Crateris). Among the dimmest of
all classical constellations is Crater, the Cup, which with Corvus,
the Crow, rides the back of Hydra, the Water Serpent. The only
proper name in the constellation, "Alkes" comes from Arabic and
means "the wine cup," the star standing in for the whole figure
(the name also clearly related to the English "alcohol.") Though
mid-fourth magnitude (4.07), Alkes received the Alpha designation
from Bayer. It takes second place to the un-named Delta star (that
is, to Delta Crateris), and is in a virtual dead heat with Gamma
Crateris. Alkes is yet one more orange class K (K0) giant star,
though one with an interesting difference. At a distance of 175
light years, Alkes shines 80 solar luminosities into space from a
4725 Kelvin surface, giving the star a calculated radius 13 times
that of the Sun. Alkes, with a mass estimated at around 2.5 times
solar, is clearly "in the clump," a set of stars that all have
about the same characteristics of luminosity and temperature and
that are all fusing helium to hydrogen in their cores, Arcturus and
Aldebaran bright examples. Unlike most helium-burning "clump
stars," unlike most stars around us, Alkes is also a modest "high
velocity" star. Most of our neighbors are going around the Galaxy
at a speed somewhat in excess of 200 kilometers per second.
However, all the orbits are a bit different, so they drift relative
to each other at speeds of 20 to 40 or so kilometers per second.
>From its rate of angular motion across the sky (480 seconds of arc
per year relative to the distant background) and its speed away
from us of 47 km/s, Alkes is moving relative to the Sun at 130
km/s, showing it to be a visitor from a different part of the
Galaxy. The star has on occasion been placed into the group of
"super-metal-rich" stars. Though the metal content is probably
more solar, it is clear that the star has come to us from the inner
metal-rich part of the Galaxy, the so-called "bulge."
Consistently, Alkes has also been dropped into an odd category of
"4150" stars, which seem to have a high abundance of cyanogen, the
CN molecule. Though many stars may look alike, none is quite the
same as the others!
****************************************************************
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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