Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 13, 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 third quarter this week on Sunday the 15th,
about the time of moonset. It then wanes through its crescent
towards new, that phase reached next week. While the crescent
begins to narrow in the early morning sky, the Moon passes north of
Neptune on Monday the 16th, then north of Uranus the following day.
Two days after the quarter, our companion passes apogee, where it
is farthest from the Earth.
The sky presents an interesting set of planetary pairings. The
outer four planets are often lumped together as "Jovian planets,"
as if they were of one kind. Jupiter and Saturn, however, the two
giants of the Solar System (which are quite similar to each other),
are very different from much smaller and more distant Uranus and
Neptune, which themselves are near-twins. The couples are also
currently paired in space. As the Sun sets and twilight ends, the
two evening planets, Jupiter and Saturn, find themselves ever
closer to the horizon. Both still in Taurus, Saturn now sets just
after 10 PM daylight time, Jupiter about an hour later. At the
same time, both are running ever faster toward the east against the
stellar background. Jupiter, the closer of the pair, is moving the
faster, causing it to pull away from Saturn. While the ringed
planet will linger in Taurus, Jupiter (which will pass north of
Aldebaran on Monday the 16th) is heading toward Gemini. Uranus and
Neptune are in much the same configuration. Neptune rises around
3 AM (Daylight time), Uranus an hour later. Both now in
Capricornus and also moving easterly, closer Uranus is pulling away
from Neptune, and will soon enter Aquarius on its 84-year journey
around the Sun, leaving distant Neptune behind. Uranus, faint but
visible to the naked eye, is now passing roughly north of Deneb
Algedi (Delta Capricorni), making it rather easy to find.
Not paired, but shining in solitary splendor, Venus rules the
eastern morning twilight sky. Our nearest neighbor has been in
retrograde motion, moving to the west against the stars. On
Tuesday the 17th it ceases retrograde and begins its direct
westerly motion once again, though at a slow pace, the planet
continuing to separate from the Sun and rising ever earlier.
Directly to the south at 8 PM, between Gemini and Leo, lies the dim
constellation Cancer, the Crab. Its greatest distinction is a
small box of 4 stars within which lies a fuzzy patch, an open
cluster called the "Beehive." Not all that much farther than the
famed Pleiades in Taurus, the Beehive (a lovely sight in a small
telescope) is older and contains intrinsically less-luminous stars
and is therefore not so prominent.
A
STAR OF THE WEEK. ACUBENS (Alpha Cancri). Though Bayer's Alpha
star, Acubens, at faint fourth magnitude (4.25), ranks only fourth
in the constellation Cancer, after Beta (Al Tarf), Delta (Asellus
Australis), even Iota, probably because of its position as a
southern claw of the celestial crab. The star's name, which it
actually shares with a northern claw (Iota), is derived from an
Arabic word that means just that, "the claw," and is the same root
from which is derived the name Zubenelgenubi (Alpha Librae), the
star that represents the southern claw of Scorpius. Acubens, 175
light years away, is something of a mystery. Seemingly a simple
white class A (A5) star, its spectrum displays ultra-strong
absorptions of particular metals that make it a "metallic line" or
"Am" star. Metallic line stars are typically strongly enhanced in
elements like zinc, strontium, zirconium, barium and others. The
phenomenon is a surface effect in which some elements sink to lower
layers under the action of gravity, while others rise, pushed
upward by radiation. To be such chemically peculiar stars, they
must rotate slowly, such that the surface gases are not stirred up.
Class A stars tend to rotate quickly however, so there should be
slowing mechanism, which for the Am stars is duplicity (each acting
tidally to slow the other). Acubens, though, rotates more quickly
than usual (at least 68 kilometers per second at the equator). It
is, however, a double star. It is close enough to the ecliptic
plane that the Moon occasionally passes over and occults it. Stars
normally wink out very quickly when the Moon covers them; Acubens
winked out twice, showing that it consists of two identical stars
only 0.1 seconds of arc (5.3 astronomical units) apart. Each is
presumably a dwarf Am star with double the mass of the Sun, each
shining with 23 solar luminosities. Given the masses and
separation, they should orbit each other every 6.1 years. Neither
can be discriminated from the other, however. Moreover, Acubens is
not just double, but quadruple. Eleven seconds of arc away is a
12th magnitude companion that is ITSELF double -- about all that is
known of it. If the two are identical, both would be dim class M
dwarfs. Separated by at least 600 astronomical units, the faint
pair must take at least 6300 years to orbit the bright pair (which
from the faint pair would appear about as far apart as the angular
diameter of the full Moon).
****************************************************************
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)
*****************************************************************
--
This is the CPS Science Teacher List.
To unsubscribe, send a message to
<[EMAIL PROTECTED]>
For more information:
<http://home.sprintmail.com/~mikelach/subscribe.html>.
To search the archives:
<http://www.mail-archive.com/science%40lists.csi.cps.k12.il.us/>
