Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, February 9, 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 Moon, now past full, fades from the evening sky. At first
waning through its gibbous phase, it passes third quarter the night
of Wednesday the 14th shortly before Moonrise in the Americas, and
then enters its waning crescent phase. The morning of Thursday the
15th finds the Moon just to the north of Mars, the pair just short
of encountering the three-star head of Scorpius.
Mercury and Uranus make an odd pair this week, as both go through
conjunction with the Sun, Uranus on Friday the 9th, Mercury on
Monday the 12th. The two could not be more different. Mercury, an
"inferior planet," one closer to the Sun than we are, passes its
"inferior conjunction" as it goes roughly between us and the Sun.
Half an orbit later (as viewed from Earth) it will pass its
"superior" conjunction," when it is on the other side of the Sun
from us. Uranus, on the other hand is a distant "superior planet"
that is outside the Earth's orbit. Its conjunction is always on
the other side of the Sun. When the Earth swings through half an
orbit (as viewed from Uranus), Uranus will then pass opposition
with the Sun, when the two are opposite each other in the sky.
Though both are in solar conjunction, Mercury is moving westward
against the stars, retrograde, whereas Uranus is moving in its
normal direction, eastward. They could hardly be more different
physically, either: Mercury is less than 40 percent the size of the
Earth, cloudless, and made of iron and rock; Uranus is four times
Earth's size, 15 times its mass, is covered with methane clouds and
haze, and is made of hydrogen, helium, water, and other light
stuff. Unfortunately, the solar glare will make viewing them
impossible.
We CAN, however, easily view brilliant Venus, high in the
southwestern sky at twilight, and still brightening, as well as the
still-close pairing of giant Jupiter (the brighter) and Saturn,
high to the south in early evening, both still set within the
confines of Taurus (not to mention the morning appearance of Mars,
which is now rising around 1:30 AM).
The north polar sky begins to turn our thoughts toward spring. At
8 PM, Perseus is nearly overhead or to the north for most in North
America. While Cassiopeia falls to the northwest, and the Little
Dipper rides beneath the pole below Polaris, the Big Dipper in Ursa
Major begins its majestic rise in the northeast, led by the four-
star "Bowl." Watch as it climbs ever higher as the Earth rounds
the Sun and the seasons advance. To the south, Orion the Hunter
still dominates, below him a distorted box that makes Lepus the
Hare, and below that the small flat triangle of Columba, the Dove.
STAR OF THE WEEK. MAIA (20 Tauri). The Pleiades, the Seven
Sisters star cluster (one of two naked eye clusters that belong to
Taurus, the other the Hyades), twinkle high in northern hemisphere
autumn and winter skies, while shining closer to the horizon in the
skies of southern hemisphere spring and summer. Maia, a proper
name, is one of the seven mythical daughters of Atlas and Pleione.
Shining at bright third magnitude (3.87) from a distance of 385
light years, she ranks fourth brightest after Alcyone, Atlas, and
Electra. Except for Alcyone (Eta Tauri), the Pleiades' stars carry
only Flamsteed numbers, Maia number 20 in the west-to-east parade
of numbered naked-eye stars within the celestial Bull. A blue-
white class B (B8) giant star, Maia radiates 660 times more energy
than does the Sun from a warm surface with a rather uncertain
temperature of 12,600 Kelvin. Its radius of 5 1/2 times that of
the Sun gives it true giant status, although the giants in these
hotter stars are nowhere near as large as their cooler orange
cousins like Arcturus and Aldebaran (which lies in front of the
Hyades). As a giant, Maia either has shut down its internal
hydrogen fusion or will do so very shortly, its mass of a bit over
four times that of the Sun giving the star a destiny as a massive
white dwarf. Like the other stars of the cluster, Maia is involved
with the Pleiades reflection nebula that peaks around Merope. Maia
appears to be a relatively slow rotator, and as such has a fairly
quiet atmosphere. As a result, different kinds of atoms drift
downward under the pull of gravity, whereas others are lofted
upward by radiation, the effects making Maia one of the "mercury-
manganese stars," in which these two and other chemical elements
are greatly enhanced (manganese in Maia up by a factor of 160
compared with hydrogen). The star also has a bit of a curious
history. Fifty years ago, the great astronomer Otto Struve
suggested that Maia was slightly variable, with a period of a few
hours. It thence became the prototype of a whole class of "Maia
variables" that included Pherkad (Gamma Ursae Minoris) and that
were in an otherwise stable realm of temperature and luminosity.
Astronomers have argued since then about the reality of the class.
Only recently has the issue been put to rest, when the prototype
(and some others) were found to be stable and not varying at all
(though others in the purported class do vary for other reasons).
****************************************************************
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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