Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 12, 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 week begins with the Moon in its waning gibbous phase. Passing
third quarter the morning of Tuesday the 16th about the time of
sunrise, it finishes the week as a waning crescent. The morning of
Wednesday the 17th finds the Moon to the northwest of Mars (now in
Libra), the morning of Thursday the 18th to the northeast of the
red planet.
The week belongs not to Mars or the Moon, however, but to Venus,
which reaches greatest eastern elongation from the Sun shortly
after it sets the night of Tuesday the 16th. "Elongation" is the
angle that any planet makes with the Sun. Venus, closer to the Sun
than we are, can never get more than 47 degrees from the Sun, its
maximum angle. The planet thus achieves its greatest visibility
this week. Since it is now at the tangent point in its orbit as
seen from Earth, it will (through the telescope) also appear as the
quarter moon does in the sky, one half seen in sunlight, the other
half in darkness. From here on to conjunction with the Sun on
March 29, the planet will appear as an increasingly thinner
crescent. At the same time, it is approaching the Earth and
appearing ever-larger, and thus brighter, maximum brilliancy to be
reached on February 21. Though the angle between the Sun and Venus
will now decrease, since the Sun is moving northward along the
ecliptic and is setting later, Venus will continue to set later as
well until early February, the "evening star" quite dominating the
early night sky.
At the other end of the planetary spectrum, Mercury passes
conjunction with Neptune (Mercury 2 degrees south) on Saturday, the 13th,
the solar glare causing the event to go unseen. In between, the
early evening eastern sky is dominated by Saturn and Jupiter, both
still retrograding (but not for long) in Taurus. The three
planets, Venus, Saturn, and Jupiter, act like a dotted line in the
sky that to a good approximation shows the ecliptic and the path
the Sun will soon follow as northern winter wanes and turns to
spring.
Look to the southeast around 8 PM to see Orion climb the sky. To
the right, on the meridian to the south, winds dim Eridanus, the
celestial River that represents the "River Ocean" of classical
times. From modest Cursa, just northwest of Rigel in Orion,
Eridanus flows west, and then curves south below the horizon of
northern latitudes, ending in brilliant Achernar, which can be seen
only south of 32 degrees north latitude. High above, the Pleiades,
the Seven Sisters star cluster, crosses the meridian as well. Most
eyes see 6 stars, but those with sharper vision might catch 8 or
even more, the view richly enhanced by binoculars.
STAR OF THE WEEK. BEID (Omicron-1 Eridani). In the middle of the
first southerly turning of Eridanus (the River) lie a seeming pair
of fourth magnitude stars, Beid and Keid, which Bayer placed far
down in the Greek alphabet as Omicron-1 and Omicron-2. The part of
Eridanus that contains Azha (Eta Eridani) and stars south, where
the River makes its second southerly plunge, were known to the
Arabs as the "Ostrich's Nest." Outlying Beid refers to "the Eggs"
and Keid to the "Egg Shells," all showing a wonderful celestial
mixture of cultures. Though to the eye they may look like a real
pair, they are very much not. Keid (Omicron-2) is very close, a
mere 16.5 light years away, where as Beid (Omicron-1) lies over 7
times farther, 125 light years distant. To be roughly similar in
apparent brightness, Beid (mid-fourth magnitude, 4.04) must be much
the more luminous. Indeed, it is a white class F (F2) giant, with
a temperature of 7100 Kelvin and a luminosity 28 times that of the
Sun. Though direct measure of mass is not possible (it has no
known companion, whose orbit would determine the mass), its current
luminosity and temperature compared with theory show its mass to be
almost exactly double that of the Sun. The star seems to have
recently left the core-hydrogen-fusing main sequence of stars, and
is now evolving with a dead (for now) helium core that is
surrounded by a shell of fusing hydrogen. With a radius of 3.5
times that of the Sun, its rather high equatorial rotation speed of
100 kilometers per second show the star to rotate in less than 2
days. Beid's real distinction, however, is that it is a notable
variable, a "Delta Scuti" star, the variation discovered only in
1971. Delta Scuti stars are less-luminous versions of the Cepheid
pulsating variables (those like Mekbuda). Unlike simple Cepheids,
however, they pulsate very subtly and rapidly with several periods
at the same time. Beid varies by at most only a few hundredths of
a magnitude (a few percent, invisible to the naked eye) with a
principal period of 1.8 hours, another of about 3.5 hours, and
certainly yet more. Such variations are extremely difficult to
study and are often quite uncertain. As Beid evolves to a cooler
giant, the instability will eventually cease.
****************************************************************
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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