Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 19, 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.
This week holds the new Moon, passed the morning of Wednesday, the
24th. Six hours later, it passes apogee, when it is farthest from
the Earth. Before that date you can see the earthlit waning
crescent in the morning hours to the east before sunrise, and
afterward the reversed waxing crescent in the west after sunset.
The Moon will be technically visible in western twilight the night
of Thursday, the 25th, nearly in conjunction with (and below) the
planet Mercury, but both will be quite difficult to see. By the
night of Friday, the 26th, however, the slim lunar crescent (now
well up and to the left of Mercury) will be quite visible.
Mercury's visibility will improve until early next week, when it
reaches greatest eastern elongation from the Sun.
Earlier in the week, on Monday the 22nd, Mercury passes close
conjunction with Uranus, a quite-invisible event. At the end of
the week (on Thursday the 25th), Uranus's near-twin, more-distant
Neptune, finally passes conjunction with the Sun to become a
morning object. Uranus will follow in early February. Though
Mercury will be, as usual, hard to find, Venus will not, as it
continues to blaze away in the southwestern evening sky.
In a highly unusual coincidence, both Jupiter and Saturn cease
their westerly retrograde motions not only the same day (Thursday
the 25th), but within about an hour of each other. It would be
difficult to figure how often this happens without running actual
orbital calculations, but it must be once in hundreds, if not
thousands, of years. Odder still, Jupiter ceases retrograde first
even though it is the more-easterly planet (ordinarily, the more-
westerly ones finish first), a result of Jupiter's being closer to
us than Saturn (4.4 as opposed to 8.6 astronomical units, the AU
the average distance between the Earth and the Sun). Both planets
will thereafter begin to move in their normal easterly direction
against the background stars (for now, of Taurus), Jupiter pulling
away from the ringed planet as it heads for the next constellation
of the zodiac, Cancer.
Taurus (Jupiter and Saturn's temporary home) is at its best in the
early evening around 8 PM. Look for the vee-shaped Bull's head
made by the Hyades cluster, the Seven Sisters (Pleiades) up and to
the right. Aldebaran, the Bull's orange eye, is only situated in
front of the Hyades and is not actually a part of it. Taurus is
one of the few constellations that link to another, the northern
horn (which extends northeast of Aldebaran) tied to bright Auriga,
the Charioteer, which with bright Capella, stands above Orion. The
other linking constellations are northern autumn's Andromeda with
Pegasus and summer's Ophiuchus with Serpens.
STAR OF THE WEEK. KEID (Omicron-2 Eridani, or 40 Eridani). An
unassuming star, Keid (Omicron-2) of Eridanus (and more commonly
known by its Flamsteed number, 40 Eridani) seems to play second
fiddle to its somewhat brighter neighbor Beid, Keid the Arabic "egg
shells" of Beid, "the eggs." The two stars are not a true binary,
Beid seven times farther than Keid. Keid, however, is by far the
more interesting star. A modest faint-fourth magnitude (4.43) to
the eye, Keid is a triple star, famed not for Keid itself but for
the much fainter companions. The 67th closest star and the 50th
closest star system, Keid lies a mere 16.5 light years away. Much
farther and the cool class K (K1) ordinary dwarf (Keid-A, one of
the very few visible to the naked eye) would not be visible. Other
than proximity, Keid A has little to offer but a cool temperature
of 5100 Kelvin, a low luminosity of 0.4 times that of the Sun, and
a mass around three-fourths solar. Such stars abound in space, but
they are so faint that few can be seen without a telescope. A
little over a minute of arc (83 seconds) away, however, and easily
seen with only a small instrument, lies the prize of the system,
Keid-B (rather, 40 Eridani B), a tenth magnitude white dwarf, by
far the most visible white dwarf of all (though Sirius-B is more
famed), its luminosity a mere 0.008 that of the Sun. White dwarfs
are the final products of ordinary solar-like evolution, and are
the spent cinders of the original stars' cores. Ordinary dwarfs
(like the Sun and Keid-A) fuse hydrogen to helium in their cores.
When the hydrogen is gone, the stars become giants and fuse the
helium to carbon and oxygen. The outer envelope is ejected, and
all that remains is the low-mass ultradense carbon-oxygen white
dwarf. Typical white dwarfs are only about the size of Earth and
have extraordinary average densities of a ton per cubic centimeter.
The distance between the white dwarf and the K star is at least 400
Astronomical Units, the orbital period at least 7500 years. More
remarkably, the white dwarf has a companion too, a dim class M (M4)
11th magnitude ordinary (hydrogen-fusing) red dwarf. The two orbit
with a 248 year period, and are now about as far in angle from each
other as they get (around 9 seconds of arc). Analysis of the orbit
shows the pair to average 34 AU apart, the distance changing from
20 AU (about Uranus's distance from the Sun) to 48 AU. From the
orbit, the white dwarf, 40 Eridani B, has a mass a bit over half
that of the Sun, the red dwarf (40 Eridani C) much smaller, 0.16
solar. Curiously the actual luminosities of the two stars are
almost the same, as the cool red dwarf (3300 Kelvin) produces most
of its radiation in the invisible infrared, making it seem dimmer
than it actually is. The white dwarf is much hotter (14,000
Kelvin, causing it to radiate much of its energy in the
ultraviolet). The white dwarf must originally have been the most
massive star of the three, with a mass about that of the Sun, to
have evolved first. (Higher mass stars live shorter lives; as a
red giant, 40 Eridani B would have quite dominated the system.)
The K star will be next to go. As fascinating as the white dwarf
may be, the dim M dwarf is not without its own charm. Like many of
its cousins, it is a "flare star," one with a magnetic field that
occasionally short-circuits, causing the star to suddenly brighten
all across the spectrum.
****************************************************************
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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