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[ISTATALK-L] Skylights
Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 14, 2005.
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.
See "The StarGazer" at a planetarium near you: visit
http://www.astro.uiuc.edu/~kaler/sg.html
Interested in Astronomy and Astro Education? Join the Astronomical
Society of the Pacific (an international organization) to get the
outstanding astronomy magazine Mercury and a variety of other
benefits. Call 1-415-337-1100, then press 1.
"Vault of the Heavens: Exploring the Solar System's Place in the
Universe," an accessible astronomy course on audio CD with 100 page
study guide narrated and written by Jim Kaler, is now available at
Barnes and Noble in stores and on line.
The Moon begins our week in its waxing gibbous phase as it heads to
full the night of Monday, January 24, the phase actually taking
place the next morning, that of Tuesday the 25th about the time of
Moonset. The near-full-Moon will thus rise just before sunset the
night of the 24th.
This full phase is particularly far north. The Moon's orbit is
tilted by about 5 degrees to the plane of the orbit of the Earth
about the Sun. As a result, the Moon can be as much as 5 degrees
north of the ecliptic (the apparent path of the Sun against the
background of the stars) and 5 degrees south of it. At this full
Moon, which will be just to the east of Castor and Pollux in
Gemini, the Moon is nearly at the full extent of its tilt to the
north and well above the ecliptic plane. The effect is made
obvious by the planet Saturn, which currently lies practically
smack on the ecliptic, and will appear 5 degrees to the south of
the Moon the morning of Monday the 24th. Two days before full, the
Moon passes apogee, where it is farthest from the Earth. The
remainder of the week sees our companion in its waning gibbous
phase.
Venus and Mercury, still close together, are effectively gone from
the morning sky. Bright as it is, rising only in mid dawn, Venus
is now near impossible to see because of the brightening sky. The
morning sky is thus left with Mars, now well east of Antares,
Jupiter (just to the west of the meridian at dawn), and Saturn,
which is near to setting at that hour. The early evening is host
to Saturn alone, the planet tucked nicely into Gemini, already
risen as evening comes to a close, and transiting the meridian
about 11:30 PM. Just as Saturn transits, Jupiter rises, the giant
planet just to the northwest of Spica in Virgo, the ecliptic just
passing between the two. Be sure to keep following the news about
Saturn's satellite Titan.
Comet Machholz, visible to the naked eye, but really best in
binoculars, is now trekking northerly through the bright stars of
Perseus. The night of Saturday the 22nd, it will pass just to the
west of Mirfak, Alpha Persei.
Though the stars of winter are now full upon us, do not forget the
so-called stars of autumn, which are still on full -- and best --
view in early evening. As the sky darkens in mid-northern
latitudes, Cassiopeia is nearly overhead and to just to the west of
the meridian, while Perseus is just to the east of it. The line
between the two passes to the south first through extreme eastern
Andromeda, then down through Triangulum, Aries (both simple
triangles), and then on to the more-or-less circular head of Cetus,
the Sea Monster, who in myth was slain by Perseus in the rescue of
Andromeda.
STAR OF THE WEEK: KAPPA-1 CET (Kappa-1 Ceti). From studies of the
apparent motions of sunspots across the solar disk, we see that the
Sun rotates "differentially." At the equator, it takes 25 days to
make a turn, while at higher latitudes it takes closer to 30, the
shearing gases coupled with convection helping to create solar
magnetic fields and the solar activity that leads to terrestrial
aurorae. Other solar type stars exhibit similar activity and
should rotate differentially too, but how can you tell when a
distant star is but a point and you cannot see the surface? Kappa-
1 Ceti -- and an extraordinary orbiting satellite -- lead the way.
Kappa-1, in far eastern Cetus off the Sea Monster's head, is an
intriguing, heavily studied, fifth magnitude (4.83) class G (G5)
solar-type hydrogen fusing dwarf that lies only 29.9 light years
from Earth. (Kappa-2, half a degree to the northeast, is an
unrelated G8 giant nearly 10 times more distant.) Kappa-1's
measured temperature of 5690 Kelvin and luminosity of 85 percent
that of the Sun lead to a radius of 0.96 solar, a mass of about 0.9
solar, and an age under 1 1/4 billion years, which allows the star
to give us a glimpse of what our own Sun might have been like in
its relative youth (the Sun now 4.5 billion years old). Though
called "strong lined" (from its strong absorption spectrum), its
metal content is only about 10 percent greater than that of the
Sun. The fascination with Kappa-1 Ceti comes from its activity.
The star has long been known to vary slightly over a period of
about 9.2 to 9.3 days, as rotation swings magnetically active
features (including starspots) in and out of sight. Spectral
indicators of activity also reveal a spot cycle (perhaps similar to
the 11-year solar sunspot cycle) of 5.6 years. Kappa-1 has also
been know to pop "flares" of huge proportion that may be some kind
of super-energetic analogue to those seen on the Sun and that are
caused by the collapse of powerful magnetic fields. A star's
magnetic field is dragged outward by its wind. Since the field is
still anchored at the star, the effect slows the stellar rotation.
Kappa-1's rapid rotation suggests a much lower lifetime of only 650
million years! It was among the first stars to be studied with
Canada's MOST (Microvariability and Oscillations of STars)
satellite, a suitcase-sized package capable of detecting a
brightness change of a mere ten-thousandth of a percent, about
0.000001 magnitude. A month's-worth of observation showed a second
(and dominant) periodicity of 8.9 days of only a few percent in
brightness that must be coming from another large spot (or spot
group) that is closer to (if not on) the star's equator, and is
thus rotating around faster, consistent with differential rotation,
making Kappa-1 the first star other than the Sun for which the
phenomenon has been seen and measured. Given the projected
equatorial rotation speed of 4.64 km/s, the stellar axis is tilted
by 60 degrees to the line of sight and the stellar radius must be
0.95 solar, almost exactly that found above from luminosity and
temperature. More and more we see solar-type behavior among stars,
which in turn will help us understand our own star, which is
crucial to understanding -- and caring for -- our own Earth and
selves. (Much of the information in this story was taken from S.
L. Rucinski et al. in the Publications of the Astronomical Society
of the Pacific, vol. 116, p. 1093, 2004. Thanks to Jaymie
Matthews.)
****************************************************************
Jim Kaler
Professor Emeritus of Astronomy Phone: (217) 333-9382
University of Illinois Fax: (217) 244-7638
Department of Astronomy email: kaler@xxxxxxxxxxxxxx
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)
The StarGazer (a new planetarium show)
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