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[ISTATALK-L] Skylights
Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, December 15, 2006.
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.
Support science literacy by joining the Astronomical Society of the
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1-415-337-1100, then press 1.
"Astronomy: Stars, Galaxies, and the Universe," an audio course on
CD with 100 page study guide narrated and written by Jim Kaler, is
available from Recorded Books.
"Vault of the Heavens: Exploring the Solar System's Place in the
Universe," an audio course on audio CD with 100 page study guide
narrated and written by Jim Kaler, is available from Barnes and
Noble.
NEW! The Cambridge Encyclopedia of Stars.
Our Moon fades away in its waning crescent phase during most of the
week, getting closer and closer to the eastern dawn horizon until
Wednesday the 20th, when it passes new and makes its transition to
the evening sky. The evening of Thursday, December 21st, you might
get your first glimpse of the slim waxing crescent in southwestern
twilight shortly after sunset. On Monday the 18th, the crescent
passes south of Jupiter, then at almost the same time both Mars and
Scorpius's Antares, but in the afternoon and evening, with the
planets out of sight.
Overwhelmed with the light of dawn, Jupiter and Mars are pretty
much out of sight anyway, although a dedicated observer with a
clear eastern horizon might still spot them, Jupiter leading the
way, Mars much fainter. And too bad, since on Monday the 18th,
Mars will pass north of its stellar look-alike and namesake,
Antares (the name meaning "like Ares," the latter the name of the
Greek god of war). Saturn remains the planet of choice, rising
ever-earlier, this week around 9 PM. Look for it just to the east
of Regulus in Leo, the separation slowly increasing, the planet now
in retrograde motion. At the other extreme of planetary visibility
is Pluto, which passes conjunction with the Sun on Monday the 18th.
The biggest planetary news is of Earth. At 6:22 PM on Thursday the
21st (7:22 EST, 5:22 MST, 4:22 PST), the Sun crosses the Winter
Solstice in Sagittarius and formal astronomical winter begins in
the northern hemisphere (summer in the southern, the celestial
point clearly named by northerners). On that date, the Sun will
lie as far south (and for northerners as low) as possible (23.4
degrees south of the celestial equator), and northern days will be
shortest. It then also rises and sets at its extreme southeasterly
and southwesterly points, and will pass overhead at the Tropic of
Capricorn. Though beginning to move slowly north, days will
continue to get colder as it will take the Sun some time to get
high enough for its heating power to gain the upper hand on the
cooling ground and waters.
As evening twilight draws to a close, look just west of south about
halfway up the sky to find the "Y" of stars that makes the heart of
Aquarius, the Zodiacal Water Bearer, most of the constellation
sprawling to the southeast of it, though two fairly bright stars
(Sadalmelik and Sadalsuud, respectively Alpha and Beta Aquarii) lie
to the west. Down and to the right is Capricornus (the "Water
Goat," while up and to the left is Pisces (the Fishes), the three
marking an ancient "wet quarter."
STAR OF THE WEEK: CHI AQR (Chi Aquarii). There is a fascination
with stellar limits, with extremes, with looking at the hottest
blue stars of class O or at the other end, at the reddish stars of
class M. Even though the colors are pale when compared to those of
the spectrum, they still stand out in a dark sky, as we can witness
by comparing Betelgeuse with the rest of Orion's family. All
constellations harbor some reddish stars, and Aquarius is no
exception. Though fifth magnitude (5.06) and thus relatively faint
to the eye, the reddish color of Chi Aquarii (a class M3 giant with
a temperature of only 3670 Kelvin) is obvious in binoculars or
through the telescope. The irregular variability of some 0.35
magnitudes (from 4.9 to 5.25) and relatively cool type are the
first clues that Chi Aqr is not just a giant, but one in an
advanced state of evolution. From a goodly distance of 640 light
years, the star shines with a luminosity of 1500 Suns, which leads
to a radius 96 times that of the Sun, or 0.45 Astronomical Units,
nearly half the size of the orbit of the Earth: a giant indeed.
Direct measure of angular diameter via interferometer gives a
larger radius of 0.60 AU. Situated just south of the ecliptic, Chi
Aquarii is periodically occulted by the Moon. From the time it
takes the lunar disk to cover the star, we derive a yet larger
radius of 0.66 AU, the discrepancies probably resulting from a
combination of errors in distance and in the estimation of
temperature and thus of the amount of infrared radiation that
factors into luminosity. Though difficult to estimate, the mass
seems be around twice that of the Sun. Most likely, the star is
brightening as a giant for the second time, now with a dead carbon-
oxygen core (the first brightening done with a dead helium core).
Such stars are commonly unstable and variable as they prepare to
slough off their outer envelope before adding to the company of
white dwarfs that flock around us.
****************************************************************
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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