Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, April 6, 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 passes through its full phase this week the night of
Saturday, April 7, somewhat after Moonrise in North America. The
Moon will therefore rise a bit before sunset, but since the full
phase is passed while the Moon is visible, it will also set a bit
after sunrise. The night of Thursday the 12th, the waning gibbous
Moon will pass to the north of Mars and make a handy reference with
which to find the red planet.
Mars now officially moves into the evening sky, rising just before
midnight (1 AM daylight time). It is also very close to bottoming
out in its current journey along the ecliptic path, having recently
entered Sagittarius. Nearly 23 degrees south of the celestial
equator, the planet rises well into the southwest. During most of
this month, as Mars crosses -- transits -- the celestial meridian
to the south, Venus rises in the northwest, but after the beginning
of twilight, so the planet will appear low in the sky. On Friday
the 6th, Venus passes 10 degrees north of Mercury, the little
planet so close to the Sun that it is not possible to see from
North America.
The evening sky still provides the best planetary view, with
Jupiter and Saturn glorious to the west among the stars of Taurus.
Saturn now sets around 10:30 (Daylight Time), Jupiter an hour
later, the two slowly drawing apart. Slowly being overtaken by the
Sun, the pair will, one at a time, disappear into twilight during
the month of May.
Daylight time has just been launched over most of the US and
Canada. The time of day refers to the "hour angle" of the Sun
(with the effects of orbital eccentricity and axial tilt smoothed
out), the angle the Sun makes with the celestial meridian, each
hour past noon corresponding to another 15 degrees. Time is really
longitude dependent. Standard time is local time at specified
meridians. Technically, everyone in a band 15 degrees wide
centered on that meridian keeps the same time (though the bands are
wildly shifted for political and social reasons). If you are west
of your meridian, the Sun seems to set late, while if east it will
seem to set early. In daylight time, we shift to the time zone to
the east of us, and as a result, the Sun seems to set an hour later
than "normal."
Dim Cancer, flanked by Gemini to the west and Leo to the east,
crosses the meridian around 9 PM. Above it is even dimmer Lynx,
below it the circlet that makes the head of Hydra, the Water
Serpent. Much farther down is Pyxis, the Compass, all a challenge
and nearly impossible to see under the glare of full Moonlight.
STAR OF THE WEEK. TANIA AUSTRALIS (Mu Ursae Majoris). Our Ursa
Major, the Greater Bear, contains remnants of ancient Arabic
constellations, the best-known example the star Alkaid, which
refers to the leader of the daughters of a funeral bier. Southwest
of the Dipper's bowl lie three obvious pairs of stars that
represent the bear's paws, but to the Arabs were the tracks of
leaping gazelles. The middle pair is the "second leap," from which
comes the name "Tania"(for "second"). In the multi-cultural mix of
constellation lore, the northern one received the Arab-Latin name
Tania Borealis, the southern Tania Australis. Bayer assigned the
three "leaps" ordered Greek letters, Tania Borealis receiving
Lambda, Tania Australis Mu. The two make a lovely contrast, Tania
Borealis a white class A subgiant, Australis a fairly rare (for
naked eye stars) red class M (M0) giant. Tania Australis shines at
mid third magnitude (3.05) from a distance of 250 light years
(double the distance of Borealis, the two only a line-of-sight
coincidence). When we account for infrared radiation from a 3950
Kelvin surface, Tania Australis is found to have a luminosity 850
times that of the Sun, which leads to a radius 62 times solar (0.28
Astronomical Units, three-fourths the size of Mercury's orbit).
Having used its core hydrogen, Tania Australis seems to be
brightening along the "red giant branch" with a contracting helium
core. Before long, the helium will fire up to fuse to carbon, and
the star will dim some and stabilize as a class K giant. Tania
Australis is an unresolvable binary, the companion (known only from
spectroscopic observations) circling the M giant every 230 days at
a distance of at least 1.5 Astronomical Units, suggesting a
combined mass over 9 times solar, double that expected on the basis
of luminosity and temperature. Tania Australis proper (ignoring
the companion) is also a rare "hybrid star." Magnetically active
stars like the Sun blow a relatively fast but thin wind from their
surfaces. Larger giants blow slower, but much thicker winds.
Hybrid stars seem to blow both. Though the star is cooler than the
dividing line at which stars seem to lose their X-rays, Tania
Australis perversely still seems to radiate them. Alas, the
southern star of the second leap is not very well understood.
****************************************************************
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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