Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, September 14, 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.
To those who have suffered losses, my heartfelt sympathies.
The Moon passes through its new phase this week on Monday the 17th,
rendering the stars wonderfully visible throughout the night
(weather and stray lights permitting). In the morning sky, the
waning crescent puts on a show, when on Saturday the 15th it passes
through the Sickle of Leo, and will be found down and to the left
of brilliant Venus, and up and to the left of the star Regulus.
The waxing crescent will then begin to make its evening appearance
in the western sky the night of Tuesday the 18th. The day before
the new phase, the Moon passes perigee, when it is closest to the
Earth at a distance of only 358,130 kilometers (222,530 miles).
Mercury makes an appearance this week, as it passes greatest
eastern elongation relative to the Sun also on Tuesday the 18th
(when it is down and to the left of the lunar crescent).
Unfortunately, the evening ecliptic lies near its flattest against
the horizon this time of year, so the little planet will appear low
and difficult to see. On Thursday the 20th, Mercury will pass only
a degree to the south of Spica in Virgo. The morning sky again
puts on a better show when Venus passes only half a degree (the
angular diameter of the Moon) north of Regulus, also on the 20th.
We thus have the odd and rare situation where the two stars closest
to the ecliptic -- which hold the autumnal equinox between them --
are being visited by the two inner planets, and not just on the
same date, but at nearly the same moment, about 3 PM Central
Daylight time.
The outer planets are not to be ignored, however. In the evening,
Mars glows in the southwest. Moving easterly against the
background, the reddish planet has shifted into the confines of
Sagittarius. In the morning, Jupiter, second only to Venus in
brilliance, shines high above that planet, with fainter but still-
prominent Saturn higher yet, the two giant planets still in fairly
close proximity.
In the evening, the Summer Triangle, made of Vega, Deneb, and
Altair, are at their best, Vega nearly overhead for those at mid-
northern latitudes as twilight fades. To the north of Vega, find
the two stars that make most of the head of Draco the Dragon, the
pair appearing as the Dragon's baleful eyes. Winding toward the
north, Draco's tail eventually passes between the Big Dipper in
Ursa Major (now going into its autumn hibernation beneath the pole)
and the Little Dipper in Ursa Minor, which in early evening stands
high upon its curved, though faint, handle. The handle ends at
Polaris, which closely marks the North Celestial Pole, about which
the northern sky seems to turn.
STAR OF THE WEEK. NODUS SECUNDUS (Delta Draconis). Not a terribly
romantic name, Nodus Secundus, Draco the Dragon's Delta star, is
less-well-known from Arabic as Al Tais, the Goat. Better to use
the Latin "Nodus," as it more refers to its host constellation,
where it marks the second (Secundus) of four loops in the Dragon's
long winding mythological body. Nicely visible at mid-third
magnitude (3.07), and one of the brighter stars of the far north,
it lies only a bit more than 20 degrees from the sky's north pole
(marked by Polaris), and is circumpolar (always visible) from
everywhere north of the Tropic of Cancer. Within its daily circle
about the North Celestial Pole, it is exceeded in brightness only
by Polaris, Kochab, and Pherkad, the Alpha, Beta, and Gamma stars
of Ursa Minor. Physically, Nodus number two is a class G (G9,
almost class K) giant star, though since it is a bit warm (4830
Kelvin) for such a giant, not quite so large as many. At a
distance of almost exactly a light-century (100 light years), the
star radiates at a rate of 63 solar luminosities, giving it a
radius 11 times that of the Sun. Its luminosity and temperature
combine to yield a mass almost exactly 2.5 times solar and an age
of 700 million years. Nodus Secundus is a fine example of a
helium-fusing giant, one that is now converting its core helium
into carbon and oxygen. Sometime in the astronomically near
future, the helium will run out and the star will brighten as it
prepares to slough its outer envelope and become a mid-mass white
dwarf. It is not clear whether Nodus-2 is a single or double star.
Lying 88 seconds of arc away is a faint twelfth magnitude
"companion" about which nearly nothing is known. The seeming
proximity may be just a line of sight coincidence. Yet the two
seem to be regarded as a real double. If so, "Delta Draconis B" is
at least 2700 Astronomical Units from the giant. Nearly 70 times
Pluto's distance from the Sun, the orbital period would be at least
87,000 years. From a hypothetical planet orbiting the giant, the
companion (which from its brightness would be a class M1 dwarf)
would shine redly at roughly the brightness of Venus, while from
the dwarf the giant would shed the light of a half-dozen full
Moons.
****************************************************************
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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