Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, January 26, 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 spends most of the week in its waxing crescent phase,
finally reaching first quarter on Thursday, February 1 a few hours
before its mid-day rise. The night of Saturday the 27th, the lunar
crescent will be seen a few degrees below brilliant Venus, which in
the Moon's absence continues to dominate the early evening sky.
Venus passed its greatest eastern elongation from the Sun over a
week ago, though little change can yet be seen. Now it is
Mercury's turn, greatest eastern elongation taking place on Sunday
the 28th, making this week a good time to find the two "inferior
planets," those closer to the Sun than we are, stretched out from
the Sun at maximum advantage. Find a clear horizon and look to the
west southwest in dusk to find Mercury, the little planet quite
bright but washed out by the thick air near the horizon and by
fading twilight. Because the sky looks so much like a bowl above
our heads, it is difficult to appreciate the effect of the Earth's
atmosphere. When we look right on the horizon we actually see
through 38 times more air than when we look directly overhead. As
a result, stars and planets appear much fainter as they rise and
set than when they are high in the sky.
Thickness of atmosphere is no deterrent to Jupiter and Saturn,
however, which are now high to the southeast at the end of
twilight, the two planets brilliant near the Pleiades of Taurus,
leading the eye to appreciate the beauty of the Seven Sisters star
cluster. Jupiter is the brighter of the pair, and after Venus (and
of course the Moon) the brightest body of the current sky. Saturn
is just a bit to the west of it. Both bodies are now in normal
direct motion, Jupiter ever-so-slowly pulling to the east of the
ringed planet.
About the time Jupiter and Saturn set, around 2 AM, Mars rises.
Now smack in the middle of the classical figure of Libra, the red
planet is being overtaken only slowly by the Earth, and will remain
a morning planet until the beginning of April, when it will finally
begin to rise before midnight.
>From mid-northern latitudes, Auriga shines high in the sky above
Orion, Perseus beginning to descend to the northwest. However,
from there to the North Celestial Pole, closely marked by Polaris,
the sky is drab indeed. This area is filled with one of the larger
constellations, and certainly one of the dimmer, the obscure modern
figure Camelopardalis, the Giraffe, its three brightest stars,
which are strung out in a line toward the pole, shining only at
fourth to fifth magnitude.
STAR OF THE WEEK. MEROPE (23 Tauri). There is little in the sky
more attractive than the marvelous cluster of the Seven Sisters,
the Pleiades of Taurus, a bright compact, fairly young "open
cluster" that is filled with bright stars of blue class B. Most
eyes see 6 stars, some 8 (or even more). Seven of the stars are
named for the sisters, who in Greek mythology were the daughters of
the god Atlas and mortal Pleione, who appear there as well. Among
the classic six visible stars is fourth magnitude (4.18) Merope,
which ranks fifth within the striking group. Of the Pleiades, only
Alcyone carries a Greek letter (Eta), the rest designated only by
Flamsteed numbers (Merope otherwise called 28 Tauri). Merope, a
mid-class B (B6) subgiant lies, along with the rest of the cluster,
at a well-determined distance of 385 light years. From its hot,
blue-white 14,000 Kelvin surface, it shines with a total luminosity
(allowing for ultraviolet light) of 630 times that of the Sun. It
is a particularly fast rotator, spinning with an equatorial speed
of at least 280 kilometers per second, 140 times that of the Sun.
Given that the star is 4.3 times the solar size, it makes a full
rotation every 18 hours. The fast rotation affects the star's
surroundings and the spectrum. Like others in the Pleiades crowd,
Merope is an emission-line (Be, class B emission) star, its rapid
rotation flinging out a disk of bright, emitting gas though
Merope's is quite thin compared with others such as Pleione.
Nevertheless, it is sufficiently dense and hot (from shock waves)
to produce observable X-rays. Merope's greatest claim to fame,
however, is not the star itself, but its surroundings. The
Pleiades is enmeshed in a cloud of dusty gas. The stars are not
hot enough to make the gas glow (as they are to make the Orion
Nebula). Instead, the tiny dust grains embedded in the cloud
scatter and reflect the starlight to make the quite-blue Pleiades
reflection nebula. It is at its brightest around Merope, so bright
as to have special names, the Merope Nebula and "IC 349," the "IC"
standing for Index Catalogue, a large addendum to the standard
"NGC" or" New General Catalogue" of celestial objects, which in
turn is an updating of the great Herschel "General Catalogue."
(The NGC and IC together contain over 12,000 clusters, nebulae, and
galaxies.) Long thought to be the remnant of the Pleiades birth,
the nebula is instead a chance occurrence, as the cluster is merely
passing through an interstellar cloud. (The cluster actually
leaves a wake as the cloud blows past.) The bright class B stars
of the Pleiades are all relatively massive, Merope (which appears
to be beginning to evolve) containing some 4.5 solar masses. Since
higher mass stars evolve first, the cluster and its stars must be
relatively young, Merope and the other Pleiads a mere 100 million
years old.
****************************************************************
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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