Skylights, University of Illinois Department of Astronomy.
Astronomy News for the week starting Friday, November 2, 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 fades this week from near full to its third quarter,
reaching the phase about the time of moonset the night of Wednesday
the 7th (or the morning of Thursday, the 8th). As it goes, it will
be seen to the west of Saturn the night of Friday, the 2nd, to the
east of it and closer the night of Saturday, the 3rd. The night of
Monday, the 5th, be sure to watch the Moon play closely with
Jupiter, our satellite less than two degrees north of the giant
planet around midnight.
The two brightest planets in the sky, Venus and Jupiter, are
highlighted this week. (Yes, Mars can get brighter than Jupiter,
but for only a very short amount of time near its favorable
oppositions.) The morning of Friday, the 2nd, Venus stands four
degrees north of Spica, which the Sun has just cleared, meaning
that Venus is now rising rather late, about 5 AM Standard Time,
just after the birth of morning twilight. On the same day, Jupiter
becomes momentarily "stationary," that is, it ceases its normal
forward motion easterly through the stars, and reverses into
retrograde, as the Earth prepares to come between it and the Sun.
Two days later, on the morning of Sunday, the 4th, Mars passes 2
degrees south of Neptune, an event only visible if you have a
telescope, Neptune rather far below naked-eye brightness. Since
Venus and Mercury maintain their close connection this week, the
two less than a degree apart for most of this period, both actually
are seen north of Spica. Go look, and use Venus to find the
smallest inner planet (Mercury), which will be the brightest body
close to bright Venus, both notably bright in morning twilight.
With the Moon now gone from the evening sky, we can look again at
the stars. Even in early evening, the Great Square of Pegasus can
be seen moving high in the southeastern sky. The Square's
northeastern star is part of both Pegasus and Andromeda, which
climbs in streams of stars to the northeast. In the middle of
Andromeda, if you have a dark sky, you might spot the fuzzy patch
of the Andromeda Nebula, which in the early twentieth century was
discovered to be a large nearby galaxy comparable to our own (our
200-billion-star assembly that makes the Milky Way). "Nearby" here
takes on a relative meaning, as this great spiral galaxy, also
called M 31, is two million light years away, the farthest thing
visible to the naked eye. Comparable in distance is another, M 33,
the great spiral galaxy in Triangulum, which under ideal
circumstances (which includes being young!) is also visible to the
naked eye, though just barely. The southern hemisphere contains
two more naked-eye galaxies, the Large and Small Magellanic Clouds
in the constellations Dorado (the Swordfish) and Tucana (the
Toucan), these two requiring the observer's latitude to be well
south of 20 degrees north latitude.
STAR OF THE WEEK. MATAR (Eta Pegasi). Pegasus (the Flying Horse)
is so well known for its Great Square that we sometimes give the
other stars little thought. Coming off the northwestern star of
the Square, Scheat, is a pair of stars that with Scheat make a
rather prominent triangle, the northern one Matar, which Bayer
called "Eta," and which (at mid third-magnitude, 2.95) actually
ranks fifth in brightness rank (ignoring Alpheratz, Delta Pegasi,
which is actually Alpha Andromedae). "Matar," from Arabic, has to
do not with a horse, but with "rain," though just what is unclear,
one source suggesting "lucky rain." At a distance of 215 light
years, Matar is double and may well be quadruple, consisting of a
very unequal pair of pairs, an unbalanced double-double. The
bright naked-eye star is actually a close pair separated on the
average by only three astronomical units (a bit over half the size
of Jupiter's orbit). The brighter, 262 times the luminosity of the
Sun, is an evolving class G (G2) 5100-Kelvin giant with a quiet,
contracting helium core, the fainter a hotter (7800 Kelvin) class
A (A5) hydrogen-fusing solar type dwarf. The measured orbit (its
period 2.24 years) reveals the stars to contain respective masses
3.2 and 2.0 times the mass of the Sun. Ninety seconds of arc away
is a much fainter (ninth magnitude) class G (G5, a bit cooler than
the Sun) star that separates into another pair only 0.2 seconds of
arc (at least 13 astronomical units) apart that take at least 34
years to orbit. That the two doubles are actually related is not
fully known, some say yes, others no, that they are a line-of-sight
coincidence. The luminosity of the dim pair, however, is close to
being right for G stars if assumed to be at Matar's measured
distance, so they are probably a true couple (of couples). If so
the two doubles are at least 6000 Astronomical Units apart and take
a minimum of 170,000 years to orbit. Even at that separation,
however, each would be separable into a double from the other (the
large pair having the combined brightness of 5 full Moons as seen
from the faint pair). The brighter of the bright pair is on its
way to becoming a much larger giant, and will eventually expand to
a radius of a quarter the distance that now separates the two
stars, streams of matter running from the brighter to the dimmer
creating quite a sight from the smaller pair. Eventually the
bright star of the brighter pair will fade to become a white dwarf,
this double perhaps looking something like Sirius does today.
****************************************************************
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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