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Starwatch NewsletterMinnesota Starwatch is a newsletter describing the night sky in the Midwest.
It is updated monthly, and is produced by the |
Minnesota Starwatch for September 2007
Greetings from the University of Minnesota, Department of Astronomy. This is Starwatch for September, 2007. Early in the evening, we are looking toward the center of our Milky Way Galaxy in the constellation of Sagittarius. Roughly an hour after sunset on the first day of fall, September 23, the Galactic center itself will be elevated about 15 degrees off the horizon just to the west of due south. The arms of the Galaxy can be seen as a faint band of light reaching up from the horizon, through the Galactic Center, and far to the north through the constellation Cygnus the Swan (alternatively known as the "Northern Cross"). This faint band of light, called the Milky Way, is the region of the Galaxy where normal stars like the sun form and spend their lives. An interesting image of the Milky Way and a telescope using Laser Adaptive optics (which compensates images for atmospheric turbulence known by astronomers as "seeing") can be found on the Gemini Observatory site: http://www.gemini.edu/files/docman/websplash/ws2006-27/fig1_med.jpg.
Our Galactic Center contains a hidden central engine that shines with enormous luminosity because of gas falling into a dust-obscured black hole that may be a million times as massive as the sun. Radio and infrared astronomers can observe this engine because the long wavelengths of infrared and radio light can penetrate the dust barrier. Many other galaxies and distant Quasi-Stellar Objects (QSOs) have black-hole central engines even more energetic than the one at the center of the Milky Way. The material falling into these massive black holes gets so hot that it emits X-rays. NASA's Great Observatory, the Chandra X-ray Observatory can observe x-rays from particles up until the last second before they fall into a black hole (see for example the Chandra image at http://chandra.harvard.edu/photo/2002/gcenter/), and it is used to study the nature of these distant energy sources, some of which are more than 10 billion light years away. A light year is 6000 billion miles, the distance that light traveling at 186,000 miles per second can travel in a year's time.
The bright red supergiant star Antares in the constellation of Scorpios can be seen setting toward the southwest. Its red color is evident even to the naked eye, and is a result of the fact that it has a greatly expanded cool atmosphere characteristic of stars that have exhausted most of their internal nuclear fuel. When the sun reaches this phase of its life cycle in 5 billion years, it will become a red giant star 10,000 times brighter than the present-day sun, and its 3000 degree Centigrade outer envelope will extend almost to the orbit of Mars! The Earth, of course, will have been vaporized by this point.
September will bring some interesting planet-watching. Venus will appear as a morning star, increasing in brilliance and prominence in the sky during the month. Early September also marks the passage of Earth through the dust trail of comet C/1991 N1, which give rise to the Aurigid meteor shower. Up to 200 meteors per hour, with magnitudes in the range of -2 to +3 (relatively bright streaks) are anticipated (see the NASA site http://science.nasa.gov/headlines/y2007/08aug_aurigids.htm). The Piscids (peaking on September 20), and the Sextantids (peaking on September 27) showers also will delight with celestial fireworks in the autumnal skies.
The Autumnal, or Fall, equinox on September 23 at 09:51 GMT represents the time when the side of the earth facing the sun is at exactly neutral tilt at noon so that the sun appears superimposed on the celestial equator. At equinox, the lengths of day and night would be equal if the sun was a point source and if the earth had no atmosphere. Daylight time actually exceeds night-time by a few minutes because atmospheric refraction causes the sun to be visible when it is actually physically below the horizon at sunrise and sunset. The large angular diameter of the sun also increases the daylight time by a few minutes. As the Earth travels further in its orbit about the sun, the daytime side of the earth will begin to tilt away from the sun in the northern hemisphere. The result is that the sun, on the equator at noon on September 23, will dip steadily lower in the sky at noon as fall progresses toward winter. In the southern hemisphere, the effect is reversed, and September 23 signals the onset of spring.
For fun Astronomy Outreach programs check out our Public Outreach link, or if you're interested in how you can help build the new Mi nnesota Planetarium, please call 612-630-6151 or visit http://www.mplanetarium.org.
The Minnesota Starwatch is produced by the University of Minnesota Astronomy Department.