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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 2009
Greetings from Minnesota Starwatch for September 2009.
Early in the evening, we are looking toward the center of our Milky Way Galaxy in the constellation of Sagittarius. 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. 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. 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 that is 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 at this point.
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 third 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, and has been 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.
September will bring some interesting planet-watching. Mercury will be low in the western horizon at twilight during Early September. Its greatest elongation on September 1 is only 22 degrees, so you will have to have a clear horizon and look for it early in the month. Venus will appear in the east as a morning star, rising about two hours after Mars on September 1st. On September 4th, Saturn's rings will disappear as they become edge on to Earth, an event unavailable to Earth observers since Saturn will be behind the Sun at the time! Jupiter will be up all night long throughout September, and will be the "star of the night" on September 2-3 when, from 11:43PM to 1:29 AM CDT Jupiter will look through a small telescope as if it has no moons at all. Callisto and Io will be behind Jupiter or in eclipse by Jupiter's shadow, and Europe and Ganymede will be in front of Jupiter. Jupiter's moons hide all at once like this only several times a century.
Speaking of tilt, the Autumnal, or fall, equinox on September 23 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 22, will dip steadily lower in the sky at noon as fall progresses toward winter. In the southern hemisphere, the effect is reversed, and September 22 signals the onset of spring.
The full moon of September 4th is called the "Full Corn Moon" and sometimes the "Fruit Moon." The October 4th full moon will be the "harvest moon," so named because farmers can work by its light late into the night during the peak of the harvest. The harvest moon is the full moon nearest to the equinox.
The alpha and delta Aurigid meteor showers will be active during 1-8 September and September 18-30 respectively. The average zenithal hourly rates (ZHR) for these showers, best seen in the wee hours after midnight, are disappointingly small (7 for the alphas, 3 for the deltas). They do produce excellent bursts in some years, with ZHRs of 30-40 being recorded in 1935, 1986 and 1994. Since full moon occurs on September 4 and October 4, moonlight will make observations of these showers somewhat problematic this year.
The Department of Astronomy's summer Universe in the Park program wraps up during the month of September with events at the Tamarack Nature Center, Afton State Park, and William O'Brien State Park. Events will be held on Friday and Saturday nights and will begin with a short presentation on one of a number of astronomy topics ranging from the Solar System to the History of the Matter, followed by telescope viewing of the night sky (assuming cooperative weather, of course). The program is free and open to the public, so feel free to bring your friends! More information can be found at www.astro.umn.edu/outreach/uitp/.
September also herald's the start of the Astronomy Department's Public Viewing Nights at the Tate Physics Building on the Twin Cities Campus. Weather permitting, each Friday night, starting on September 18th, astrophysics graduate students will open up the dome on the roof of the Physics building to observe the night sky with a 10.25" refracting telescope built in the late 1890's! This is an excellent opportunity to see the night sky, learn about historical and present day telescopes, and ask astronomy questions of (semi-)professionals. More information can be found at www.astro.umn.edu/outreach/pubnight.
The Como Planetarium in St. Paul's Como Park offers limited star shows. For more information, call (651) 293-5398 or check their website at www.planetarium.spps.org. If you're interested in how you can help build the new Minnesota Planetarium, please call 612-630-6151 or visit www.mplanetarium.org.