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Astronomy From Your Back Yard - 9/1 to 9/7 2010

By Dave Grosvold

The Last-Quarter Moon occurs at 12:22 PM CDT on Wednesday afternoon. Fortunately, moonset occurs before dusk until early next week so we are lucky to have dark skies in the evening for observations.

On Wednesday and Thursday evenings, Venus, Spica, and Mars form a fairly straight line about 5° long, low in the west-southwest about a half hour after sunset as shown in the illustration. Despite how it appears in the image, Venus is over 175 times brighter than Spica, even though Spica is the brightest star in the constellation Virgo!

Beginning in September, the Great Square, an asterism in the constellation Pegasus has risen well up in the eastern sky after dark. The Great Square appears to be balanced on one corner, the sign of autumn to come. This year, bright Jupiter becomes a helpful landmark, shining to the lower right of the Great Square. The lower left corner star in the Great Square is Alpheratz, which is actually in the constellation Andromeda, rather than Pegasus.

From Alpheratz, follow the stars of Andromeda in a diagonal line down to the left of the Great Square, to Mirach, the second star from Alpheratz in the line. Then follow a line straight up from Mirach to the next brightest star, Mu Andromedae (μ And), and then beyond that to an even dimmer star, known as Nu Andromedae (ν And). Directly above and to the right of ν And, you will find the Great Galaxy in Andromeda, or M31. Also known as the Andromeda Galaxy, at a distance of 2,500,000 light years from Earth, M31 is one of the most distant galaxies in the night sky that can be seen from Earth with the naked eye.

If you find the right combination of a dark night, good transparency, and you've taken the time to get your eyes dark-adapted (1.5 hours or more,) you should be able to see this wonder of the heavens as a dim smudge just above ν And. A good pair of binoculars will bring the smudge out to a very prominent hazy streak across the field of view.

M31 is actually 6 times larger than the Full Moon when viewed through a large telescope, but the surface brightness is so low that you only see the center of the galaxy with the naked eye or binoculars. With a small telescope, see if you can pick out the two companion galaxies, M32 and M110. In a medium-sized telescope (8" - 12",) you should be able to see a wonderful image of the galaxy, including M32 and M110. It's almost as good as a photograph!

Like M31, M33, the Triangulum Galaxy, Is another great deep sky target, and it lies just below Mirach in Andromeda, and above and to the right of the constellation Triangulum. M33 is also visible to the naked eye in the right conditions. The Triangulum Galaxy is somewhat more distant from us than M31 at 3 million light years. That's a long way to see without any kind of optical aid!

Although both are spiral galaxies, the Triangulum galaxy presents itself to us face-on, so we can see the spiral structure much more prominently than we can with M31, which is situated at an angle to us. Along with the Milky Way and the Andromeda Galaxy, the Triangulum Galaxy is part of the Local Group of galaxies, being the third largest. The Local Group also includes about 30 smaller galaxies. The galaxies in the Local Group are gravitationally bound together, so as a whole, they move through space as one, even though the individual members are moving around within the group - sort of like a swarm of bees!
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