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Northern Rockies Skies for September

August 30, 2011

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

September is an excellent time to see the changes in the seasonal constellations.

After dusk you are opened up to the summer vision of the Milky Way. With Vega almost directly overhead anchoring the summer triangle, you can see the full extent of the Milky Way stretching to the North via Cygnus the Swan (the Northern Cross) and Cassiopeia, the giant stretched W on the northeast horizon.

At the southern horizon "via latte," the Milky Way, passes through Sagittarius and the Scorpion, with its unique "stinger" outline. As the night continues, you will see the great square of Pegasus. At midnight, the winter constellations start to arise. With Jupiter beaconing their arrival, you will see the Pleiades, Taurus, and then Orion.

At 3 a.m., Orion the Hunter rises toward the southeast. Rising at that time but closer to the northeast is Mars in the constellation Gemini. The Gemini brothers Castor and Pollux form a near straight line with Mars. This month before dawn view the winter constellations and follow Jupiter and Mars.

September, 2011 Interest: Stars VI: The Main Sequence

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Stellar masses are measured in "solar mass" units -- the sun has one solar mass, or two nonillion (2 followed by thirty zeroes) kilograms. The range of stellar masses is roughly from 100 to 1/100 solar masses. The more massive a star, the more luminous it is.

Above this range, more massive stars would be unstable due to the high rates of fusion overcoming gravity and blowing the star apart; and below, less massive stars are not hot enough in their cores to ignite the fusion processes that convert hydrogen into helium. When stars are "born," having condensed from gas in the interstellar medium, and while they continue to make radiant energy by fusing hydrogen, they are said to be on the "main sequence."

In addition to embodying the mass-luminosity relationship, this stellar sequence relates mass to temperature and color:  More massive stars are hotter with core temperatures measured in millions of degrees, and their colors are bluer. The stellar colors span those of the visible spectrum --just as seen in rainbows.

Two energy-generation processes dominate in the cores of stars that are on the main sequence. Stars more massive than two solar masses fuse hydrogen into helium using carbon, nitrogen and oxygen (present in small amounts in stars) as catalysts. The most massive stars have very high energy production rates, and so they use up their hydrogen supply very quickly, living for only a few million years.

Stars as massive as the sun, and less so, fuse hydrogen one nucleus at a time) into helium. The least massive stars produce energy at much reduced rates, and have estimated lifetimes of more than a trillion years. Most stars live most of their lives on the main sequence, fusing hydrogen.

Next time we'll discuss what stars do after they exhaust their hydrogen supply -- how they continue to remain luminous objects until eventual stellar death.

To view this month's sky chart, click here.

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