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Northern Rockies Skies for October

September 28, 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.

You have been watching the summer triangle over the past few months and although it is prominent in the western skies, it will be gone for another year. This triangle of bright stars (Vega, Deneb and Altair), together with the great square of Pegasus (the winged horse) is our sign that autumn has arrived and winter unfortunately is not too far behind.

Jupiter rises after 9 p.m. and will remain in sight all through the night. It is at its closest approach to the Earth and so it should be very bright and with good binoculars or a small telescope quite a pleasant sight to study. Around 2 a.m. Mars rises and is present in the early morning. Venus is an "evening star" and very close to the sun so it may be difficult to spot.

This month offers another opportunity to view the Orionids meteor shower, which peaks around Oct. 21. You should see about 20 meteors per hour; it is best viewed after midnight toward the east in the direction of Orion.

October, 2011 Interest-- Stars: VII:  Giants
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As we described last month, stars fusing hydrogen into helium are said to be on the stellar "main sequence." Such stars are the most numerous. A giant star is much rarer, and much larger and brighter than a main sequence star of similar temperature. However, because the surface area of giants is larger than that of most main sequence stars -- roughly 100 to 10,000 times larger -- giants produce higher luminosities commensurate with their size and therefore can be seen from much larger distances. Thus amongst the stars we see with the naked eye, giants are overrepresented compared to their actual numbers.

Giant stars, often with surface temperatures that make them appear red or orange, have exhausted the hydrogen supply in their cores and have begun to fuse helium instead. Helium fusion occurs at higher temperatures and inflates these stars to their giant sizes.

A different kind of giant -- very massive, still fusing hydrogen and on the main sequence -- is referred to as a blue supergiant, also large and bright. Blue supergiants evolve in surface temperature and color into red supergiants as they exhaust their hydrogen and move off the main sequence.

Two prominent examples of giant stars can be found in the constellation Orion. The red supergiant Betelgeuse in the upper left of Orion lies at a distance of 640 light-years (1 light-year equals six trillion miles), and has a diameter roughly the size of Jupiter's orbit (almost a billion miles).

Rigel, a blue supergiant, lies in the lower right of Orion. Its distance is 770 light-years and its diameter smaller, about half the size of Venus' orbit (130 million miles). Betelgeuse is rated as the eighth and Rigel as the sixth brightest stars in the whole sky. Betelgeuse's luminosity varies on a timescale of several years due to contraction and expansion of its atmosphere effectively changing the size of the star. Rigel varies less so and therefore Betelgeuse, although infrequently, can be brighter than Rigel.

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

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