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Northern Rockies Skies for December

November 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.

Directly overhead at sunset is the very distinctive "W"-shaped constellation Cassiopeia. Never setting below the horizon at our latitudes, Cassiopeia represents the Queen of Ethiopia, the "Lady in the Chair."

Just east of Cassiopeia lies Perseus, the Hero or Champion. This term comes from Perseus using the severed head of Medusa to save Andromeda, the chained princess, from a sea monster.

The brightest star in Perseus is Algol, which is actually two stars orbiting each other. Its variable brightness is due to one star eclipsing the other.

Later in the evening on the east horizon we find Taurus (the bull), Orion (the hunter) and Gemini (the twins). Notice a small grouping of stars near Taurus called the Pleiades or the "seven sisters." This is a relatively young star cluster actually containing more than 1,000 stars. After midnight Sirius, the brightest star in the sky, rises in the eastern sky.

Interesting December features: The winter solstice falls on Dec.22; the Geminid meteor shower peaks Dec. 13-14; Venus is the bright evening star seen immediately after sunset; Jupiter can be seen throughout the evening; and Mars rises around 1 a.m. and Saturn at 4 a.m.

December, 2011 Interest: Stellar Death I -- Red Dwarfs Live Forever, Almost

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In the end stars die -- fusion of hydrogen into helium, and on to heavier elements, ceases as the fuels run out. If an object remains after fusion ceases, its status is no longer "stellar."

However, if the star has a small initial mass, less than about 40 percent of our sun (and greater than brown dwarf masses, 7.5 percent of solar), its lifetime can be very long. The rate of production of energy by fusion in the star's core is roughly proportional to its mass raised to the power 3.5; hence red dwarfs burn up their hydrogen very slowly compared to our sun.

At the resulting lower luminosities, 1/10 to 1/10,000 that of the sun, their hydrogen supplies last for hundreds of billions to trillions of years. The best estimate for the age of the universe is about 13.7 billion years. Thus the smallest red dwarfs would seem to live "almost forever" -- at least a great many times the universe's age. In comparison the sun's expected total lifetime is about 10 billion years, twice its current age.

While red dwarfs are not very luminous, they are plentiful -- about two-thirds of all stars are red dwarfs. Also, because they are dim, red dwarfs are much more easily detectable and inspectable at nearby distances. In fact, some red dwarfs that astronomers study are so near, planets have been detected orbiting them. Some of these planets could be inhabited by life, except for problems that arise in such systems.

Red dwarfs tend to generate variable starspots and stellar flares, which make for variable luminosity received by their planets -- perhaps too variable for some kinds of life. Also, like our moon that keeps one face aligned toward Earth, similar "tidal locking" is expected to occur in that portion of a red dwarf's planetary system close enough to the star to otherwise support life. Without planetary rotation the hot and cold sides of the planet may be too inhospitable for life.

The eventual stable fate of red dwarfs -- and most other stars after they cease fusing hydrogen and helium -- is to become a white dwarf, to be discussed next month.

Have a great holiday season.

To view sky chart, click here.

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