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Northern Rockies Skies for January

December 21, 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.

After sunset, our cold January evenings provide a transparent and crystal-clear window to our Rocky Mountain skies. We find Venus on the eastern horizon, Jupiter near the meridian, and the most intriguing constellations of all, the winter constellations.

You see the Milky Way arching over the sky from the southeast horizon, through the zenith and landing on the northwestern horizon. Directly overhead is queen Cassiopeia and to its south is Pegasus, the winged horse.

Rising brilliantly on the east are the constellations Taurus the bull; Orion the hunter; the seven sisters, the Pleiades; the Gemini twins, Castor and Pollux; followed by Sirius, the Dog Star and the most luminous star in the sky after the sun.

It may be cold outside but a quick look in the early evening skies brings a warm wonderment from the heavens.

January, 2012 Interest: Stellar Death II -- White Dwarfs.

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The vast majority of stars, about 97 percent, "die" a particularly well-defined death ending up in the state called white dwarf (WD). These are stars with masses ranging from 0.07 to 1.4 times the mass of the sun. In this end state they no longer fuse light elements into heavier ones.

Pressure necessary to support the WD against gravitational collapse is provided by "electron degeneracy." The Pauli Exclusion Principle -- fundamental to the periodic table of the elements -- requires that no two electrons may share the same energy state. As gravity attempts to compress the WD, the electrons effectively push back, preventing further collapse.

WD radii are 0.008 to 0.02 times the sun's radius. (Earth's radius, about 4,000 miles, is comparable to that of some WDs.) Thus WDs are very compact with densities on the order one million times that of the sun -- about one million grams per cubic centimeter. The famous Indian astrophysicist, Subrahmanyan Chandrasekhar, received the Nobel Prize for developing stellar theory related to WDs.

WDs for more than 200 years have been known to be somehow different from ordinary stars undergoing fusion. The original clue to their nature came from WD colors and luminosities. Spectral color (temperature) is related to luminosity for ordinary stars, but WD luminosities are far too low compared to their colors.

WD luminosity arises from residual thermal energy that is not replenished for lack of fusion in the WD's core. As a WD cools over trillions of years, its color passes from the blue to the red part of the spectrum (so not all WDs actually appear white in color) and will eventually fade out completely, as the WD becomes a "black dwarf."

Mostly hidden in the luminosity of the brightest star in the sky, Sirius A, is its orbiting companion, the WD Sirius B -- one of the first WDs discovered. Sirius B's existence was previously suspected due to its gravitational effect on Sirius A.

As a sufficiently massive WD evolves, it passes through giant phases while fusing helium to carbon and oxygen and on to heavier nuclei. In these later phases the system may appear as a planetary nebula as the star puffs off its outer layers, eventually leaving behind the dead, nonfusing core of heavier nuclei (up to magnesium) -- the WD itself. Some WDs destruct as type Ia supernovae, to be discussed next month.

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

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