Bear's Leg

StarDate: February 28, 2009

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This is a great time of year for watching the Big Dipper. It's in the northeast in early evening, wheels high to the north in the wee hours of the morning, and is well up in the northwest at first light.

The dipper doesn't form a constellation in its own right. Instead, it's part of the large constellation Ursa Major, the great bear. The dipper's bowl forms his body, with the handle forming the bear's long tail.

None of Ursa Major's other stars is nearly as impressive as those of the dipper -- both because they're fainter, and because they're not part of an easy-to-see pattern.

The brightest of the bear's non-dipper stars is known as Psi Ursae Majoris. It's in one of the bear's legs, well below the line that forms the bottom of the dipper's bowl.

Psi Ursae Majoris is only about 300 million years old -- less than one tenth of the Sun's age. Yet it's already entering the final stages of life. It's puffed up to about 20 times the Sun's diameter. Eventually, its outer layers will drift away into space, leaving only its hot, dense core, known as a white dwarf.

The star has aged so quickly because it was born several times as massive as the Sun. Mass is the key factor in a star's evolution -- heavier stars evolve much faster than lighter ones. They use up their nuclear fuel at a prodigious rate, so they shine bright but burn out fast. So Psi Ursae Majoris will soon fade away, leaving the great bear with a slightly gimpy leg.

Script by Damond Benningfield, Copyright 2008

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