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Outcast Star

July 22, 2014

The star that anchors the Big Dipper’s handle is a bit of an outcast. The five stars that make up the dipper’s middle are members of the same family — the Ursa Major Moving Group. They were born from the same cloud of gas and dust, and they move through space together.

The stars at the ends are different. Tens of thousands of years from now, they will have moved a good distance from the others as seen from here on Earth, pulling the “dipper” apart.

Alkaid is the end of the dipper’s handle. It’s farther away than the members of the moving group — 100 light-years. And it’s only about 10 million years old, which is far younger than the members of the moving group.

Ten million years isn’t long on the astronomical time scale — the Sun is four-and-a-half billion years old. Even so, Alkaid is getting well along in life because it’s more than six times as massive as the Sun.

The cores of heavy stars are much hotter than those of lighter stars, which revs up their nuclear reactions. They consume their nuclear fuel at a much faster rate, so they burn out much more quickly. Alkaid, for example, will live a “normal” lifetime of less than a hundred million years, versus 10 billion years for the Sun — a short life for a stellar outcast.

Look for Alkaid in the north as night falls, with the rest of the dipper hanging below it. The dipper rotates down toward the northern horizon during the night, ready to scoop up a dipperful of stars.


Script by Damond Benningfield, Copyright 2014

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