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Moon and Pleiades

April 6, 2011

Like characters in a fairy tale, the stars known as the Seven Sisters are the great beauties of a big family. The family consists of hundreds of sibling stars, most of which are so small and faint that you need a telescope to see them. But a few are big and bright, so they're easy to see even though they're more than 400 light-years away.

The stars form the Pleiades star cluster. It's in the west as night falls this evening, directly above the crescent Moon. The brightest members of the cluster form a tiny dipper.

Although those stars are known collectively as the Seven Sisters, only six of them are easily visible to the unaided eye. All six are among one of the showiest classes of stars -- stars that are bigger, heavier, and hotter than the Sun -- and much brighter.

And four of those stars are giants. They're nearing the ends of their lives, so they're undergoing changes in their cores. That causes their outer layers to puff up like giant balloons, so the stars get much bigger and brighter.

Over the next few million years, the other two bright stars will also become giants. Eventually, though, they'll cast their outer layers into space, leaving only their hot but tiny cores. So the bright sisters will fade away. By then, though, some of the smaller stars in the cluster will begin entering their own giant phases. So just as in a fairy tale, the less-flashy siblings will someday outshine their showy sisters.

Script by Damond Benningfield, Copyright 2011


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