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May 9, 2010

The star at the end of the handle of the Little Dipper is the most famous star of all: Polaris, the North Star. Although it's not all that bright, it points the way to true north, so it has a big reputation.

The star at the end of the handle of the Big Dipper isn't nearly as famous, even though it's actually a little brighter.

The star is Alkaid. It's almost exactly 100 light-years away, so the light you see from it tonight left the star around the start of 1910.

Alkaid is much bigger and brighter than the Sun. It's also thousands of degrees hotter. That high surface temperature makes the star glow blue-white -- a color that you can see if you look at the star under a dark sky.

Alkaid is so hot because it's several times heavier than the Sun. The gravity of all that extra mass squeezes the star tightly, heating its core. That speeds up the nuclear reactions that take place in the core, generating a tremendous amount of energy. The energy heats up Alkaid's outer layers, making the star shine blue-hot.

Hot stars like Alkaid pay a price for their showiness, though. They burn through the nuclear fuel in their cores in a hurry, so they live fairly short lives. Alkaid, for example, will live less than a hundred million years before it burns out. It'll leave behind a small, dense corpse known as a white dwarf -- a cosmic ember that will slowly cool across the eons. The same fate awaits the Sun -- but not for several billion years.

Script by Damond Benningfield, Copyright 2010

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