StarDate: October 14, 2009

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One of the brightest stars in the northern half of the sky is climbing to prominence this month: Capella, the she-goat, the leading light of Auriga, the charioteer. It rises in early evening, and is in good view in the northeast by around 10 o'clock.

Capella actually consists of two stars, not one. Both of them are bigger and heavier than the Sun, and both are nearing the ends of their lives.

Astronomers describe such stars as "giants." They've used up the hydrogen fuel in their cores, triggering a series of changes.

For one thing, their cores have gotten smaller and hotter. That allowed both stars to "burn" the helium ash created as they burned the hydrogen. But both stars appear to have used up the helium, too, so now they're moving on to other elements.

In response to the changes in their cores, the stars' outer layers have puffed up like balloons. They've also gotten cooler, so the stars have about the same surface temperature and color as the Sun.

The stars are only about one-tenth as old as the Sun. But they're also more massive than the Sun, and that's the key in determining how quickly a star ages. Heavier stars consume their nuclear fuel at a much faster rate, so they burn out more quickly.

Because the Sun is less massive than the stars of Capella, it's still burning through its hydrogen. And it will continue to do so for billions of years more -- shining long after the stars of Capella have faded away.

Script by Damond Benningfield, Copyright 2009

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