Moon and Gemini

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The Moon hangs out with a pair of twins tonight -- Pollux and Castor, the twin stars of Gemini. As they climb into view in mid evening, Pollux lines up to the upper left of the Moon, with Castor a little farther along the same line.

Although they represent a pair of mythological twins, the stars themselves are quite different.

Pollux is classified as a red giant -- a star that's reached the end of its "normal" life and has puffed up to many times its previous size. As its outer layers puffed up they cooled, giving the star a distinctive orange color that's easily visible to the unaided eye.

In time, Pollux will lose its outer layers completely, leaving only its hot, dense core -- a white dwarf. As the cast-off outer layers expand into space they'll surround the core with a colorful nebula. But eventually they'll fade away, and Pollux will fade from view.

Castor, on the other hand, is actually a system of at least six stars, which are grouped into three pairs. The most impressive pair is known as Castor A. Both of its stars are hotter, brighter, and more massive than the Sun. And they're both still a long way from Pollux's stage of life. They'll continue to shine brightly long after Pollux has faded away.

Again, look for Pollux and Castor to the upper left of the gibbous Moon as they rise this evening. The whole lineup climbs high across the sky later on, and is in the west at first light tomorrow.

Script by Damond Benningfield, Copyright 2008

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