Moon, Mars, and Gemini

StarDate: November 7, 2009

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The "twin" stars of the constellation Gemini are more fraternal than identical -- a fact that you can see just by looking at them. Tonight, they stand to the upper left of the Moon in late evening.

The star that's closer to the Moon is Pollux, while the other is Castor. Each represents the head of one of the mythological twins.

But if you look at them carefully, you quickly see a couple of differences between them. For one thing, Pollux is about twice as bright as Castor. And for another, Pollux looks orange, while Castor is white.

Pollux is an old, bloated star known as a red giant. It's much bigger and heavier than the Sun. As it neared the end of its life, it puffed up like a big balloon. Fairly soon -- in astronomical terms, anyway -- it'll cast its outer layers into space, leaving behind only its hot, dense core -- a white dwarf.

Castor is actually a system of six stars, which are separated into three pairs. The brightest of the bunch are a good bit hotter and brighter than the Sun, while the dimmest are cool cosmic embers known as red dwarfs. The system is about 50 light-years away -- half again as far as Pollux.

Look for the unrelated "twins" of Gemini lining up to the upper left of the waxing gibbous Moon late this evening. The bright orange planet Mars is to the lower left of the Moon. It'll line up closer to the Moon tomorrow night -- and we'll have more about that tomorrow.

Script by Damond Benningfield, Copyright 2009

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