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Moon and Spica

July 17, 2010

The Sun is a beacon of light and warmth in the vast reaches of the Milky Way galaxy. It provides enough energy to sustain life on our planet, even though it's more than 90 million miles away.

Compared to some stars, though, the Sun is like a candle next to a bonfire.

An example is Spica, the leading light of Virgo. It's a little above the Moon at nightfall, and sets around midnight.

Spica actually consists of two stars that are locked in a tight orbit around each other. Each star is a good bit hotter, brighter, and more massive than the Sun.

It would be pretty much impossible for a planet to orbit one of the stars and survive. A planet could survive, though, if it orbited both of the stars. But it would have to be much farther away from the stars than Earth is from the Sun.

For each square foot of its surface to receive as much visible light as Earth does, a planet would need to be about 45 times farther than Earth is from the Sun.

But because both stars are so hot, they also produce a lot of ultraviolet light. When you add that to the visible light, the two stars shine thousands of times brighter than the Sun. So to receive as much total energy as Earth does, a planet would have to be about 120 times farther from the stars of Spica than Earth is from the Sun -- much farther than any of the planets in our own solar system.

At that range, each star would look tiny -- but too dazzling to behold with the naked eye.

Script by Damond Benningfield, Copyright 2010

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