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Moon and Spica

December 28, 2010

A bright star tags along with the Moon before dawn tomorrow. Spica, the leading light of Virgo, is to the left of the Moon as they rise around 2 a.m., and directly above it at first light.

As bright as Spica is, it would look far brighter if our eyes were tuned to ultraviolet light instead of visible light.

That's because Spica is quite unlike our own star, the Sun. What we see as Spica is actually two stars that are locked in a mutual orbit. They're so close together that it takes just four days for them to orbit each other.

Both stars are much bigger and heavier than the Sun. Their great mass makes them tens of thousands of degrees hotter than the Sun. To the eye alone, that extreme heat makes them look blue-white.

But because they're so hot, both stars emit most of their energy at wavelengths that are much too short for the eye to see: the ultraviolet. So while they shine about 2,000 times brighter than the Sun at visible wavelengths, they're about 14,000 times brighter in the ultraviolet.

Even if our eyes were tuned to the ultraviolet, though, we couldn't see the stars of Spica from here on Earth, because our planet's ozone layer absorbs the ultraviolet. And that's a good thing, too, because ultraviolet energy from the Sun causes sunburn and other nasty effects.

From high above the atmosphere, though, Spica would blaze as one of the brightest objects in the sky -- a brilliant star that tonight tags along with the Moon.


Script by Damond Benningfield, Copyright 2010


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