Venus and Spica

StarDate: November 17, 2012

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There's much more to most stars than meets the eye. A good example is Spica, the brightest star of the constellation Virgo. It's in the east at dawn tomorrow, just to the right of Venus, the brilliant "morning star."

Spica is pretty impressive to the eye alone — it's one of the brightest stars in the night sky. But what our eyes can't see is that Spica is actually two stars. Both stars are a good bit bigger and heavier than the Sun, but they're only a few million miles apart, so they're impossible to see as individual stars even through a large telescope.

There's something else our eyes miss, although they do give us a hint.

Spica shines blue-white, indicating that its stars are quite hot — thousands of degrees hotter than the surface of the Sun. Such hot stars produce most of their light not at visible wavelengths, but in the ultraviolet. When you combine the visible light from Spica's two stars, they shine about 2,000 times brighter than the Sun. But when you throw in the ultraviolet, they're about 14,000 times brighter than the Sun.

Even if our eyes were sensitive to ultraviolet as well as visible light, though, Spica wouldn't look any brighter than it does now. That's because Earth's atmosphere absorbs almost all ultraviolet light before it can reach the surface. So the only way to study the ultraviolet is to loft telescopes on balloons or spacecraft — adding extra "eyes" to help us see the wonders of the universe.

Script by Damond Benningfield, Copyright 2012

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