Moon and Spica
If you could stand on the surface of the Sun, you'd feel not only the searing heat, but also crushing gravity. If you weigh a hundred and fifty pounds, for example, you'd weigh about two tons at the Sun's visible surface.
But the surface gravity of other stars varies by quite a bit. In fact, some of the most massive stars have the weakest gravity -- which can create some bizarre effects.
An example is the star Spica, which is to the lower left of the Moon this evening. It's actually two stars in a tight orbit around each other. Both stars are a good bit more massive than the Sun.
Such stars are not only heavy, they're huge, so their outer layers are quite thin. Their surfaces are a long way from their centers, too, so they feel a weaker gravitational pull than the surface of the Sun does.
The surfaces of these stars also feel a pull from the opposite direction -- from the other star. The two stars actually bulge toward each other, so they're shaped a bit like eggs. So as we view the stars from different angles, the changing shape makes Spica appear a tiny bit brighter or fainter.
The change is so small that you need sensitive instruments to detect it, though. In fact, you also need those instruments to tell that Spica consists of two stars. They're so close together that even the biggest telescopes see them as a single point of light -- the same view you'll have if you look at Spica close to the Moon tonight.
Script by Damond Benningfield, Copyright 2008
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