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Moon and Spica
Stars that live alone are pretty predictable. Their evolution is controlled strictly by their mass -- how "heavy" they are. More-massive stars shine brighter, age more quickly, and produce a greater variety of chemical elements in their cores than their lighter-weight kin.
When a star has a companion, though -- and particularly if it's an especially close companion -- then things get more complicated.
Consider Spica, the brightest star of Virgo, which rises just above the full Moon tonight and follows the Moon across the sky.
What we see as Spica is actually two massive stars that are separated by only about 10 million miles -- only a fraction of the distance from Earth to the Sun.
The heavier star is about 10 times as massive as the Sun. On its own, it likely would blast itself to bits as a supernova. Before that happens, though, it will puff up to dozens of times its current size. As it does so, it'll engulf its companion, which will orbit inside the bigger star's outer layers.
And no one can be sure just how that will change the evolution of the two stars. The less-massive star will probably steal gas from the outer layers of the heavier one, but it'll also probably get dragged toward the core of the heavier one. How this will balance out -- and what else will happen as the two stars mingle and merge -- we just don't know. No matter what happens, though, it'll be a spectacular show to watch -- millions of years from now.
Script by Damond Benningfield, Copyright 2011