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December 10, 2016

For a star like the Sun, which travels through the galaxy alone, the future is pretty certain. But for stars with close companions, the future is tougher to see. Interactions between the two stars change their evolution, fuzzing the view through the astronomer’s crystal ball.

Consider Capella, one of the brightest stars in the night sky. The yellow star is in the northeast at nightfall, and soars high overhead later on.

What we see as Capella is really two stars. Each star is bigger, brighter, and heavier than the Sun.

The stars are only about 10 percent the age of the Sun. But because of their greater heft, they’ve aged much more quickly. Both stars have completed their “normal” lifetimes. Now, thanks to changes in their cores, their outer layers are puffing outward — especially the heavier star.

The stars are close together — only about two-thirds the distance from Earth to the Sun. So as the stars puff up, their surfaces will get closer and closer to each other. Eventually, the heavier star will begin to dump some of its gas on the companion. It’ll also generate strong “winds” that’ll surround both stars in a cloud of gas.

And that’s likely to alter the evolution of the smaller star. As it adds mass, its own nuclear reactions will crank up. The two stars also may spiral closer together — and could even merge. That could create a bigger star, or it could trigger an explosion. For now, though, just how it all plays out remains hard to foresee.


Script by Damond Benningfield



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