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Two stars that are undergoing a series of changes shine boldly through the autumn night. They form the system known as Capella, which is a bit more than 40 light-years away. It climbs into good view in the northeast by 8 or 9 p.m., and looks like a single bright yellow-orange star.

What we see as Capella consists of two stars that are similar to each other. Each is a good bit bigger and heavier than the Sun. And even though they’re much younger than the Sun, they’re already near the ends of their lives. Both are undergoing changes in their cores that are causing them to puff up.

During its “normal” lifetime, a star shines by fusing together the hydrogen atoms in its core to make helium. About one percent of the hydrogen is converted to energy, which makes the star shine.

Eventually, the star uses up the hydrogen, so the nuclear reactions shut down. Without the outward push of the radiation from these reactions, the core shrinks and gets hotter. When it gets hot enough, a new series of reactions starts, converting the helium to heavier elements. At the same time, the star’s outer layers puff up, expanding the star to dozens of times the diameter of the Sun.

The stars of Capella are going through this process, with the heavier of the two a little farther along. The stars are so close together that as the heavier one puffs up even more, it’ll engulf its companion — setting up a hard-to-predict future for this impressive duo.

Script by Damond Benningfield

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