Two stars that are undergoing a rapid series of changes shine boldly through the autumn night. They form the system known as Capella, a yellow-orange star that’s in the northeast at nightfall. It will stand well to the upper left of the Moon as it rises a couple of hours later.
Capella consists of two stars that are quite similar to each other. Each is a good bit bigger and heavier than the Sun. And even though they’re a good bit 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 to giant proportions.
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 nuclear reactions stop for a while. 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 outward, 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 more, it’ll engulf its companion — setting up a hard-to-predict future for this impressive duo.
Script by Damond Benningfield, Copyright 2013