From most of the United States, Capella is the fourth-brightest star in the night sky. And it’s close by, too, at a distance of just 43 light-years. Yet until recently, we couldn’t see that Capella is actually two stars, not one. They’re so close together that even the biggest telescopes couldn’t show us the individual stars. Even so, by using astronomy’s full repertoire of techniques, those same telescopes did reveal a lot about the stars.
For example, astrometry, which measures the positions of astronomical objects, revealed that Capella is moving at more than 60,000 miles per hour relative to our own solar system.
Spectroscopy, which breaks light into its individual wavelengths, told us that the system consists of two stars. It also told us how quickly the stars orbit each other, what they’re made of, and how hot they are.
And photometry, which measures changes in brightness, revealed that even though the stars are close together, they don’t “bulge” out toward each other.
Astronomers combined these techniques to learn the masses of the two stars, and to discover that both stars are nearing the ends of their lives.
In recent years, new techniques have produced sharp images of Capella, finally showing us what astronomers have known for a long time: that Capella is two stars that shine as one in the night sky.
Capella is in the northeast at nightfall. It arcs high overhead later on, and is in the northwest at first light.
Script by Damond Benningfield