Capella Aa is the cooler of the two giant stars that make up Capella, a close pair seen as a single golden star, the brightest in Auriga.
At a glance
| type | orange giant (class K) |
|---|---|
| distance from the Sun | 42.8 light-years (13.1 pc) |
| its light left it | 42.8 years ago |
| luminosity | 78.7 × the Sun |
| radius | 12 × the Sun |
| surface temperature | 4,970 K |
| constellation | Auriga (Charioteer) |
Read off the piece’s own catalogue, the same values it draws with.
About Capella Aa
To the eye Capella is one star, but it is two giants in a tight orbit: Capella Aa, the cooler and more evolved, and Capella Ab, a little hotter and spinning faster. Both have left the main sequence and swollen, and together they make one of the brightest points in the northern sky.
The pair is too close to separate with an ordinary telescope. It was one of the first binaries resolved with an interferometer, which combines light to see finer detail than a single mirror. A separate, much fainter pair of red dwarfs shares Capella’s motion at a great distance.
What you see in the piece
With HUMAN EYE on Capella is a bright golden-yellow point high in the winter sky. Fly to it and the two giants separate.
Measured or modelled
Capella Aa is thought to be burning helium in its core, a later stage than its companion, which is still crossing between hydrogen and helium burning; this comes from comparing the two stars with models of stellar evolution.
Questions
Is Capella one star or two?
Capella is two giant stars in a close orbit, seen as a single point by the eye, with a much fainter pair of red dwarfs far away.