An ageing red clump star, Capella Aa is fusing helium to carbon and oxygen in its core. Ab is slightly smaller and hotter and of spectral class G1III; it is 72.7 ± 3.6 times as luminous as the Sun and 8.83 ± 0.33 times its radius. It is in the Hertzsprung gap, corresponding to a brief subgiant evolutionary phase as it expands and cools to become a red giant. The Capella system is one of the brightest sources of X-rays in the sky, thought to come primarily from the corona of the more massive giant. The second pair, around 10,000 au from the first, consists of two faint, small and relatively cool red dwarfs. They are designated Capella H and Capella L. Several other stars in the same visual field have been catalogued as companions but are physically unrelated.
