Alpha Aurigae - Capella

Capella is the brightest star in the constellation Auriga, marking the left shoulder of the Charioteer - a prominent irregular pentagon of stars. Capella is of magnitude 0.08, almost as bright as Vega, making it the 6th brightest star in the night sky. It distinguishes itself by being the first-magnitude star closest to the pole. Five degrees southwest of Capella is a small triangle of stars known as "The Kids", or baby goats.

Capella's name in Latin means "the little she-goat". It rises on the northeastern horizon during the latter part of August, and for this reason is sometimes known as the "Harvest Star".

Components

Capella, at a distance of 42 light years, is one of the sky's most famous double stars. Its two components are both yellow class G stars with roughly the same surface temperature as the Sun, but are much larger and brighter. One is 50 times more luminous than the Sun, the other 80 times more luminous. Both are about 10 times the Sun's diameter. The two stars, just below the edge of visible separability in the best telescopes, are about 60 million miles apart - about 2/3 the distance between Earth and Sun - and orbit each other with a period of 104 days. From this, we find their masses to be about 2.5 times the Sun's.

Capella also has a faint companion that is itself a double, made up of two dim red class M dwarfs which orbit a good fraction of a light year away.

Evolution

Both stars are dying giants that have ceased fusing hydrogen in their cores. The brighter star, slightly more massive and hence more evolved, has almost certainly begun fusing its internal helium into carbon. The dimmer component seems to have a contracting helium core that has not yet "fired up". Capella is a source of X-rays, probably from surface magnetic activity similar to that seen on the surface of the Sun, but which star is responsible is uncertain.