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CAPELLA

In early mid-northern winter evenings, Capella shines almost directly overhead, and is one of the three bright stars spread around the northern sky, the others Arcturus of spring and Vega of summer. All are close to the same brightness, Arcturus cool and orange, Vega, hot and white, Capella yellow-white and in the middle of the temperature range. Barely the faintest of the three, it distinguishes itself by being the first- magnitude star closest to the pole. Capella, meaning "the She- Goat" from old Roman times, is the Alpha star of the constellation Auriga, the Charioteer, a prominent irregular pentagon of stars. Just to the south of Capella is a triangle of fainter stars, an asterism called "The Kids." 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 temperature as the Sun, but are much larger and brighter, one 50 times more luminous than the Sun, the other 80 times brighter, each having about 10 times the solar diameter. Each is therefore a dying giant that has ceased hydrogen fusion in its core. The two stars, just below the edge of separability in the best telescopes by eye, are only about 60 million miles apart - - about two-thirds the distance between Earth and Sun -- and orbit each other with a period of just 104 days, from which we find masses about 2.5 times that of the Sun. The brighter star, slightly the more massive and the more evolved, has almost certainly begun the fusion of its internal helium into carbon. The dimmer of the pair 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. Capella also has a faint companion that is itself a double made of two dim red class M dwarfs that orbits a good fraction of the light year away from the star that graces northern winter.