Capella

Capella is a star system of four stars in the constellation of Auriga, appearing as a single star to the naked eye. It is the third-brightest star or star system in the northern celestial hemisphere, after Arcturus and Vega. Always above the horizon for observers north of 44°N, the Capella system is only 42.9 light-years from the Sun. Its two largest stars are Capella Aa and Capella Ab, bright yellow giant stars in a binary pair, both around 2.5 times as massive as the Sun. They are in a very tight circular 104-day orbit, some 0.76 astronomical units (au) apart. Capella Aa is the cooler and more luminous of the two, with around 79 times the Sun's luminosity. An ageing red clump star, it is fusing helium to carbon and oxygen in its core. Ab is slightly less massive, smaller and hotter, about 73 times as luminous as the Sun. 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 other two stars, a binary pair around 10,000 au from the first two, are faint, small and relatively cool red dwarfs.

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