Capella

Capella (α Aurigae, α Aur, Alpha Aurigae) is the brightest star in the constellation Auriga, the eleventh brightest star in the night sky and the third brightest star in the northern celestial hemisphere, after Arcturus and Vega.

Although it appears to be a single star to the naked eye, it is actually a star system of four stars in two binary pairs.  The first pair consists of two bright, large type-G giant stars, both with a radius about 10 times the Sun's, in close orbit around each other.  These two stars are thought to be cooling and expanding on their way to becoming red giants.

The second pair, around 10,000 astronomical units from the first, consists of two faint, small and relatively cool red dwarfs.Kaler, James B. 2002. Capella, in The hundred greatest stars, Springer.  (see §18); .Ayres T.R. 1984. Capella HL. In Cool stars, stellar systems, and the Sun, edited by Sallie L. Baliunas and Lee Hartmann, Berlin/Heidelberg, Springer-Verlag, 1984, Lecture Notes in Physics, 193,  pp. 202–204.  The Capella system is relatively close, at only  from Earth.

The system is a member of the Hyades moving group, a group of stars moving in the same direction as the Hyades cluster.Famaey B. et al. 2007. The Hyades stream: an evaporated cluster or an intrusion from the inner disk? Astronomy and Astrophysics 461, #3, 957–962. 

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Category:Binary stars