Xi Ursae Majoris (Xi UMa, ξ Ursae Majoris, ξ UMa) is a star system in the constellation Ursa Major. On May 2, 1780, Sir William Herschel discovered that this was a binary star system, making it the first such system ever discovered. It was the first visual double star for which an orbit was calculated, when it was computed by Félix Savary in 1828.
The two components are yellow G-type main-sequence stars. The brighter component, Xi Ursae Majoris A, has a mean apparent magnitude of +4.41. It is classified as an RS Canum Venaticorum type variable star and its brightness varies by 0.01 magnitudes. The companion star, Xi Ursae Majoris B has an apparent magnitude of +4.87. The orbital period of the two stars is 59.84 years, and they are currently separated by 1.2 arcseconds, or at least 10 AUs.
Each component of this double star is itself a spectroscopic binary. B's binary companion, denoted Xi Ursae Majoris Bb, is unresolved, but the binary star is known to have an orbital period of 3.98 days. The masses of both A and B's companions (Ab and Bb) (deduced by the sum total mass of the system minus the likely masses of Aa and Ba determined by their class) indicate that they are probably MV stars (red dwarfs), Bb being on the cool end of the M spectrum, not much hotter than a brown dwarf.
It also has the proper name Alula Australis (and erroneously Alula Australe).
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