Xi Ursae Majoris - Alula Australis

Xi Ursae Majoris is a star system of magnitude 3.79 in Ursa Major. It has the traditional name Alula Australis; Alula comes from an Arabic phrase meaning "first leap"; the distinction Australis ("southern") is added in Latin.

Observation

Xi Ursa Majoris is a nearby binary system, 27.3 light years away, whose stars orbit each other in a period of 59.88 years, and is of profound historical importance. It was the first binary system discovered, by William Herschel in 1780. It was also the first binary system to have its orbit calculated, by Felix Savary in 1828.

The components have apparent magnitudes of +4.32 and +4.84, and were at their closest, only 1.6" apart, in 1995. At their widest separation the stars are 3" apart. Their orbit has an average radius of 21.2 AU, but a rather high eccentricity (e=0.412) takes them as close as 13.4 AU to as far as 29.6 AU.

Properties

The system is a double star whose components are both sunlike, yellow main sequence stars. The brighter component, ξ UMa A, is of spectral type G0 Ve. The secondary, ξ UMa B, is class G5 V. They have respective temperatures of 5740 and 5720 K, luminosities of 1.1 and 0.72 suns, radii of 1.04 and 0.9 suns, masses of 1.0 and 0.98 suns, and an age of 6 billion years - 1.5 billion years older than our Sun. ξ UMa A is classified as an RS CVn-type variable star; its brightness varies by 0.01 magnitudes. ξ UMa B is also magnetically active, with a hot corona measured to be between 2 and 6 million K. The major difference between these stars and our Sun is a metal abundance relative to hydrogen only half solar.

Each component of this double star is itself a spectroscopic binary. ξ UMa A's companion orbits every 1.833 years, contains 0.5 solar masses, and must be a cool class M dwarf. More interesting is ξ UMa B, which was is orbited every 3.98 days by a low mass companion that was once thought to be a brown dwarf; more recent analysis indicates that is is a dim class M red dwarf with a mass of 0.15 suns. There is some evidence that ξ UMa B may have another low-mass companion, with a period of 2.2 - 2.9 years, but this has not been confirmed.

Nearly an arcminute away is a 15th magnitude star that, if a true companion, is separated by at least 450 AU, takes at least 5600 years to orbit the inner quadruple, and must also be a cool, low mass class M8 red dwarf. From that distance, the inner pair of G stars could appear as far as 2.7° apart, on the average each shining with the light of five full Moons. Herschel, who died in 1822, would have loved it.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]