ALULA AUSTRALIS (Xi Ursae Majoris). Ursa Major walks on legs identified by three pairs of close but unrelated stars that the ancient Arabs called the "springs (leaps) of the gazelle" that lie north of Leo Minor. From west to east they are Talitha and Kappa UMa, Tania Borealis and Australis, and Alula Borealis and Australis, the last a bright-end-fourth magnitude (3.78) star better known as Xi UMa. The proper name comes from an Arabic phrase meaning the "first spring" ("leap" or "jump," not the season), the other two names in reverse order meaning "second spring" and "third spring" (the "Borealis" and "Australis" denoting the northern and southern stars of the pairs). Alula Australis is of profound historical interest. While Mizar was the first known "double star," our Alula was the first double to be identified as physically related (by no other than William Herschel, the discoverer of Uranus in 1781), when he saw that the two orbited each other. Xi UMa was also the first star for which an orbit was actually determined, such orbits (with gravitational theory) allowing the all-important measure of stellar mass. Alula Aust, just 27 light years away, is made of near-twin class G hydrogen- fusing dwarfs similar to the Sun: brighter fourth magnitude (4.33) class G0 Xi UMa A coupled with lesser fifth magnitude (4.80) class G5 Xi UMa B, with respective (but not well- determined) temperatures of 5740 and 5720 Kelvin, luminosities 1.1 and 0.72 solar, radii 1.04 and 0.9 solar, masses (from the theory of stellar structure and evolution) of 1.0 and 0.98 solar, and an age of 6 billion years, 1.5 billion years older than the Sun. The major difference between these stars and the Sun (other than duplicity) is a metal abundance relative to hydrogen (of Alula Aust B) only half solar. The two stars orbit each other with a period of 59.878 years at an average separation of 21.2 Astronomical Units. A rather high eccentricity takes them from as far as 29.6 AU to as close as 13.4 AU. From the orbit, the
Xi Uma Xi UMa B orbits Xi UMa A every 59.84 years. The orbit (five seconds of arc across and the best fit to the observations) is seen as projected through 122 degrees onto the plane of the sky (making "B" appear to go around "A," at the cross, in the clockwise direction), which offsets the primary star from the focus of the ellipse. The dashed line is the true major axis of the rectified ellipse, what we would see were the orbit viewed from overhead. The two stars were closest in February of 1995. (From an article in the Astronomical Journal by B. D. Mason, H. A. McAlister, W. I. Hartkopf, and M. M. Shara, as presented and modifed by W. I. Hartkopf and B. D. Mason, US Naval Observatory.)
total mass of the system is 2.65 times that of the Sun, notably more than the sum gotten from structure theory. No surprise, as both stars are themselves "spectroscopic binaries" with low mass companions. Of the two, the more interesting is Xi UMa B, as it is orbited by a low mass star with a very short period of 3.98 days that was once thought to be a brown dwarf, a "substar" with such a low mass that it cannot fully fuse hydrogen to helium. Masses determined from the spectrum alone, however, are lower limits. Alula Aust B is (like the Sun) also magnetically active, with a hot corona with a temperature measured to be between 2 and 6 million Kelvin. Variations caused by surface activity lead to a rotation period of 4.0 days, which when compared with the projected equatorial velocity (2.8 kilometers per second) allows the axial and presumably orbital tilts to be found, which in turn allows for a true estimation of mass and shows the dim companion to be a cool-end class M dwarf with a mass of 0.15 Suns, placing it well above the brown dwarf limit of 0.075 solar masses. Alula Aust A's companion orbits in 1.833 years. To reconcile the sum of all masses, Alula Aust A's companion must carry a mass of 0.5 solar masses, rendering it a cool class M dwarf as well. (As an additional complication, there may be another star in the Xi-B system around which the known pair goes, which would reduce the mass of the companion to Xi-A.) So Herschel's double is actually quadruple, may be quintuple, may even be sextuple. Off in the distance, nearly a minute of arc 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 degrees apart, on the average each shining with the light of five full Moons. Herschel, who died in 1822, would have loved it. (Thanks to Jerry Diekmann for suggesting this star.)
Written by Jim Kaler. Return to STARS.