ALULA BOREALIS (Nu Ursae Majoris). Leaping across the sky south of Ursa Major's Big Dipper bounds the Arabic gazelle, marked by three unrelated pairs of stars, the First-Leap Alulas (Borealis and Australis, respectively Nu and Xi), the Second Leap Tanias (Borealis and Australis, Lambda and Mu), and the Third Leap made of Talitha (to the north) and Kappa (to the south). Talitha and friend lie just north of southern Lynx, the Tanias just north of Leo Minor, while the Alulas can be found just to the west of the Smaller Lion. In Greek tradition, they are the feet of the Great Bear as he plods around the North Celestial Pole. Physically, Alula Bor is a higher mass example of one of the more common kinds of naked eye stars, a class K (K3) giant, a star that has given up core hydrogen fusion and is now quietly pursuing the fusion of helium into carbon and oxygen. From a distance of 421 light years, it shines at us with a luminosity 1355 times that of the Sun from a coolish surface of 4100 Kelvin, which together lead to a substantial radius of 76 times that of the Sun, or 0.35 Astronomical Units, close to the size of the orbit of Mercury. Measurement of the star's angular diameter give a reasonable supporting measure of 66 times that of the Sun. Were the star to replace our Sun, it would appear more than 33 degrees across, roughly the angle from southern Orion to the upper tip of Taurus. Theory gives a mass of some five times solar. Having begun life as a blue class B5 dwarf, the star -- now 100 million years old -- gave up core hydrogen fusion a mere five million years ago. While it's been classed as a "semi-barium star" (the barium stars those that have been contaminated with by-products of nuclear fusion by now-dead companions), but with more or less normal abundances, it really does not fit the category. Perhaps of greatest significance, Alula Borealis has a sunlike tenth magnitude (10.1) companion, a class G1 dwarf that lies 7.4 seconds of arc away from it. Over the past 175 years, the two have maintained a near-constant separation, which implies a real relation. With a luminosity only about 1.3 times greater than the Sun, the star is just a hair more massive. With a separation of at least 950 Astronomical Units, the companion must take at least 12,000 years to orbit. From Alula Bor proper, the companion would appear about as bright as our full Moon, whereas from the companion, the giant would appear starlike and shine with the light of nearly 300 full Moons. (Thanks to Jerry Diekmann, who suggested this star.)
Written by Jim Kaler 1/25/08. Return to STARS.