GAMMA LIB (Gamma Librae). On the bright side of fourth magnitude (3.91), Gamma Librae anchors an eastern corner of the traditional figure of Libra, the Scales, which in ancient times held the autumnal equinox in its weighing pan. Gamma Lib, however, has the misfortune to be not only a common sort of star, a class G (G8.5) helium-fusing giant (some say a K0 giant, making it even more common), but has to compete with two of the most beloved stars, or at least star names, in the sky: Zubenelgenubi (Alpha Librae, to the west of Gamma) and Zubeneschamali (Beta Librae, to the northwest of Gamma), which in mythology represent the outstretched claws of Scorpius, the Scorpion, found to the southeast of Libra. Yet common as the star is, an air of mystery lingers about it. Various "Zubens" (from "claw" in Arabic) have been applied to it, Gamma at one time or another garnering Zuben Elekrab, Zuben Hakraki, Zuben el Hakrabi. Since we've already assigned a version of the latter to Sigma Librae, it's best to let any proper names go and just use Bayer's Gamma. A look at the star itself deepens the mystery. Observation from decades ago makes the star out to be a close double, its components nearly equal (differing by 0.2 magnitudes) and separated by 0.1 second of arc. Yet no modern observation, including spectroscopic, has confirmed such duplicity. So for now we proceed as if Gamma is single. An accurate distance of 163 light years (good to just 2) and a well-determined temperature of 4730 Kelvin (needed to assess the amount of infrared radiation) leads to a luminosity of 82 times that of the Sun and a radius of 13.5 times solar. There is no measure of rotation. Application of the theory of stellar structure and evolution suggests a mass around 2.5 times that of the Sun (not all that far from 2.15 solar masses as previously measured) and clearly shows the star to be a "clump giant" that is quietly fusing helium into carbon and oxygen and that joins a large number of others with similar properties. Having spent some 580 million years as a hotter class A hydrogen-fusing dwarf, Gamma is now around 700 million years old. The radius presents yet another problem. Direct measure of angular diameter gives radii that range from close to that given above to twice as large. (If Gamma Lib were composed of two nearly identical stars, which seems unlikely, the luminosity of each would be 41 Suns, the radii about 10 solar, the masses of each would drop to about double solar, and they would be some 50 AU apart.) Though Gamma Librae itself does not seem to be a close double, it does indeed appear to have a companion. At a current separation of 42.5 seconds of arc, eleventh magnitude Gamma Librae B has been tracking its giant mate pretty well since the first measure in 1878. From its brightness and distance, "B" is probably a K7 dwarf with a mass a bit above half solar. If the two are truly related, Gamma Lib B must be at least 2100 Astronomical Units from "A" and has to take more than 55,000 years to orbit. Remarkably, such fragile links are fairly common. Given the above separation, from Gamma B, Gamma A would be just a bright stellar point six times brighter than our full Moon, while from Gamma A, Gamma B would shine with the light of eight times that of Venus at her best.

Written byJim Kaler 1/27/14. Return to STARS.