GAMMA HYA (Gamma Hydrae). Hydra's (the Water Serpent) luminary is properly the Alpha star, second magnitude Alphard (which from Arabic means "The Solitary One," having nothing to do with the Greek "Alpha."). Skipping over obscure (at least in brightness) fourth magnitude Beta Hydrae (in far southern Hydra), exact third magnitude Gamma Hydrae (3.00) comes in at number two, just beating out a host of slightly lesser stars (such as Nu, Zeta, Pi, and Epsilon). The star is remarkably easy to find, as all you need to is to follow, as the lower two stars of Corvus the Crow point eastwardly right to it. While seemingly just another coolish class G (G8) giant, Gamma Hya provides us with a guide to an interesting phase of stellar evolution. From a distance of 132 light years, the star -- with a temperature of 5100 Kelvin -- shines with a luminosity of 105 times that of the Sun, not much for a giant star. Temperature and luminosity then give a radius of just 13 times solar, and with the theory of stellar structure and evolution reveal a star with a healthy mass of 2.7 times that of the Sun. Rather than being a classic helium fusing giant of the kind that fills the sky, Gamma only recently shut down its hydrogen fusion. Currently with a dead helium core, the star is in a state of transition in which it is about to make its run to growing to much larger proportions and greater luminosity as the helium core shrinks. By the time the helium core, mashed to vastly higher temperature (100 million Kelvin) and density, fires up to fuse to carbon and oxygen, Gamma will be six times brighter (mostly in the infrared part of the spectrum) and will be nearly five times bigger. It will thereafter shrink to more normal giant proportions rather like Pollux and Aldebaran. Stable Gamma Hydrae serves nicely as a standard calibrator for those making interferometric measures of stellar angular diameters, and is listed as having a distant tenth magnitude companion some two minutes of arc away. Wrong. Over a 110 year period, the separation has decreased by some 10 seconds of arc, revealing the two to be merely a line-of-sight coincidence.
Written by Jim Kaler 7/04/08. Return to STARS.