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.