GAMMA HYI (Gamma Hydri). Third in brightness rank, as it should be, third magnitude (3.24) Gamma Hydri marks the southeastern apex of a large triangle of stars that dominates the modern constellation of Hydrus, the Water Snake (Beta and Alpha, also third magnitude, in reverse brightness order). While not differing much in brightness from Alpha and Beta, Gamma is of a different kind altogether, as seen from its reddish color. Alpha and Beta are both yellow to white mid-temperature stars, the latter a dwarf, the former a subgiant, that lie relatively nearby. In stark contrast, Gamma is a luminous class M (M2) red giant that has completely given up core hydrogen fusion and is well along in the process of its death, though exactly where is not known. At a healthy distance of 214 light years, three times farther than Alpha, nine times more than Beta (once again showing the sky to be in three dimensions), Gamma Hyi shines with the light of 655 Suns, most of it radiated in the infrared from its cool surface (from the star's color, 3820 Kelvin). Luminosity and temperature give a good-sized radius of about 60 times that of the Sun, which would stretch the star two-thirds of the way to the orbit of Mercury. Roughly carrying between 1.5 and 2 solar masses, the star's evolutionary status is uncertain. It may be brightening (over an astronomically long time scale) with a dead helium core, it may have already fired up its helium to fuse to carbon and oxygen and is now fading toward a stable configuration, or it may have used up all its helium and is brightening again with a carbon and oxygen core in preparation for sloughing off its outer hydrogen envelope and becoming one of the huge number of dead white dwarfs that flock the telescopic sky (those such as Sirius B, Procyon B, and 40 Eridani B). Seemingly single, Gamma Hydri is an "unresolved Hipparcos problem star," as the Hipparcos satellite (which was designed to measure distances from parallaxes) suggested duplicity that was not found in ground-based observations.
Written by Jim Kaler. Return to STARS.