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, which includes Beta (the luminary) followed by Alpha, both also third magnitude (respectively 2.80 and 2.86). (Don't confuse Hydrus with ancient and much more northerly Hydra, the Water Serpent.) As Beta Hydri is to the Small Magellanic Cloud, Gamma is to the Large Cloud (our brightest satellite galaxy, about 175,000 light years away), shining 10 or so degrees to the southwest of it. While not differing that 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 with a temperature of about 3800 Kelvin. At a healthy distance of 214 light years (give or take 2), three times farther than Alpha, nine times more distant than Beta (once again showing the sky to be in three dimensions), Gamma Hyi shines with the light of 675 Suns, most of it radiated in the infrared where we can't see it without special instrumentation. Luminosity and temperature give a good-sized radius of about 60 times that of the Sun, 0.28 Astronomical Units, 72 percent the size of Mercury's orbit. 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. One of the first two options seems by far the more likely. Whatever its current status, the star will eventually expose its core to become one of the huge number of dead white dwarfs that flock the telescopic sky (those such as Sirius B, Procyon B, and 40 Eridani), but one not terribly massive, around 60 percent of a solar mass. 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. That seems unlikely too.
Written byJim Kaler 1/06/06; updated 6/19/15. Return to STARS.