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.