GAMMA HYA (Gamma Hydrae). Hydra's (the
Water Serpent's) luminary, properly the Alpha star, is second magnitude Alphard, which from Arabic means "The
Solitary One," as it appears rather by itself to the south-southwest
of Regulus in Leo. Skipping over obscure (at least in
brightness) fourth magnitude Beta Hydrae
(the southernmost of Hydra's outlining stars), 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). A dozen degrees south of first
magnitude Spica in Virgo, Gamma Hya is
remarkably easy to find. Moreover, the southern two stars of Corvus, the Crow, point southeasterly right
to it. From a distance of 134 light years, the class G (G8)giant, with a temperature of 5090
Kelvin (needed to assess a bit of infrared radiation), shines with
a luminosity of 109 times that of the Sun, not
all that much for a giant star. Temperature and luminosity then give
a radius of only 13.1 times solar, and with the theory of stellar
structure and evolution reveal a star with a healthy mass of 3.1 or
so times that of the Sun. Gamma Hya is either just beginning to
brighten as a giant with a dead helium core or is in a stable state
in which it is fusing its helium into a mix of carbon and oxygen. It's
difficult if not impossible to tell from just temperature and
luminosity. Gamma Hyi began life on the "zero-age main sequence" as
a class B8 dwarf. Class B
stars are known for their high rotation velocities, which can get into
the hundreds of kilometers per second. As they expand to become
giants, the rotation speeds slow as a result of the conservation of angular momentum (in
reverse, the reason a skater's spin speeds up as she brings in her
arms). Gamma Hya's projected equatorial rotation speed is now down
to as low as 2.8 kilometers per second, which means the rotation period
could be as long as 240 days. After ejecting its outer envelope, which
will briefly be lit by the exposed core to create a planetary nebula, Gamma Hya's core will die as a
white dwarf with a mass
just over 70 percent that of the Sun. Stable Gamma Hydrae serves
nicely as a standard calibrator for astronomers making interferometric
measures of stellar angular diameters, and is listed as having a
distant twelfth magnitude companion a bit over two minutes
of arc away. Wrong. Over a 110 year period, the separation has
decreased by some 10 seconds of arc, revealing the so-called companion
to be merely a line-of-sight coincidence.
Written byJim Kaler 7/04/08; revised 6/22/15.
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