ZETA HYA (Zeta Hydrae). Given a good imagination, the head of Hydra, the Water Serpent, can be a
bit scary, with a pair of stars that look like glaring eyes staring
down at the Earth. In spite of its naming after the sixth letter
of the Greek alphabet, third magnitude
(3.11) Zeta (the eastern-most star of hydra's head) comes in tied
for third place with Nu Hydrae in brightness, following Alphard (Alpha) and Gamma Hya (which lies in the far eastern and southern reaches of the long constellation). Lying 150 light years away,
this class G (G9) bright giant
shines with a luminosity 138 times that of the Sun from a surface with a well-determined
temperature of 4820 Kelvin. Together, these parameters give a
radius 16.9 times that of the Sun. Direct measure of angular
diameter (coupled with distance) then gives a closely consistent
value of 16.0 times solar, showing that the values are all in good
order with little error. A slow projected rotational speed
(expected for a giant star) then yields a rotation period of under
265 days (the upper limit the result of not knowing the tilt of the
rotation axis). Luminosity, temperature, and theory clearly show
the star to be a 3.0 solar mass "clump giant" that is quietly
fusing its core helium to carbon and oxygen (the term "clump"
coming from the fact that there are so many stars that graphically
fall together on a plot of luminosity vs. temperature). The metal
abundance, just a bit under solar, is nothing to get very excited
about. Zeta Hydrae has a fast 6-10 hour surface oscillation that
is nowhere near visible without the use of highly sophisticated
instrumentation. With a current age of 397 million years, the star
(which began life as a mid-class-B dwarf) has about another 36 or
so million years left to it before its helium runs out and it
begins to swell and brighten, as it prepares to become a pulsating
long-period variable like Mira, sloughs off its outer layers, lights up
a "planetary nebula" (its old, rejected outer
envelope), and eventually dies as a white dwarf, the fate of
stars born with a mass under 8 or 10 times that of the Sun.
Written by Jim Kaler 2/20/09. Return to STARS.