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.