NU HYA (Nu Hydrae). Hydra, the Water Serpent, is so long and faint that most of its stars -- barring second magnitude Alphard, the Alpha star -- live in relative obscurity. Perhaps we can except third magnitude Gamma Hydrae, south of Corvus, as well. But then the remainder are hardly known at all. Tied for third place with Zeta Hydrae in Hydra's head, third magnitude (3.11) Nu Hydrae is well down in the body of the giant mythological snake. The star's major positional significance is that it serves as a gateway to one of the truly obscure constellations of the sky, Crater, the Cup, which lies on Hydra's back to the west of Corvus. Look well south and a bit east of the Sickle of Leo to find lonely Alphard, then to the southeast of Alphard for modestly bright Nu Hydrae. Fourth magnitude Alkes, Alpha Crateris, lies immediately to the southeast of it. Physically, Nu Hya is one more class K (K2) orange giant. From a distance of 139 light years, and with a well-determined temperature of 4375 Kelvin, it shines with the light of 156 Suns, much of its radiation emerging in the invisible infrared. While there are no direct measures of angular diameter, temperature and luminosity combine to give a radius 22 times that of the Sun. A slow, projected equatorial rotation velocity of just 1.8 kilometers per second (close to the solar value) then gives a hugely long rotation period of 619 days -- 1.75 years. It could be well under that if the rotation pole is pointed more toward us. We can't tell. Theory gives a mass probably between 2 and 2.5 times solar and tells of a star that is quietly fusing its core helium into carbon and oxygen. It's hard to tell the exact state. The star could have just started fusing the helium, or it could be in the final stages of core helium fusion, the two states not differentiating themselves well as to surface properties. Nu Hya might even have finished with its core helium fuel (the helium eventually to burn in a shell, surrounded by another shell within which hydrogen fuses to helium), the star brightening to become a yet larger giant. The chemical composition is a bit subsolar, the iron-to-hydrogen ratio roughly half that found in the Sun, as are carbon and oxygen, which is rather consistent with a space motion relative to the Sun about three times the local average, both of these telling us the star is perhaps a bit of a visitor from another, though nearby, place in the Galaxy.
Written by Jim Kaler 5/22/09. Return to STARS.