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.