BETA HYA (Beta Hydrae). Among the toughest constellations to trace out in the sky is Hydra, the Water Serpent, the heavens' longest, so long and dim, and far enough to the south, that northerners have trouble seeing the whole thing, let alone photographing it. The Serpent's irregular head, south of Cancer, is fairly easy to locate, as is the luminary, Alphard, south of Regulus. To the east, Corvus (which along with Crater rides Hydra's back) helps out. Box-like Corvus (the Crow) is famed for its "pointers," the two northern stars that point the way east to Spica in Virgo. But two other sets are rarely noted. The two southern stars point the way east to Gamma Hydrae, which is one star to the west of Pi Hydrae, the star that marks end of the tail of the Serpent, while the two western ones point the way south to our Beta Hydrae, the constellation's southernmost outlining star. Hardly Hydra's second brightest, Beta shines to us at a mere fourth magnitude (4.28, going on fifth) from a substantial distance of 310 light years (give or take 18). It's actually a close, and much understudied, double (consistent with its general obscurity), the pair at most seen to be 1.7 seconds of arc apart. Beta Hya A is a class B (B9) fifth magnitude (4.67) giant (so called, but see below) in orbit with Beta Hya B, which at dim fifth (5.47) is unclassified, but for simplicity assumed here to be at the same temperature (11,100 Kelvin) as Beta A. Allowing for significant ultraviolet light, Beta A has a luminosity of 151 times that of the Sun, which leads to a radius of 3.4 solar and a substantial mass of 3.3 times that of the Sun, the star really a hydrogen-fusing dwarf. Classed as "peculiar," Beta Hya A is rich in silicon, chromium, and strontium, the result of separation of elements through gravitational settling and radiative lofting plus concentration into magnetic patches that for our star have strengths a couple hundred times that of Earth. As the Beta-A rotates, the patches move in and out of sight, which causes the star to be slightly variable with a period of 2.357 days and which is identified with the stellar rotation period. This rotation period is in turn consistent with a measured equatorial rotation speed of 72 kilometers per second and a vertical rotation axis. Beta B (which is almost certainly a dwarf) is about half as luminous as Beta A, slightly smaller, and carries a mass of perhaps 2.8 solar. The orbit is not determined. An angular separation of 1.7 seconds of arc leads to a physical separation of at least 170 Astronomical Units and a period of at least 900 years, though even these parameters are highly uncertain.

Written byJim Kaler 8/23/08); revised 7/04/15. Return to STARS.