BETA HYA (Beta Hydrae). Among the toughest constellations to trace
out in the sky is Hydra, the Water
Serpent, the heavens' longest constellation, so long and dim, and far
enough to the south, that northerner's 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 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, which marks end of the tail of the
Serpent, while the two western ones with
Alchiba (Alpha Crv) point the way
south to our Beta Hydrae, the constellation's southernmost
star. Hardly Hydra's second brightest star, Beta shines to us
at a mere fourth magnitude (4.28, going on fifth) from a
substantial distance of 365 light years. 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 assumed here to be at the same temperature
(11,100 Kelvin) of Beta A. Allowing for significant ultraviolet
light, Beta A has a luminosity of 210 times that of the Sun, which leads to a radius of 4.0 solar, a
substantial mass of 3.3 times that of the Sun, and an age of just
220 million years (the star really a hydrogen-fusing dwarf).
Classed as "peculiar," Beta Hya A is rich in silicon, chromium, and
strontium, the result of concentration into magnetic patches (with
a strength a couple hundred times that of Earth) that cause the
star to be slightly variable with a period of 2.357 days, which is
thus the stellar rotation period (which is consistent with a
measured rotation speed of 72 kilometers per second and an axial
tilt to the plane of the sky of 59 degrees). Beta B is about half
as luminous as Beta A, slightly smaller, and carries a mass of
perhaps 2.7 solar. Given that it is certainly a dwarf, its
absolute brightness suggests that it is actually somewhat hotter
than its bigger mate, which would raise the mass to 3.2 solar. The
orbit is not determined. An angular separation of 1.7 seconds of
arc leads to a physical separation of at least 190 Astronomical
Units and a period of at least 1000 years, though even these
qualified parameters are highly uncertain.
Written by Jim Kaler 5/23/08. Return to STARS.