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