ETA HYA (Eta Hydrae). Eta Hydrae, in the raggedly roundish head of Hydra,
the Water Serpent, is part of the six-star set made of Delta, Epsilon,
Zeta, Eta, Rho, and Sigma Hydrae that
(from Allen) the Arabs called Min al Az'al, oddly meaning "from the
uninhabited spot." Notably fainter than the two brightest stars of
the group (Epsilon and Zeta), and at first indistinguishable among
the quartet that makes the rest of it, fourth magnitude (4.30) Eta
Hydrae stands out as by far the bluest (and hottest) of them all.
A class B (B3) dwarf, its
closest rival in surface heat is the fourth magnitude (4.36) class
A0 dwarf Rho Hydrae, and even here the gap is wide. The only
reason Eta appears faint is its rather large distance of 587 (give
or take 25) light years. Put it where Rho Hya is (at 354 light
years) and it would shine a full magnitude brighter. Distance, a
high temperature of 18,400 Kelvin (to allow a lot of ultraviolet radiation), plus
correction for a small (six percent) amount of dimming by interstellar dust yield a luminosity
of just under 2500 Suns, which in turn gives
a radius of 4.9 times solar. Like most stars of its class, Eta
Hydrae is a fairly fast spinner, though with a projected equatorial
velocity of 107 kilometers per second far from the top, which is
more than triple that speed. That Eta shows no evidence of being
a B-emission ("Be") star with a surrounding disk (like Zeta Tauri and many others) suggests that
it really is not turning all that much faster, that is, that its
rotation axis is more or less perpendicular to the line of sight
and that we are getting at least somewhat close to a true
rotational velocity. Were that to be the case, Eta would make a
full turn in just 2.3 days (as opposed to 25 for the Sun) or a bit
less. The theory of stellar structure and evolution then tells
that the star is a middle-aged dwarf (younger than the 50 million
year hydrogen fusing lifetime) with a goodly mass of 6.5 times that
of the Sun. After losing its outer envelope
as a vastly bigger growing
giant that has not only fused its internal hydrogen to helium,
but the latter into carbon and oxygen, Eta Hydrae should die as
dense, Earth-sized white
dwarf of about a solar mass similar to Sirius B, though without a companion to mark its
evolutionary progress (or for that matter to get in the way of it).
Not only is Eta Hydrae excluded from the populous ranks of binaries
(though surely one could be hidden in its glare) and "Be" stars, it
was once thought at least to be a Beta
Cephei type of subtle, rapidly pulsating variable. Alas, no
more, as the star has been rejected as a "poor candidate." But if
you are looking for color and mass, Eta Hya clearly tops the local
crowd.
Written by Jim Kaler 4/05/13. Return to STARS.