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.