THETA HYA (Theta Hydrae), with a nod toward OMEGA HYA. Bright fourth magnitude (3.88), Theta Hydrae plays a prominent role as the first (and easternmost, though one might pick Omega) star in the long body of Hydra (the Water Serpent). Just seven or so degrees southeast of the fearsome animal's roundish head, 2.3 degrees north of the celestial equator, the star rather surprisingly carries no proper name. Its significance is not so much in the star itself, but in its history and companionship. A common hydrogen-fusing dwarf at the cool end of class B (B9.5), Theta Hydrae lies 113 light years away (give or take 6), which with a modest calculated addition of ultraviolet light from its 10,460 Kelvin surface yields a luminosity of only (for a class B star) 37 Suns. Luminosity and temperature then give a radius of 1.9 times solar. From a well-determined projected rotation speed of 89.5 kilometers per second, not particularly fast, Theta Hya must rotate in under a day. Theory gives a mass of 2.5 Suns, and shows the star to be quite young, just starting out along its 600 million year dwarf lifetime. At first Theta disappoints. It's listed in older literature as a rare Lambda Bootis star. Lambda Boo stars have weird metal-deficient chemical abundances. They are suspected of having accreted surrounding birth gases in which various chemical elements (e.g. calcium, titanium, nickel, and iron) have been depleted by attachment onto dust grains, the dust and the elements in question then wafted away by stellar winds. Alas, 'tis not so. Modern analyses don't reveal any particular Lambda Boo spectral signatures. But then there are the three companions, tenth magnitude Theta Hya B 20 seconds of arc away from A, thirteenth magnitude C at 101 seconds, and twelfth magnitude D at 82 seconds (all approximately as of this writing), making a veritable cluster. Alas alack, they are all moving much too fast relative to the primary star to be actual travelling companions, and thus they are all just line-of-sight coincidences.

So what's so special? There is a deficiency of close, hot white dwarfs attached to warmer, brighter stars. They are devilishly difficult to find. In the visual realm, the light of the primary overwhelms them. A number of class A stars have been found from space-based observations to be anomalously bright in the far ultraviolet, revealing invisible hot companions that could only be white dwarfs. Class B is more difficult, however, as these hotter stars have their own strong ultraviolet components. But the white dwarfs are slowly revealing themselves, Theta Hya among them, joining Regulus and 16 Draconis, in Theta's case the far ultraviolet suggesting a tiny white dwarf with a temperature between 25,000 and 31,000 Kelvin. Such pairings are important. Since higher mass stars die faster, the initial mass of the now-defunct white dwarf constrains the upper limit to white dwarf production and the lower limit to that of supernovae, not to mention additional information on the statistics of binary stars and their possible interactions among their companions. Maybe now Theta can have its proper name.

Oh yes, Omega. Omega Hydrae (we might say in the neck of the beast) is a fairly common, understudied class K2 giant-bright giant more recently classed a K2 supergiant. It stands out for its luminosity and mass. At a distance of 896 light years (give or take 76), it shines with the light of 982 Suns from a 4650 Kelvin surface, from which we calculate a radius 48.5 times that of the Sun, or nearly a quarter of an Astronomical Unit. It weighs in at 4.5 to 5 solar masses depending on whether it is cooking its helium into carbon and oxygen or about to begin the process. Without sufficient mass to make a supernova, the star will die as a fairly heavy white dwarf.

Written byJim Kaler 5/08/15. Return to STARS.