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 supergianSt. 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 did as a fairly heavy white
dwarf.
Written byJim Kaler 5/08/15. Return to STARS.