HYADUM I (Gamma Tauri). Much and deservedly famed are the Hyades (in mythology half-sisters to the Pleiades), a nearby cluster that makes the vee-shaped head of Taurus the Bull. Right at the point of the "vee" lies just-barely-fourth magnitude (3.65) Gamma Tauri, whose Latin name of Hyadum I indicates it to be "first of the Hyades." Consistently, in Arabic lore the star was "premier" too, as it was known as Awwal al Dabaran, "first of the followers of the Pleiades," as it led first magnitude Aldebaran (Alpha Tauri), whose name means more simply "the follower." We now understand Gamma Tauri (by which it is far more commonly known) to be one of the four ageing giants of the middle-aged (650 million year old) cluster. Of them, Gamma ranks second in brightness, just after Ain (Epsilon, 3.53) and just before Hyadum II (Delta-1, 3.76) and Theta-1 (3.84). Along with its fainter giant brothers, Gamma is class K0, while brighter Ain comes in a bit ahead at G9.5 (though all are usually lumped together as the "K giants"). While the directly-measured distance is 154 light years, it is probably more accurate to take the Hyades' average distance of 151 light years for Gamma as well. With a temperature measured at 4970 Kelvin, "Hyadum the First" is then found to radiate at a rate 79 times greater than does the Sun, from which we can derive a radius 12 times solar, and from the theory of stellar structure and evolution, a mass of about 2.6 Suns. Though the star has a reputation of varying a bit, up to 0.1 magnitude, there is no confirmation, nor is anything known of the variation period. The rotation is so slow that the equatorial spin speed is difficult to measure, various estimates giving 2.4, 1, and under 1 kilometer(s) per second. Adopting an average of 1.7, the rotation period could be as long as 150 days. Yet the star has a surrounding magnetically-active outer layer, perhaps a corona, with very highly excited X-ray emissions from iron, oxygen, and neon, which is odd, since cool-star magnetic fields are believed to require much more rapid spins for their creation. Like the other Hyades giants, Gamma Tau is richer in metals than the Sun, its iron content (relative to hydrogen) up by about 25 percent. And like the rest of its gang, it will eventually slough off most of its outer hydrogen envelope and expire as a white dwarf with about two- thirds of a solar mass.
Written by Jim Kaler 3/02/07. Return to STARS.