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.