ZETA TAU (Zeta Tauri). Many are the brighter stars that carry no
"western" proper names (the names really coming from ancient Greece
and the middle east). The "Zeta" star of Taurus is one of them. Though a Babylonian name exists
(from Allen, "Shurnarkabti-sha-shutu", the Star in the Bull towards
the South), it is never used. A pity too, as Zeta Tauri is among
the most prominent of any stars that make constellation figures,
the star representing one of the celestial Bull's outstretched
horns. Defining mid-third magnitude (3.00), Zeta Tauri is in truth
a brilliant hot blue class B (B2 or B4) giant or subgiant (we adopt
a B2 subgiant). At a distance of 420 light years, it radiates the
light of (if a B2 star, the class needed to account for ultraviolet
radiation) 5700 Suns from a surface heated to 22,000 Kelvin.
Together, temperature and luminosity yield a radius 5.2 times that
of the Sun. What makes the star really
special, however, is not its high temperature and luminosity, but
its rotation and mass loss. The equatorial rotation speed has been
measured as high as 330 kilometers per second, 115 times that of
the Sun, the star spinning around with a period of only one day (as
opposed to the 25-day solar rotation period). The rotation, which
is still well short of that needed to break up the star, is somehow
related to a thick disk of matter that surrounds it. The disk
radiates bright emissions from hydrogen in the red and blue parts
of the spectrum, making Zeta Tauri one the sky's best-known "B-
emission" or "Be" stars. (Another is Gamma
Cassiopeiae, which also carries no proper name, as if the
ancients were somehow unknowingly discriminating against such
stars.) The star and disk are both large enough to have had their
angular diameters actually measured. The disk is some 64 solar
diameters across, while the star is seen to be 5.5 solar diameters
wide, in nice accord with that found from temperature and
luminosity. Typical of all Be stars (which actually abound), the
radiation from the disk is variable (here over a period of several
years), though no one is quite sure why. Neither do we understand
the mass loss process very well, though it is surely related to
high spin, as all Be stars rotate quickly. Zeta Tauri, which
itself varies by a tenth of a magnitude, is accompanied by a much
lower mass companion that orbits with a period of 0.36 years at a
distance of only about an astronomical unit. A planet at that
distance would surely be fried. A nine solar mass star around 25
million years old, Zeta Tauri is now evolving, and is close to
giving up core hydrogen fusion, if it has not done so already. It
is just under the limit at which stars probably explode, a relief
perhaps to its smaller neighbor.