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.
Written by Jim Kaler. Return to STARS.