ZETA CEN (Zeta Centauri). Though the second brightest third magnitude star (after Menkar, Alpha Ceti), Zeta Centauri is really too far south to have received a classical proper name. Lying among a wonderful collection of similar hot blue stars, it helps give Centaurus (the Centaur) its marvelous sparkle. While just one of many such stars, it somewhat stands out for being almost exactly east of the great globular cluster Omega Centauri (by roughly five degrees), the grandest of them all and that some suggest is the remnant of a small galaxy that merged with ours. In almost any other constellation, this hot (21,090 Kelvin) class B (B2.5) subgiant would be notable. From a decent distance of 385 light years, with allowance for a lot of ultraviolet radiation and a six percent dimming by interstellar dust, Zeta Cen shines with the brilliance of 7100 Suns, which with temperature yields a radius 6.6 times bigger than the solar value. The theory of stellar structure and evolution then gives a hefty mass of 9 solar. Though the star is classed a subgiant, which implies that it has (or will soon) run out of core hydrogen, it is really clearly a dwarf that, with an age of 20 million years, is about halfway through core hydrogen fusion. With a stellar wind appropriate to its high luminosity, it is losing mass at a rate about 500 times that of the Sun. As do many stars of its class, it is a fast rotator, spinning at the equator with a speed of at least 225 kilometers per second, which gives it a rotation period of less than 1.5 days. The rotation probably flattens the star somewhat into an oval shape, though no such deviation has yet been measured. Not alone, it possesses a companion that orbits in a mere 8.02 days, implying serious proximity. Detected only spectroscopically (through back and forth movements of the big star, Zeta Cen A), nothing at all is known about Zeta Cen B. At minimum, it orbits with a distance of 0.08 AU, about 20 percent Mercury's distance from the Sun. If its mass is as much as half that of Zeta-A's, the separation would be 0.19 AU. The luminosity of such a star would reduce that of Zeta-A to 6800 Suns but have little effect on its calculated mass. But companionship ends there. Once thought to be a member of the huge Upper Centaurus-Lupus association of hot blue stars, it now seems not to be. Zeta Cen lies at the 8-10 solar mass dividing line between stars that blow up as supernovae and those that make massive white dwarfs. Perhaps it will develop just far enough to turn itself into a rare neon-oxygen white dwarf, the result of the fusion of a more-normal carbon-oxygen core. Perhaps a devastating explosion will eject the companion to make a "runaway star" like Zeta Ophiuchi. Only time will tell.
Written by Jim Kaler. Return to STARS.