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.