OMEGA VIR (Omega = 1 Virginis). Lying in far northwestern Virgo (the Maiden), Omega Vir is a modest
fifth magnitude star (at 5.36, nearly sixth) that escapes notice
but for a few curiosities. It is named with the last letter of the
Greek alphabet, Omega, while at the same
time -- as the most easterly "named" star of Virgo -- it carries Flamsteed's number 1. As a
class M (M4) red giant, it is
among the cooler stars of the sky, and has a curiously high iron
content (relative to hydrogen) measured to be 32 percent greater
than that found in the Sun. The advanced
spectral class and semi-regular variability of nearly three tenths
of a magnitude strongly suggests a state of fairly advanced
evolution (which is backed up by the numbers below). At 480 light
years, the star is fairly distant (new efforts giving an improved
value of 496 light years). Measures give a low temperature of 3500
Kelvin, which is right on the mark for an M4 giant. Correction for
a lot of infrared radiation then gives a relatively modest (for
such a star) luminosity of 1105 times that of the Sun, which with
temperature speaks of a star with a radius 91 times solar. The
projected rotation speed is so slow that it has never been
measured. Luminosity and temperature along with the theory of
stellar structure and evolution then conspire to give a mass of 1.5
times that of the Sun. But something is amiss. Direct measure of
angular radius shows the star to be 106 times bigger than the Sun,
16 percent larger than the figure derived above. To be that large
at 3500 Kelvin (which seems secure), the luminosity must be 46
percent greater, or 1500 times solar, which in turn gives a mass of
just double that of the Sun. The culprit may well be the
correction for infrared radiation. The star is either near the
point of helium ignition, has just ignited its core helium to fuse
to carbon, or is brightening for the second time with a dead
carbon-oxygen core. The semi-regular variability, with dual
periods of 30 and 275 days, suggests the last case. The star then
well be preparing to become a much brighter long-period variable like Mira, after which it will produce a
surrounding planetary nebula and die as a
modest white dwarf. In
any case, Omega Vir is the only star in the sky that takes on both
extreme names, compliments of Bayer and
Flamsteed.
Written by Jim Kaler 5/08/09. Return to STARS.