SIGMA VIR (Sigma Virginis, actually more commonly known by its Flamsteed number, 60 Vir). Cool
class M stars, whether dwarfs,
giants, or supergiants, are usually of
interest, especially those that seem isolated. Sitting in a relatively
blank area of Virgo (the Maiden), between
Vindemiatrix and Heze (Epsilon and Zeta Vir), fifth magnitude (4.80)
Sigma Virginis, a class M (M2) giant, seems to fit the bill. A slight
variable it can dim to 4.85, which is not noticeable to the naked eye.
Rather far away, 675 light years (plus or minus 27), with a temperature
of 3800 Kelvin, Sigma Vir shines with the light of 1600 Suns, from which we derive a radius of 93 times
that of the Sun (0.43 Astronomical Units, making it a true giant for
sure). Sigma Vir is used as a calibrator for measures of angular
diameter, the accepted value leading to a significantly higher radius
of 138 times solar. In spite of the star's position off the Milky Way to the southeast of the giant Virgo
cluster of galaxies, Sigma
Virginis seems to suffer from a slight degree of dimming by interstellar (or perhaps
circumstellar) dust, as much as 0.22 magnitudes. If so, and it's
certainly the extreme limit, the luminosity jumps to 1960 Suns and the
radius to 102 times solar. Theory fits the undimmed version with a mass
of 3.0 Suns, the star most likely rising as a giant for the second time
with a dead carbon and oxygen core. If that be the case, Sigma Vir is
450 or so million years old, and gave up core hydrogen fusion around
90 million years ago. (If we apply the correction for interstellar
dust, the mass increases to 3.5 Suns). If the star follows the usual
path, and there is little reason to think that it would not, it will
brighten and expand eventually to become a Mira-type Long Period Variable. It will
slough off its outer layers, probably pop an ephemeral planetary nebula, then die as cooling white dwarf with a mass of
around 0.7 solar masses. Along with a variety of class M giants, Sigma
Vir has been examined to see how many of them have spectroscopic companions. An orbiting neighbor
would pull the bright star back and forth, resulting in Doppler shifts
in the absorption lines, alternately toward shorter then longer
wavelengths. Sigma failed the test but gave a significant lesson, as
convective, up and down motions in its atmosphere seemed to mimic
orbital motions. But they apparent movements are not quite enough to
declare a companion, so the star is sadly most likely single.
Written byJim Kaler 7/15/16. Return to STARS.