TAU VIR (Tau Virginis). Virgo, the
Maiden, sprawls across and to the north of the ecliptic, the constellation marked by bright first
magnitude Spica. To the northwest of the
star lies the Autumnal Equinox, which
the Sun crosses on the first day of northern autumn. Some 15
degrees to the northeast of Spica, seven degrees east-northeast of
Zeta Vir (Heze), just a degree and a half
north of the celestial equator, is
much dimmer fourth magnitude (4.26) Tau Virginis, listed as a
rather standard class A (A3) dwarf. As we will see, though, not
quite. With a decently-defined temperature of 8225 Kelvin
(compromised by a rather high projected equatorial rotation speed
of 168 kilometers per second, which flattens the star a bit and
makes temperature a function of stellar latitude) and a distance of
225 light years (give or take just 3), the star has a radiance of
73 Suns, which leads to a radius of 4.2
solar. Estimates of angular diameter give a satisfyingly close
value of 4.4. From the rotation speed we then get a spin period
under 1.3 days, and from theory find that the star carries a mass
of 2.5 to 2.6 times that of the Sun depending on the exact state of
its evolution. Whatever that may be, Tau Vir is close to giving up
its core hydrogen fusion and might as well be considered as a
subgiant. It's interesting to compare it to a recently-posted
star, Delta Hydrae. With a hotter (A1)
spectral class, one would naturally think Delta Hya would be the
more luminous, yet it radiates at just two-thirds the rate of
cooler (A3) Tau Vir. The difference shows the power of mass and
evolution, as Tau Vir carries just a bit more heft and is
relatively farther along its life's path, dwarf stars notably
brightening as they age to become subgiants. If Tau Virginis is
marked by anything, though, it's the list of its "companions," which flock around
it like stellar flies. Five are given in the catalogs, Tau Vir B
through F. Thirteenth magnitude Tau C, nearly 180 seconds of arc
away, is clearly an "optical" mate that is just in the line of
sight. Tenth magnitude "D," nearly six minutes of arc distant, is
almost certainly one too. Twelfth magnitude "E" (14 seconds away)
has just one measure, so for now we can dismiss it. That leaves
15th magnitude "F" at 15 seconds, which if real orbits at a
distance of at least 1000 Astronomical Units. A probable mid-M
dwarf, it would take at least 20,000 years to make a complete turn
about Tau proper. Ninth magnitude "B," a possible solar type star
at 80 seconds, might then go about the inner double at five times
the distance would then take ten times the time. They could also
all be illusions and just be "in the way," leaving Tau Vir quite
single. Nobody really knows.
Written by Jim Kaler 4/20/12. Return to STARS.