THETA VIR (Theta Virginis). A significant part of the outline of
the constellation Virgo, a bit over six
degrees to the northwest of Spica, fourth
magnitude (4.4) Theta Vir (of no proper name) is a marvelous
multiple star. The nineteenth century authorities Smythe and
Chambers refer to it as a "triple star, A pale white, B violet, C
dusky." In fact Theta A is itself a close double whose components are well
under a second of arc apart and were too close for early observers
to note, Theta Vir A made of a two white class A stars. Aa Vir is
a fourth magnitude (4.5) A1 subgiant, while Ab Vir is a seventh
magnitude (6.8) A5 dwarf that holds a "metallic" label, rendering
the actual class pretty problematic (spanning F2 to A9). With
temperatures of 9450 and (from the class) 8300 Kelvin, from a
distance of 316 light years (give or take 33), the Aa and Ab pair
shine with luminosities of 135 and 14 Suns,
the resulting radii coming in at 4.3 and 1.2 solar, the masses at
2.5 and 1.85 Suns. Aa is indeed a subgiant or close to it, the
system about 560 million years old. Separated by an average of 39
Astronomical Units, the two take roughly 116 years to orbit each
other. With an equatorial rotation velocity of at least 13
kilometers per second, Aa rotates in under 16 days, which is pretty
slow for the class. "Ab" is probably a slow rotator too, which
would be consistent with the "metallic" designation. Unstirred by
spin, the chemical elements then can separate, some lofted up by
radiation, others settling under the force of gravity, giving the
star weird abundances, which makes classification difficult. There
is no evidence of a significant magnetic field that can further
skew the abundances. Ninth magnitude (9.4) Theta Vir B, 7 seconds
of arc from A, is not really violet, but a yellowish solar class G
(probably G0) star, the seeming visual color coming from faintness
and contrast effects with its brighter neighbor. Its parameters
are all just a little greater than those of our Sun, the luminosity
about 1.4 Suns. Orbiting at least 690 AU from A, it must take at
least 7800 years to make a circuit of the inner AaAb pair. Ten
times farther out is the 10th magnitude (10.4) "dusky" one, Theta
Vir C, from its brightness probably a sub-solar G8 dwarf with an
orbital period that must be at least 230,000 years around AaAb-B.
Both the outer ones seem to track the inner pair fairly well and
are probably real companions. From C, the AaAb pair could be some
three degrees apart, while from B, they would be ten times closer.
It would be quite a sight were anyone present on an orbiting
earthlike planet (which seems highly unlikely).
Written by Jim Kaler 6/8/12. Return to STARS.