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.