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THETA VIR (Theta Virginis). A significant part of the outline ofthe constellationVirgo, a bit over sixdegrees to the northwest ofSpica, fourthmagnitude (4.4) Theta Vir (of no proper name) is a marvelousmultiple star. The nineteenth century authorities Smythe andChambers refer to it as a "triple star, A pale white, B violet, Cdusky." In fact Theta A is itself a closedouble whose components are wellunder a second of arc apart and were too close for early observersto note, Theta Vir A made of a two white class A stars. Aa Vir isa fourth magnitude (4.5) A1 subgiant, while Ab Vir is a seventhmagnitude (6.8) A5 dwarf that holds a "metallic" label, renderingthe actual class pretty problematic (spanning F2 to A9). Withtemperatures of 9450 and (from the class) 8300 Kelvin, from adistance of 316 light years (give or take 33), the Aa and Ab pairshine with luminosities of 135 and 14Suns,the resulting radii coming in at 4.3 and 1.2 solar, the masses at2.5 and 1.85 Suns. Aa is indeed a subgiant or close to it, thesystem about 560 million years old. Separated by an average of 39Astronomical Units, the two take roughly 116 years to orbit eachother. With an equatorial rotation velocity of at least 13kilometers per second, Aa rotates in under 16 days, which is prettyslow for the class. "Ab" is probably a slow rotator too, whichwould be consistent with the "metallic" designation. Unstirred byspin, the chemical elements then can separate, some lofted up byradiation, others settling under the force of gravity, giving thestar weird abundances, which makes classification difficult. Thereis no evidence of a significant magnetic field that can furtherskew the abundances. Ninth magnitude (9.4) Theta Vir B, 7 secondsof arc from A, is not really violet, but a yellowish solar class G(probably G0) star, the seeming visual color coming from faintnessand contrast effects with its brighter neighbor. Its parametersare all just a little greater than those of our Sun, the luminosityabout 1.4 Suns. Orbiting at least 690 AU from A, it must take atleast 7800 years to make a circuit of the inner AaAb pair. Tentimes farther out is the 10th magnitude (10.4) "dusky" one, ThetaVir C, from its brightness probably a sub-solar G8 dwarf with anorbital period that must be at least 230,000 years around AaAb-B. Both the outer ones seem to track the inner pair fairly well andare probably real companions. From C, the AaAb pair could be somethree degrees apart, while from B, they would be ten times closer. It would be quite a sight were anyone present on an orbitingearthlike planet (which seems highly unlikely). Written byJim Kaler 6/8/12. Return toSTARS.
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