KAPPA TAU (Kappa Tauri). Here is a two for one special, two stars together known as Kappa Tauri, a seeming naked-eye double a bit north of Aldebaran in Taurus. Kappa-1, at fourth magnitude (4.22) the brighter, is very slightly to the west of fifth magnitude (5.28) Kappa-2 and 5.6 minutes of arc to the north. There is no question that the two are related, as they lie within the Hyades cluster, which (at a distance of 151 light years) spreads rather well beyond the main vee-shaped head of the celestial bull, a generally under-appreciated fact. But are the Kappas really a pair? They are remarkably similar, both white class A stars, Kappa-1 an A7 subgiant-dwarf (meaning that it is about to give up core hydrogen fusion), Kappa-2 an A7 hydrogen- fusing dwarf. Moreover, both are classed as multiperiodic Delta-Scuti type variables, which change brightness by a few percent over periods of hours (though this pairs' variations are not well studied). Kappa-1 has a measured temperature of 8290 Kelvin, a luminosity of 34 times that of the Sun, a radius 2.9 solar, and an estimated mass 2.2 solar. Oddly, though the stars are of the same class, Kappa-2 is cooler, only 7600 Kelvin. That and the calculated luminosity of 13 solar give a radius 2.1 solar and a mass 1.8 solar. They may not, however be single stars, which leaves these numbers somewhat problematic. Kappa-1 seems to have a spectroscopic companion, while lunar occultations (in which the Moon covers the stars) suggest close companions to both. There may be as many as 6 stars in the system (assuming that Kappa-1 and Kappa-2 are a real pair). The class A stars seem to dominate, however, and the parameters are probably not far from wrong. But back to the original question. The two are moving more or less through space together. Assuming they are at the same distance (which they may not be), they are 16,000 Astronomical Units apart, which implies a period of 900,000 light years and an orbital speed of only 0.6 kilometers per second. Measures of the motion, however reveal a relative speed of 4 kilometers per second, much too high for them to be a real pair, which given the gravitational tugs from other stars within the half-billion-year-old cluster makes sense. Even factoring in observational errors, the relative speed cannot be brought down far enough. Mere proximity is not enough to believe a seeming pair to be an actual double. Both of the Hyades family, they seem to be cousins rather than siblings.
Written by Jim Kaler. Return to STARS.