KAPPA OPH (Kappa Ophiuchi). As anyone who studies the sky for any time knows, star names can confuse, even to the point of mistaken identity. A good example is the occasional confusion between X ("ex") and Chi Herculis, the two letters, one Roman the other Greek, looking very much alike. Here's another. Kappa Ophiuchi is a relatively ordinary third magnitude (3.20) class K (K2) giant in northern Ophiuchus to the southwest of Rasalhague (Alpha Oph) that lies 91 and a half (give or take a half) light years away. From that and a temperature of 4620 Kelvin (needed to account for some infrared radiation), we find that the star shines with the light of 53 Suns, which gives it a radius of 11.4 times solar. A slow projected rotation speed of just one kilometer per second leads to a rotation period that could be as long as 1.6 years (though the axial tilt is not known). The mass from luminosity and temperature is ambiguous, but falls roughly around 1.5 solar. A much better mass of 1.2 solar is derived from the careful observation of minute stellar oscillations, the star perhaps 5 billion or so years old (just a bit older than the lower- mass Sun) and quietly fusing helium into carbon and oxygen in its deep core. Now to the heart of the matter. Many decades ago, it was "discovered" to be an irregular variable (by about a half a magnitude), and it became so listed in various catalogues (though skeptically given as "constant?" in the Russian variable-star catalogue and thus in the Bright Star Catalogue as well). In 1991, however, Tristram Brelstaff in "The Variable Star Observer" suggested that Kappa Ophiuchi had been confused with CHI Oph (a heavily reddened fourth magnitude class B2 subgiant in southern Ophiuchus), the two Greek letters, if written without care, looking somewhat alike. Chi Oph is indeed variable, with an amplitude that well exceeds half a magnitude. Hardly variable, though, Kappa Oph is listed among the "least variable stars" as observed by the Hipparcos parallax satellite. The confusion thus seems rather plausible. So rather unlike 2 and T Centauri where (forgive it again) it's "T for 2," "Chi for Kappa" does not really work. One might argue that Kappa Oph was indeed at one time variable, but has stopped pulsating. That seems unlikely, as the pace of evolution in human terms is far too slow, and variable giants do not really behave that way. (Yes, Polaris did indeed cease or rest its pulsations, but that is probably a case of a Cepheid-type star switching from overtone to fundamental.) Other than its notoriety as a result of human error (so it is then supposed), the star has going for it that it is moving relative to the Sun at a fairly high speed of 68 kilometers per second, more than four times normal, suggesting that it is a visitor from a somewhat different part of the Galaxy. (Thanks to Phil Bagnall for suggesting this star, for providing the background about the confusion, and for information regarding "The Variable Star Observer.")
Written by Jim Kaler 6/18/10. Return to STARS.