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KAPPA CEN (Kappa Centauri).Centaurus,the Centaur, blends smoothly into the stars ofLupus, the Wolf, the twoconstellations filled with bright, prominentstars. The two are commented through mythology, as in one storyCentaurus sacrifices Lupus uponAra, theAltar. The main stellar connection between them is made by notablethird magnitude (2.97) Kappa Centauri and, about a degree to thesouth, somewhat brighterBeta Lupi. Though perhaps looking paired, they are not, Beta Lup more than 500light years away, Kappa Cen much closer at 383 light years (give ortake 28). Kappa Cen is, however, a member of the Upper Centaurus-Lupusassociation of hot starsthat is centered at a distance of 460 light years, showing howspread out these associations are. Beta Lupi could be a member as well, thus reconnecting them. A close look, then a very closelook, reveals more relations, that Kappa Cen is actually a triplestar. Visually, the bright star, Kappa Cen A, a hot blue class B(B2) subgiant, ispaired with11th magnitude Kappa B. Lying four seconds of arc away, from itsbrightness Kappa B is a class K2 dwarf. Kappa Cen A is then splitwith sophisticated instrumentation into two just a tenth of asecond of arc apart, and is made of the hot B star (now Kappa CenAa, of magnitude 3.4), and Kappa Ab, a 4.7 magnitude star that isprobably a class A0 dwarf. Aa and Ab are difficult to separate, tosay the least. With respective temperatures of 19,845 Kelvin, and(from the class) 9500 K, after about a 15 percent correction fordimming by interstellar dust, Aa and Ab have luminosities somewherearound 3500 and 190 times that of theSun,which lead to radii of 5.0 and 5.1 solar. With a slow projectedequatorial rotation speed of 20 kilometers per second, the rotationperiod of Aa is under 13 days (the rotation probably more or lesspole-on). Theory then allows masses of 7.5 and 3.0 Suns. It alsoshows Aa to be not a subgiant (a star that has given up corehydrogen fusion, or is about to), but a dwarf about two-thirds ofthe way through its lifetime of 33 million years, such divergencesin classification not uncommon. However, Ab then seems to be asubgiant, which cannot be, as lower mass dwarfs take longer to burnout than do those of higher mass. Moreover, Aa should be largerthan Ab, not smaller, both of these anomalies showing that one ormore of the input parameters is wrong. We can probably rely onthose of Aa more than those of Ab. From distance and separation, Ab is separated from Aa by about a dozen Astronomical Units, and assumingthe masses to be correct, take a dozen years to orbit each other. Out in the distance, Kappa Cen B lies at least 470 AstronomicalUnits away from the inner pair. Adopting 0.75 solar for Kappa CenB, the little class K star must take at least 3000 years to orbitthe inner pair. From its perspective, Aa and Ab would get as faras a bit over a degree apart. Not big enough to explode as asupernova, Kappa Cen Aa willdie as a fairly massivewhitedwarf rather likeSirius B. Written byJim Kaler 6/29/12. Return toSTARS.
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