KAPPA LUP (Kappa Lupi). This fourth magnitude (3.63) star in far southern Lupus (the Wolf), found just over three degrees to the north of the constellation's southern anchor, Zeta Lupi, provides us with a nice surprise in that it is two stars rather than one. The brighter one, Kappa-1 Lup (magnitude 3.9), shines just to the west of the fainter (5.7, measured as bright as 5.5) Kappa-2, the pair a wide 27 seconds of arc apart. Through the telescope, both appear white, Kappa-1 a class B (B9.5) hydrogen- fusing dwarf, the other, though classified as an A3 subgiant, most likely a dwarf. Not a line of sight coincidence, the two have long tracked each other through space, though they are far enough apart so as not to show significant orbital motion. Over 185 years they have changed position by just 2.5 or so seconds of arc. The brighter is measured at a distance of 180 (give or take 5) light years. That of the fainter is subject to large measurement error (probably because of the proximity of the brighter), so we adopt 185 light years as the value for the pair. Temperatures of Kappa-1 and 2 are respectively 12,200 and 8600 Kelvin (estimate), the pair shining at 130 and 26 Suns with radii of about two and a half solar. Both spin quickly, with equatorial velocities of about 175 kilometers per second, giving rotation periods under three-quarters of a day. Theory then yields masses of 3.3 and 2.1 Suns, and clearly reveals both to be true dwarfs. Kappa-1 is a good part of the way through its 280 million year lifetime, while less massive Kappa-2 has another billion or so years to go before it evolves as a giant. At least 1500 Astronomical Units apart, they must take more than 25,000 years to make a full orbit around each other. From Kappa-2, Kappa-1 would shine with the light of at least 14 full Moons. The binary was at one time thought to be related to more southern Zeta Lupi, but the distance between Zeta and Kappa is too great for them to be gravitationally bound.
Written by Jim Kaler 8/17/12. Return to STARS.