KAPPA DEL (Kappa Delphini). Kappa Delphini, in Delphinus (the Dolphin), a couple degrees southwest of Epsilon (the fingertip of the hand that appears to point south), at first appears to be a bit of a mess. The Bright Star Catalogue lists Kappa Del as a fifth magnitude (5.05) star with a combined spectral class, a G5 subgiant (more recently given as G1) plus a K2 subgiant, giving us some semblance of solar types to look at. Or perhaps not. The dimmer K2 star, Kappa B, was last measured 45 seconds of arc from Kappa Del A. But it's moving so fast relative to Kappa A that it's clearly an "optical" line of sight coincidence and does not belong to Kappa A at all. Much farther (212 seconds, 3.5 minutes of arc) is ninth magnitude (8.62) Kappa Del C, which for 160 years has been keeping a fine pace with Kappa A and clearly DOES belong to it. Its spectral class, however, is not known. Given an accurate distance of Kappa A (and thus C) of 98.2 light years (the uncertainty but 0.8), it should be a K2 or so dwarf, both B and C then confusingly sharing part of a spectral class. Given a recent magnitude of 5.15 for Kappa A, respective temperatures for Kappa A and C of 5675 (measured) and 5000 (estimated) Kelvin, we get respective luminosities of 6.85 and 0.34 times that of the Sun and radii of 2.7 and 0.8 times solar. The theory of stellar structure and evolution then indeed shows Kappa A to be a subgiant 50 percent more massive than the Sun, one that after a hydrogen-fusing lifetime of 2.7 billion years is ready to brighten as a true giant, following which it will slough off its outer layers to become a white dwarf. The companion, Kappa C, comes in at 0.8 solar masses, theory showing it not to be a subgiant as is Kappa "B," but a common low mass hydrogen-fusing dwarf. If that is not enough, subtle wobbles in Kappa A's motion detected by the Hipparcos parallax satellite suggest a much closer faint companion (more than 4 magnitudes dimmer than Kappa A) at a separation of half a second of arc (corresponding to 16 AU), probably a white or red dwarf with a mass of under 0.4 Suns that takes 45 years to orbit. We thus have ourselves a triple star. Given the huge separation between A and C, at least 6400 Astronomical Units, some 200 times greater than Neptune is from the Sun, it's amazing that they have stayed together and that Kappa C has not broken loose of its gravitational bonds, leaving the inner two (if indeed there ARE two) to themselves. If left undisturbed, Kappa C would have an orbital period around Kappa A of more than 300,000 years. Such "fragile" binaries are not all that uncommon. We have a more extreme example in Proxima Centauri, the nearest star, which orbits Alpha Cen (third brightest in the sky) with a period of perhaps three-quarters of a million years. From Kappa A, Kappa C would shine a magnitude or more brighter than our Venus, while from Kappa C, Kappa A would be brighter than a quarter Moon, Kappa B entirely out of the picture as it sails away. (Thanks to Bill Hartkopf for extensive discussion of this star.)

Written byJim Kaler 10/10/14. Return to STARS.