KAPPA PEG (Kappa Pegasi). Fourth magnitude (4.13) Kappa Pegasi, of no proper name, 112 light years away (give or take 3), is the second-most western star with a Greek letter in Pegasus (the Flying Horse, of course). It's just barely beaten out for the western honor (such as it is) by second magnitude Enif (Epsilon Peg). No matter, position is irrelevant to the Kappa's nature as a nifty very close triple that requires interferometry to resolve and study. The AB pair are but a couple tenths of arc apart, if that. Defying convention, Kappa Peg B is the brighter of the two rather than the fainter, which has led to considerable confusion, allowing A and B to get mixed up. Both are given as class F subgiants (combined, F5). The two orbit every 11.567 years at a mean distance between them of 8.04 Astronomical Units (not far from Saturn's distance from the Sun), a high eccentricity taking them between 10.6 and 5.5 AU apart. Application of Kepler's laws yields a total mass to the system of 3.88 Suns. A closer look shows Kappa Peg B also to be double, its components only a few thousandths of a second of arc apart. (Kappa-A was at one time also thought to be binary, but it apparently isn't.) The lesser companion (Kappa Bb) orbits brighter Kappa Ba with an amazingly short period of just 5.971 days!, the two in near-circular orbit (as would be expected) just 0.086 AU apart, a quarter Mercury's distance from the Sun. These parameters then give a total mass to Kappa B of 2.39 Suns.

Kappa Peg Kappa Peg B and A orbit each other every 11.6 years averaging about 8 Astronomical Units apart. Here we see the apparent orbit of Kappa A around Kappa B, which is not at the focus of the apparent ellipse because of the orbit's tilt and orientation relative to the plane of the sky. Note the small scale on the axes, the two stars very close to each other, at least as seen from Earth. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

Kappa Peg B and A have more recently been catalogued at magnitudes 4.94 and 5.04, which combine to 4.24, a tenth of a magnitude fainter than found earlier, the difference of little consequence. Temperatures and individual classes are also a bit insecure. Kappa-B may be as hot as F2, but temperature measures suggest more like F5. No matter, B and A have respective luminosities of 1.5 to 1.6 times that of the Sun (depending on whether they are real subgiants or old dwarfs) with radii of 2.4 solar. Subtracting the mass of Ba alone from the total given by the orbit gives a mass to the companion (Bb) of 0.8 to 0.9 Suns, making it out to be a K0 or G8 dwarf. To be proper, we ought to subtract its brightness from the magnitude of B given above, but it really makes no significant difference. The evolutionary masses of the whole system then add up to around 3.9 to 4.0 Suns, which is the value given by Kepler's Laws, suggesting that the orbits are pretty good (not to mention theory, though there is some circular reasoning involved). The real interest is in what is going to happen in the future. Near or at their main sequence hydrogen-fusing lifetimes of some 2.5 billion years, both Kappa B and A will turn into giants with helium cores and then into more advanced giants with carbon cores. While Kappa A will not cause a problem with B, the consequences for the little one that orbits Kappa B will be severe, as the two will probably merge. Mass loss combined with binary action may someday also produce a highly structured planetary nebula before one or both Kappa B and A turn into white dwarfs.

Written byJim Kaler11/21/14. Return to STARS.