KURHAH (Xi Cephei). The rough pentagon that makes the heart of Cepheus, the King, is outlined by a set of half a dozen modest stars. Almost smack in the middle is another, fourth magnitude (4.29) Kurhah, better known as Xi Cephei. The meaning of the reduced Arabic name (sometimes Al Kurhah) is somewhat uncertain, but is usually taken as "the white spot on the face of a horse," having nothing to do with a King, but instead most likely related to Arabic tradition. It's also a fine triple star, and if we push things, may even be quadruple. Through the telescope, it first appears as a "splendid double" made of fifth (4.8) and sixth (6.3) magnitude stars separated by 8 seconds of arc. Xi Cephei A is a class A "metallic" (A3m) dwarf star, while Xi-B is an F7 dwarf (both oddly called "bluish" by nineteenth century astronomers). The spectrograph then shows Xi-A to be a spectroscopic double with another sixth magnitude (6.2) class F7 companion (Xi-Aa) in an 810.9 day orbit. Off in the farther distance, 13 seconds away, lies 13th magnitude (12.7) Xi-C, which may or may not belong to the system. As a metallic line star, Xi-A is classed as too hot, it's actual temperature 7200 Kelvin, making it more like a class F1 star. Such stars have atmospheres that are enriched in copper, zinc, and rare earths (like europium), but are deficient in calcium, all the result of chemical separation by gravity and radiation in a relatively quiet environment. Rotation, which would stir things up, is not high, the projected equatorial velocity but 20 kilometers per second. A modest luminosity of 9.5 times that of the Sun leads to a radius double solar, a rotation period of under five days, a mass 1.7 solar, and an age of about a billion years. The respective parameters of Aa are even more modest, 2.6 solar luminosities, 1.4 solar radii, and 1.2 solar masses. The 2.22 year orbital period of Aa around A gives a mean separation of 2.4 Astronomical Units (0.08 seconds of arc, which fits with that observed). Xi-B is almost identical. From observations of but a small part of the orbit, Xi-B seems to take some 3800 years to make a circuit about the inner pair at a mean separation of 359 AU, a 24 percent eccentricity taking it as close as 273 AU and as far as 445 AU. From these data, the total mass of the system would be 3.2 solar, whereas from the theory of stellar evolution the actual total mass is 4.1, showing the orbital parameters to contain significant, but not surprising, error. From Xi-B, the inner pair would appear no more than 0.4 degrees apart and as bright as 30 and 10 full Moons. More-distant Xi Cephei C is most likely a line-of-sight coincidence. If it is a real member, it would be an M3 dwarf at least 3000 AU off that takes at least 80,000 years to orbit the inner trio.
Written by Jim Kaler 12/28/07. Return to STARS.