AL KAPHRAH (Chi Ursae Majoris). Al Kaphrah, far better known as Chi UMa, has three things of interest about it in addition to lying along one of the legs of Ursa Major, the Great Bear: its erroneous name, its relatively low metal content, and that in an evolutionary sense it is not fully stable, but is DOING something. Called Al Kaphra, El Kophrah, Al Kafzah, the name is clearly misplaced, as it refers to the "springs" of the gazelle, which are three pairs of stars that lie well to the south of it and consist (from west to east) of Iota-Kappa, Lambda-Mu, Nu-Xi. Far better to use "Chi" for this modest fourth magnitude (but at 3.71 nearly third) star. Lying 196 light years away, this seemingly rather standard class K (K0.5) single (so far as we know) giant comes with a well- determined surface temperature of 4415 Kelvin, giving it an orange- yellow color. From temperature (to allow for infrared radiation) and distance, we get a luminosity of 172 times that of the Sun, which leads to a radius of 22.5 times solar. Direct measure of angular diameter with a sophisticated interferometer (which makes use of the interfering qualities of light) coupled with distance give a very satisfying 22.2 times solar, showing that all is in order. Typical of giant stars, the rotation is slow. A projected equatorial rotation speed of 1.15 kilometers per second gives a ponderous rotation period of nearly 1000 days (2.7 years), which is actually an upper limit because we do not know the tilt of the rotation axis and the real equatorial rotation speed could be higher. The iron abundance relative to hydrogen is rather low, falling at around 37 percent solar. This value goes along with a modestly high speed relative to the Sun of 44 kilometers per second, around three times normal, both of these findings suggesting that the star is a visitor from another part of the Galaxy. Finally, the theory of stellar structure and evolution then show that the star must have a mass almost exactly double that of the Sun and that it started life at least a billion years ago as a much hotter class B dwarf. Most class K giants (and there are a number of them in Ursa Major, including Dubhe at the top of the Big Dipper's Bowl) are quietly fusing helium into carbon and oxygen. Chi UMa, however, is in one of the preceding or succeeding states. It is not readily possible, however, to tell which one it might be. The star might be brightening with a dead helium core, it might be dimming just after having fired up its helium, or it might possibly be done with helium fusion and starting to brighten once again to become a much larger giant like Mira. If we wait a another hundred million years or so we might be able to tell, though by then the star will have moved far away and probably be invisible.
Written by Jim Kaler 4/24/09. Return to STARS.