CEBALRAI (Beta Ophiuchi). At the northern tip of Ophiuchus, the Serpent Bearer, lies Rasalhague, the bright Alpha star. Helping to form the huge rough pentagon that makes the classic figure is Cebalrai, at the pentagon's northeastern corner, its position and bright third magnitude status (2.77) causing Bayer to call it "Beta." Unusual among star names, "Cebalrai" is pronounced with a soft "C," even though it derives from a hard "K" sound in the original Arabic. The name has nothing to do with a serpent or a serpent bearer, but comes from ancient Arabic patterns that reflect an Arabic pasture that lies from Hercules to southern Ophiuchus. Here, Rasalhague (which refers to the serpent bearer) is the shepherd, and Cebalrai his dog, the name deriving from a phrase that means exactly that. Physically, Cebalrai is a common orange class K (K2) giant, one of many relatively cool (4600 Kelvin) "K stars" (headed by brilliant Arcturus) that populate the naked-eye sky. At a distance of 82 light years, Cebalrai releases 64 solar luminosities into space from a sphere with a radius 12.5 times that of the Sun. From these data we find that the star has a very uncertain mass of about twice solar and that it is most likely now fusing helium into carbon in its deep core, having long ago given up the hydrogen fusion that powers more common ordinary stars like the Sun. At first glance, Cebalrai is known more for its setting as anything else, as it makes a guidepost to the old, no-longer-recognized constellation "Poniatowski's Bull," a vee- shaped figure just to the east of the star, and to an attractive large star cluster (IC 4665) just to the northeast. Closer physical examination, however, reveals Cebalrai to be oddly variable, not in brightness but in size. Doppler measures (which reveal surface motions through tiny shifts in the wavelengths of light) show the star to jitter with periods of 0.26, 13.1, and 142 days. The longer period is caused by rotation and dark spots that swing in and out of view, and is consistent with the rotation velocity of 2 kilometers per second and an axial tilt. The 13-day period, however, seems to be a real, though quite subtle, pulsation, whose origin is not well understood. A similar kind of pulsation has also been discovered to take place in Arcturus, though with a shorter 8.3 day period. Sufficiently detailed observations may show many of these stars to be slightly unstable.
Written by Jim Kaler. Return to STARS.