SABIK (Eta Ophiuchi). At the dim end of second magnitude (2.43), and (after Rasalhague, Alpha Ophiuchi) the second brightest star in Ophiuchus, Sabik still received the lowly Eta appellation from Bayer. The anomaly is the result of Bayer's distributing the Greek letters not so much in order of brightness in the constellation but in order of position: Alpha, Beta, and Gamma lie at the northern end of the sprawling figure, while Delta through Eta form the Serpent Bearer's lower "skirt," Sabik the southern-most. The star's Arabic name, something of a mystery, refers to one that "precedes" or "comes in first," and may perhaps have to do with Sabik's position at the end of the stream of stars at the bottom of the constellation (though if it were a race, it would come in last as Ophiuchus moves toward the west). The star is a close and rather unusual double that is very difficult for the amateur to resolve. Two third magnitude class A stars (A2 at magnitude 3.0 and A3 at magnitude 3.5) swing around each other in mutual orbit every 88 years. The angular orbital size is only 1.3 seconds of arc, and the two are usually much closer than that. At the star's distance of 84 light years, 1.3 seconds corresponds to 33.5 Astronomical Units, a bit farther than Neptune is from the Sun. The most unusual aspect of the system is the very high orbital eccentricity of 0.94, which means that the stars come as close as 2 AU (0.5 AU farther than Mars is from the Sun) and then only 44 years later are 65 AU (over half again farther than Pluto is from the Sun) apart, the separation varying by a factor of 32. The gravitational disturbances caused by such an orbit would make planets impossible (and indeed there is no evidence for circumstellar dust). Otherwise, the stars are rather ordinary. The brighter has a temperature of around 8900 Kelvin, a luminosity 35 times that of the Sun, and a radius 2.5 times solar; the fainter is 300 Kelvin cooler, and is 21 and 2.0 times brighter and bigger than the Sun. The masses of the stars can be derived from their luminosities, temperatures, and theory (which give 2.3 and 2.0 solar masses for the brighter and fainter respectively) or from the gravitational solution of the orbit, which gives a sum of the masses of 4.8 solar, 12 percent higher, the result of natural errors in the observed orbit and in distance. There is some evidence that one or both have enhanced metals, common among slowly rotating A stars (only about 30 km per second), the result of chemical diffusion in the stellar atmospheres.
Written by Jim Kaler. Return to STARS.