AL DHIBAIN "PRIOR" (Eta Draconis). The name Al Dhibain really belongs to a pair of stars in Draco, the one that Bayer called "Eta Draconis" and its fainter neighbor Zeta Draconis, the name referring to two hyenas. (They are only spatially aligned, Zeta four times farther away than Eta). To distinguish between the two, we will call Eta "Al Dhibain Prior," as it is east of Eta and precedes it across the sky, and therefore lies "before" it. Bayer clearly gave the Alpha designation to Draco's famed ex-pole star Thuban, which lies far from the top of the brightness pile. Draco's other bright stars are labelled more in order of where they fall within the Dragon's body. The luminary, Eltanin (Gamma), lies in the head, while Al Dhibain Prior, a bright third magnitude (2.74) and the constellation's second brightest star, received lowly Eta. Al Dhibain Prior is a "yellow" coolish class G (G8) giant only 88 light years away with a temperature of 5000 Kelvin and a luminosity 61 times that of the Sun. For a giant it is really not all that big, and only 10 times the solar diameter. The radius calculated from temperature and luminosity (10.6 solar) is nearly the same as found from the small angular diameter (0.0038 seconds of arc), which gives 9.8 solar, showing that all the measures are in good accord. Eta Dra is a classic example of a dying 2.5 solar mass "clump star," one of the many that concentrate into a similar zone of temperature and luminosity and that are quietly fusing their interior helium into carbon and oxygen. It seems to rotate slowly with a period of some 400 days. Like many similar stars, Eta Dra weakly radiates X-rays, which reveal a surrounding shroud of very hot gas associated with a magnetic field. The star is graced with a small faint companion that, given its proximity of 5.3 seconds of arc, is quite difficult for the small telescope. The companion, a much lower mass class K (K1) dwarf, shines at only 9th magnitude (roughly 8.8). Lying at least 140 Astronomical Units from the bright giant, the companion takes at least 1000 years to make a complete orbit. From the giant, the little one would shine with the brightness of roughly nine full Moons. Eta Dra proper may be slightly variable. Oddly, the companion has a reputation of being variable as well, something not expected for such a star, and probably an artifact of observation.
Written by Jim Kaler 6/21/02. Return to STARS.