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.