Epsilon Ophiuchi is a star of magnitude 3.24 in the constellation Ophiuchus. It has the traditional name Yed Posterior; the star δ Oph, with which it forms a naked eye optical double, is Yed Prior. The name Yed is derived from the Arabic "al-yad", meaning "the hand", as the two stars are the left hand of the Serpent Bearer. Separated by only 1.5°, Yed Posterior is the western of the two stars, the "follower" as the stars cross the celestial sphere on their daily paths.
Properties
While the two stars may at first appear to be a widely-spaced naked-eye double, they have nothing to do with each other. The brighter Yed Prior, is at a distance of 170 light years, while Yed Posterior is much closer, at 108 light years. Yed Posterior is a less-luminous class G9.5 III giant. With a well-determined temperature of 4850 K, Yed Posterior shines with a luminosity of 61 suns, and has a radius of 11 suns. With a mass around triple the Sun's, Yed Posterior began life as a blue-white class B main-sequence star. It is now leaving the main sequence, and is probably just beginning to fuse helium in its core.
Unusual for a glass G giant, Yed Posterior is cyanogen-deficient and carbon-deficient, which implies that it belongs to the older star population that occupies the "thick disk" of our Galaxy. Yed Posterior is also a weak source of X-rays, not uncommon among giants of its class.
Companion
Some 2' away is a possible 12th magnitude companion. If truly related, it is a small red dwarf, at least 3600 AU away from the giant, and it orbits with a period of at least 125,000 years.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]