Delta Ophiuchi - Yed Prior

Delta Ophiuchi, at magnitude 2.74, is Ophiuchus's fourth brightest star; it is one of the two stars that represent the left hand of the Serpent Bearer. δ Oph has the traditional name Yed Prior; ε Oph, with which it forms a naked eye optical double, is Yed Posterior. Yed Prior and Yed Posterior are compound names, formed from the Arabic "al-yad", meaning "hand", with the Latin words Prior and Posterior added to give names meaning the "leading" and "following" part of the hand. Yed Prior is the western of the two, and leads Yed Posterior across the sky as the Earth rotates.

Properties and Evolution

The two stars are unrelated; Yed Prior lies at a distance of 170 light years, while Yed Posterior is much closer at 107 light years. Yed Prior is a cool class M0.5 III giant, with a surface temperature of only 3800 K. Including infrared radiation, Yed Posterior's luminosity is 630 times the Sun's, giving it a radius 58 times solar. Its measured angular diameter (0.0095") gives about the same result. If placed at the center of our Solar System, the star would extend two-thirds of the way to Mercury.

Though Yed Prior is quite evolved and no longer fusing hydrogen to helium, its evolutionary status is uncertain. It may be brightening with a now-quiet helium core; or the core may have begun fusing helium to carbon. Yed Prior has no known companions nor any significant variation, and as such it is an important star for defining the stellar temperature scale. The star is rather rich in iron, with about double the Sun's content; and also has almost three times as much nitrogen as the Sun, enriched by internal nuclear processes.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]