YED PRIOR (Delta Ophiuchi). Two stars represent the left hand of the Serpent Bearer, Ophiuchus, which holds Serpens, his entwining Serpent. Both, Yed Prior and Yed Posterior, mix languages, "Yed" coming from the Arabic word for "hand," "Prior" and "Posterior" Latin for "in front" and "behind." Yed Prior is the western of the two, and leads its mate Yed Posterior across the sky as the Earth rotates. In fact the two have nothing to do with each other except for alignment, Yed Prior at a distance of 170 light years, Yed Posterior much closer at 107 light years. As befits Bayer's Delta designation, Yed Prior, at bright third magnitude (2.74), is Ophiuchus's fourth brightest star; however, it is beat out not by Beta and Gamma, but by the more lowly-lettered stars Eta and Zeta. It is clear that Alpha (Rasalhague), Beta, and Gamma were used for the northern stars, and the next five in sequence for the line of stars that makes the bottom of the constellation, much in the way the letters are used in the Big Dipper. Yed Prior deviates some from the norm in being one of the sky's few relatively bright cool class M (M0.5) giants, rather similar to (but not as luminous as) Scheat, Beta Pegasi. At the warmer end of class M, the temperature is still only 3800 Kelvin. From its distance, and allowing for invisible infrared radiation, we find it to be 630 times brighter than the Sun, giving it a radius 58 solar. Its measured angular diameter (0.0095 seconds of arc) and distance give 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 still be brightening with a now-quiet helium core; the core may have begun fusing helium; or it may be in a more advanced state fusing helium to carbon. Star- studies are often statistical in nature and it is much more difficult to tell what any individual might be doing. Yed Prior has no known companions nor significant variation, and as such it is an important star for setting up stellar temperature scales. It is, however, rather rich in iron (about double the Sun's content) and also has almost three times as much nitrogen as the Sun, the nitrogen enriched by interior nuclear processes.
Written by Jim Kaler. Return to STARS.