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.