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YED POSTERIOR

(Epsilon Ophiuchi). From the southwest corner of the giant constellation Ophiuchus, the Serpent Bearer, two stars stare down at us like a pair of eyes, both named Yed, meaning "the hand." The Arabic name refers to one of the hands of the Healer (Ophiuchus the celestial embodiment of Asclepius, the physician) that holds Serpens, the Serpent. Separated by only about a degree and a half, the western of the two, the "leader" as the stars cross the celestial sphere on their daily paths, is Yed Prior (the one that precedes), the eastern Yed Posterior (the one in back). Ophiuchus is anchored at its base by a line of four stars that were given Greek letters not in order of brightness, but from west to east, Yed Prior the Delta star, Yed Posterior Epsilon. While the two may at first appear to be a widely-spaced naked-eye double, they have nothing to do with each other. The brighter, bright-third- magnitude Yed Prior, is at a distance of 170 light years, while faint-third-magnitude (3.24) Yed Posterior is much closer, at 108 light years. Both, however, are giants, though rather opposite, Yed Prior a more luminous red class M giant, Yed Posterior a less- luminous but more-unusual class G (but just barely, G9.5) giant. With a well-determined temperature of 4850 Kelvin, Yed Prior shines (after accounting for a fair amount of infrared radiation) with a luminosity 61 times that of the Sun, from which we derive a modest radius of 11 solar, not all that large for a giant. Class G and K giants are known for having the cyanogen molecule (CN) in their atmospheres (indeed, the molecule is important in their classifications). Yed Posterior stands out a bit as a "cyanogen-deficient" star, which implies that it belongs more to an older population than does the Sun, one that occupies a thicker disk in our flat Galaxy. (The Sun is a member of the "thin-disk" population, in which stars are vigorously being born.) Consistently, carbon seems somewhat underabundant as well. Yed Posterior is also a weak source of X-rays, not uncommon among giants of its class. Though not involved with its Ophiuchan "companion" Yed Prior, Yed Posterior does have a possible companion. Some two minutes of arc away is a dim 12th magnitude star about which nothing is known. If a true companion, which seems doubtful, it is a small red dwarf, is at least 3600 astronomical units away from its bigger brother, and orbits with a period of at least 125,000 years. With a mass of somewhere around triple that of the Sun, Yed Posterior began life as a sparkling blue class B star. It is now on its way out, and is probably (but uncertainly) just short of beginning to fuse helium in its core.