ZETA OPH (Zeta Ophiuchi). Just over the line into third magnitude (2.56), and third brightest star within the constellation Ophiuchus (the Serpent Bearer), Zeta Ophiuchi is oddly not graced with a proper name, which is odder still since it is in the middle of the line of stars that make the bottom border of the constellation, the others, from west to east being Yed Prior (Delta Ophiuchi), Yed Posterior (Epsilon), and Sabik (Eta) (though Zeta has been known to share that name with just-barely-brighter Eta). Too bad too, as Zeta Oph is truly the magnificent star, a blue-white class O (though at 09.5 just barely) hydrogen-fusing "dwarf" (a strange term for a star with a diameter 8 times that of the Sun). The star (which is very slightly variable), however, does not LOOK so blue- white. Zeta Oph is one of the brighter stars in the sky to be significantly affected by absorption and reddening of its light by passage through interstellar dust (which lies everywhere within the Milky Way). At a distance of 460 light years, the star is deeply involved with dust gas clouds (and even illuminates one of them), and is used as a background light source with which to examine the stuff of interstellar space. If the dust were not in the way, Zeta would shine at almost first magnitude. From its distance and great temperature of 32,500 Kelvin (from which we can account for the star's fierce ultraviolet light), we calculate a magnificent luminosity of 68,000 times that of the Sun, and from that a mass of 20 times solar, the star about in the middle of its short (8 million years) hydrogen-fusing lifetime. Like most luminous stars, Zeta Oph is losing mass through a strong wind that in this case blows at about 1600 kilometers per second at a rate of about a hundredth of a millionth of a solar mass per year. The star's only fate seems to blow up as a supernova. Among Zeta Ophiuchi's most interesting properties is that it is one of the sky's most famed "runaway stars," stars that used to be together and are now fleeing from a once-common point. The prime examples are Mu Columbae and AE Aurigae, which are running away from Na'ir al Saif (Iota Orionis) after an exchange and expulsion when two massive double stars encountered each other. Zeta Ophiuchi, on the other hand, seems to have been expelled from a double star system when its one-time and clearly more-massive companion exploded and is now a tiny "neutron star" about the size of a small town. The explosions that make supernovae are apparently off-center, so that when one of the stars goes off and is blasted like a bullet to one side, the other one can, if conditions are right, be shot off as well. Zeta Ophiuchi is now single, however, so that the scene cannot repeat itself.
Written by Jim Kaler. Return to STARS.