MU OPH (Mu Ophiuchi). Just outside the eastern stretch of the classical "dotted-line" outline of Ophiuchus, fifth magnitude (4.62) Mu Ophiuchi lies between the Serpent Bearer and the eastern branch of Serpens, Serpens Cauda (the Serpent's Tail). While faint, three things recommend it: an interesting state of evolution, an odd chemical composition, and a location within a dark rift in the Milky Way. The last comes first. At a substantial distance of 753 light years (give or take 31), this class B (B8) bright giant/giant is dimmed by nearly 0.7 magnitude by interstellar dust. Given its foreground of interstellar matter, Mu Oph is a fine target as a background against which to study interstellar gas. As to the star's state, from its distance and temperature of 12,200 Kelvin (with which we can estimate the amount of its invisible ultraviolet radiation), we find a luminosity 2170 times that of the Sun, a radius 10.5 times solar, and from a projected spin rate of 114 kilometers per second, a rotation period under 4.5 days. Theory then tells of a star with a significant mass of 5.2 times that of the Sun, that it is some 80 million years old, and that it has just recently given up core hydrogen fusion and has just entered a core contraction phase in which the deep internal helium ash is being squeezed and heated prior to fusion into carbon and oxygen. As such, Mu Oph is more a rather rare type of subgiant than a giant, one that is destined someday to become a pulsating Cepheid variable and ultimately, after it loses its outer envelope as a REAL luminous giant, will die as a rather massive white dwarf of some 0.9 solar masses, the star having whittled itself down to 20 percent the mass it started with. Finally, vastly understudied Mu Oph is classed as a "manganese star," one with a high abundance of the element. In more modern parlance, it is really more of a "mercury- manganese" (HgMn) star, the early observations probably just not good enough to have detected the mercury. The flagships of the class are Alpheratz (Alpha Andromedae) and Chi Lupi. Though there is no formal abundance analysis for Mu Oph, such stars (which fall into the cool end of class B) typically have manganese up over solar (compared to hydrogen) by factors of thousands, mercury up by 100,000 or more, various other rare elements way up, and such things as zinc way down. (Mercury has never even been detected in the Sun; its abundance is estimated from meteorites.) The explanation is one of separation of elements, with some kinds falling under the force of gravity (making them relatively rare in the outer stellar skin, where we can analyze them), others lofted up by stellar radiation pressure, but only in stars that have relatively slow rotations to avoid stirring effects (which Mu Oph rather oddly has not).
Written by Jim Kaler 9/17/10. Return to STARS.