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.