THETA OPH (Theta Ophiuchi). As the most southerly of the brighter
stars (third magnitude, 3.27) of Ophiuchus, Theta Oph extends the large constellation
across the ecliptic, helping to make the Serpent Bearer the
"thirteenth" constellation of the Zodiac. The blue, hot, class B
(B2) subgiant lies 560 light years away, enough for the intervening
dust in the Milky Way to dim its light by some 14 percent. Even
though not in the classical misshapen pentagon that makes
Ophiuchus, the star is easy to find, located smack between Antares
and the Little Milk Dipper of Sagittarius.
After a considerable correction for ultraviolet light, the very hot
(22,500 Kelvin) star is seen to shine with the light of 11,500 Suns, from which we calculate a radius 7.3
times solar. The minimum rotation speed of only 31 kilometers per
second gives a maximum rotation period of 11 days (which could be
much shorter, since we do not know the axial tilt). As a subgiant,
this 9 to 10 solar mass star is nearing the end of its core
hydrogen-fusing state. Only 25 million years old (massive stars
burn out quickly), it will shortly become a true large red giant.
Theta Oph is right at the dividing line between stars that make
massive white dwarfs and those that explode -- only time will tell.
Like so many of its kind, it belongs to the "Beta Cephei variables" that pulse with
multiple periods, this one by 0.04 magnitudes (about 4 percent)
principally over 0.140531 days, the pulsation driven by a deep
layer in which metals are becoming ionized (having their electrons
removed by the high internal temperature). Theta Oph is at least
a binary (and probably a multiple) star, but the data are
conflicting as to how many and how widely separated the components
are. Only two degrees south of the ecliptic, it is occasionally
occulted (covered) by the Moon, from which observation we find a
companion two magnitudes fainter than Theta proper that at the time
of observation lay a mere 0.00003 seconds of arc away (0.005
Astronomical Units). The luminosity would make the companion a 7
solar mass class B2 (or so) Star. At the same time, Theta is a "spectroscopic double" with a period
of 11.44 days, which would place the companion out at about 0.25
AU. Since the occultation observation may have foreshortened, this
may or may not be the same star. But there is also evidence for
one or two other stars in the system. It is amazing at times what
we do not know about even bright stars. Perhaps Theta Oph's most
prominent role is that, lying in the Milky Way not far (in
projection) from the center of the Galaxy, it draws the eye to vast
dark dust clouds that lie in the neighborhood, in particular to the
striking "Pipe Nebula" that lies just below it,
all these and
others giving great beauty to the extraordinary encircling band of
light that is the disk of our Galaxy.