RHO OPH (Rho Ophiuchi). As we admire Scorpius, we focus on Antares and perhaps its flanking stars Sigma and Tau (both named "Al Niyat"), as well as the Scorpion's
three-star head. Few notice a fainter fifth magnitude (at the
bright end, 4.63) interloper from Ophiuchus named Rho Ophiuchi. Just northeast of Sigma
Sco, it is largely overwhelmed by its neighbors. And too bad, as
it is quite the wondrous hot (22,400 Kelvin) class B (B2) double,
which shines at us from a large distance of 395 light years.
Though each star carries the same spectral class, one is notably
brighter than the other (5.0 vs. 5.9). Separated by just three
seconds of arc as projected on the sky, the two are at least 400
Astronomical Units apart and take at least 2000 years to orbit each
other. The brighter (at 4900 solar luminosities and a mass of
around 9 times that of the Sun), is classed
a subgiant, the fainter (at 2100 solar luminosities and 8 solar
masses) a dwarf. Both are probably still in the late hydrogen
fusing stage. Typical of hot B stars, they rotate quickly, over
300 kilometers per second at the equator. Though a pretty sight
through a small telescope, and an interesting hot double in its own
right, Rho is far more famed for its setting within dusty clouds of
the Milky Way, the luminous band that
is caused by the combined light of the stars in the disk of our
Galaxy. Surrounding Rho is a bright "reflection nebula" caused by
starlight that is scattered from a cloud of interstellar dust
grains. Surrounding the nebula are vast dark unilluminated clouds
extending 100 light years to the east toward Sagittarius that are active seats of star formation. The dark dust
(which dims Rho by two magnitudes, a factor of 6) is a marker for
what are really thick "molecular clouds" made mostly of hydrogen in
molecular form. Compression of the dusty gas by stellar winds and
shock waves from exploding stars causes clumps to gather together
that will eventually collapse under their own gravity to form new
stars. The process is happening literally as we watch, as we can
penetrate the clouds with observations of infrared and radio waves.
Rho Oph and all the other stars in the neighborhood were born this
way. As Rho evolves to become a pair of massive white dwarfs, it will be
replaced by new stars that will someday escape their birth clouds
to take their places within the Galaxy's Milky Way. (Rho Oph is
included in Jim Kaler's Hundred
Greatest Stars.)