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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.