67 OPH (67 Ophiuchi). Look to the northeastern part of Ophiuchus to find an exquisite five-star vee-shaped asterism that for awhile was its own constellation. Rather reminding one of the Zodiac's classic Taurus, the figure got the name Taurus Poniatovii, which honored the late-eighteenth century King of Poland, Stanislaus Poniatowski. Few remember it now by its more common name "Poniatowski's Bull." Had the constellation formally survived, its brightest (mid-fourth magnitude, 3.97) member, Flamsteed's 67 Ophiuchi, would be Alpha Tauri Poniatovii (the classic binary 70 Oph becoming Beta "TaP," a totally unofficial abbreviation). But it didn't, and it isn't, so 67 Oph will have to do. And too bad, as this very luminous blue-white class B (B5) supergiant would have made a magnificent "alpha star." It is far enough (1420 light years, but with an associated 33 percent error), as well as close enough to the Milky Way, that interstellar dust dims its light by a third of a magnitude. Unfortunately the star is almost as obscure to astronomers as the one-time constellation. Two temperature estimates have very different values of 13,180 and 16,000 Kelvin. The former seems to be the better, however, as it is much closer to that expected from other, similar, supergiants. When dust absorption and ultraviolet light are accounted for, the star is seen to shine with the light of 12,100 Suns, which then yields a radius of 21 solar and (from theory) a mass of 9 times that of the Sun. (The higher temperature gives 10 times solar.) A minimum equatorial rotation velocity of 40 kilometers per second gives a maximum rotation period of under 26 days. With an age of 26 million years following its class B2 beginning, 67 Oph is now in a transition state in which, with a dead helium core, it is expanding to become a much larger red supergiant, at which point it will fire up its internal helium and fuse that into carbon and oxygen. The star is right in the transition region where it could evolve as either a massive white dwarf or explode as a supernova. At present there is no way to tell except to wait it out, and it's a very long wait, even for a star evolving as fast (astronomically) as this one. Clumping around it is a set of four faint "companions" that are most likely all just line-of-sight coincidences. At 67 Oph's distance, "B" (a 14th magnitude B1 dwarf 7 seconds of arc away) and "C" (8th magnitude, 15 seconds of arc) are too faint for their given spectral classes, and "E" (much more distant) seems to have moved too much ("D" a seeming companion to "C"). Then back to probably-single 67 Oph itself, which seems to vary slightly, by 0.5 percent, over a short period of 2.3 days. Of much greater significance, 67 Ophiuchi seems to be at the peak of a young, very extended (by several degrees) open cluster with a curious history called Collinder 359 (from the astronomer Per Collinder, 1890-1974). Of the original brightest members, Gamma=62, 66, 67, 68, and 73 Oph, all but 67 Oph were later shown to be NON-members, while a large number of other much fainter stars were then included. An estimated age, however, of 60 million years is not fully consistent with that of 67 Oph. Collinder 359 may more be classed as a "moving group," one that is not gravitationally well-bound together. Poniatowski's Bull then takes on an even more interesting aspect rather similar to Vulpecula's "Coathanger" (Collinder 399), which was also once thought to be a cluster, but isn't.
Written by Jim Kaler. Return to STARS.