OMI SCO (Omicron Scorpii). Fifth magnitude Omicron Scorpii (at 4.55 just over the border from fourth), in northern Scorpius, is just barely the westernmost star of the "Scorpion's Pentagon" that (going clockwise) includes third magnitude Sigma Scorpii (one of the "Arteries"), first magnitude famed Antares, obscure 22 Scorpii, and the outlier double star Rho Ophiuchi. Three of them (Rho, actually both "Rho's," 22, and Sig Sco) are hot, massive blue-white class B2 stars, while Antares is a cool M1.5 supergiant and Omi Sco (the second faintest) is a white class A (A5) "bright giant" that falls somewhere in between. Adding to the lot, Antares has a hot class B (B2.5) companion. Evolving, Antares and Omi Sco were once hot dwarfs, making them even more part of the small family. The Pentagon is within one of the most complex and beautiful parts of the Milky Way, a vast region filled with bright diffuse and reflection nebulae and dark interstellar clouds. Especially well-known is the Rho Oph dark cloud, a star-forming region that stretches to the east of the eponymous duplicitous star. Though second-faintest of the five, Omi Sco stands out as both the most distant, 880 light years (give or take 130), and thus not surprisingly as the most reddened and obscured by interstellar dust. Interstellar dimming becomes stronger with shorter wavelengths, which makes stars look "redder" than they actually are. Interstellar reddening (which can be measured by comparing the observed technically-defined color with that expected from the spectral class) is closely tied to the visual obscuration, which can then be found. Omicron Sco is dimmed by 2.27 magnitudes. Were the line of sight clear, the star would shine at a prominent second magnitude (2.28) and might even have a proper name.

From distance and a temperature of 8390 (which places nearly all the radiation in the visual band of the spectrum), we find a luminosity of 7045 times that of the Sun, consistent with the star's classification as a bright giant. From luminosity and temperature, the radius comes out to be 40 times that of the Sun, about half the size of Mercury's orbit. The rotation period (from an equatorial rotation speed of at least 23 kilometers per second) is under 86 days. There seems to be some sort of infrared excess, which implies a surrounding disk of some sort. The mass of Omicron Scorpii is close to eight times that of the Sun, which places it near the limit above which stars explode as supernovae. Once it sloughs away its outer layers it will more likely die as a massive white dwarf not far below the white dwarf mass limit of 1.4 solar masses, perhaps one made of neon and oxygen rather than of oxygen and carbon. Now just over 28 million years old, Omi Sco was born as yet another class B2 dwarf, or at least close, fitting it right into the other stars of the Pentagon, except of course for more massive Antares, which was born as a class O star. Omicrom Sco may become unstable as it cools and spend some time as a Cepheid variable, and will briefly also grace the sky as a red supergiant, though one not as grand that which dominates the Pentagon right now.

Written byJim Kaler 9/19/14. Return to STARS.