OMEGA-1 AND OMEGA-2 SC0 (Omega-1 and Omega-2 Scorpii), another two-for-one special. In Scorpius, seven and a half degrees northwest of Antares and just one degree south-southeast of Beta Sco (the northernmost star in the trio that makes the Scorpion's head), lies a prominent duo of stars, fourth magnitude (3.96) Omega-1 Scorpii (the western of the pair) and fainter-fourth magnitude (4.32) Omega-2. Only a quarter of a degree apart, at first they seem like a naked-eye double. But once again the eye is fooled, as they have no relation to each other except chance alignment, as Omega-1 is 471 light years away (give or take 18), Omega-2 60 percent as far, at 291 light years (plus or minus 8). They are radically different physically as well. Omega-2 is a yellowish class G (G3, though classed as cool as G8) ageing giant, whereas Omega-1 is a blue, hot class B (B1) youthful dwarf. Unfortunately Omega-1's true color does not come through as a result of interstellar dust absorption and reddening that makes it appear more white. If the path to the star were clear, Omega-1 would be just over a magnitude brighter, shine at a nice third magnitude of 2.94, and perhaps even have a proper name. Distance and a temperature of 26,530 Kelvin (from which we calculate a lot of ultraviolet radiation) gives a whopping luminosity of 11,700 Suns, a radius 5.13 time that of the Sun, and a mass of just under 12 Suns (which agrees well with a published value of 11.1), the star roughly 80 to 100 million years old, well along to the cessation of core hydrogen fusion. A projected equatorial rotation speed of 105 kilometers per second (nowhere near the limit) yields a rotation period of under 2.5 days. It may be a subtle "Beta Cephei" variable with a period of 0.667 days but of unknown amplitude. Omega-1 Sco is part of the expanding Scorpius OB association of young luminous stars, specifically belonging to the "Upper Scorpius" subgroup along with Antares, Sigma, Pi, and Nu Scorpii, Chi and Rho Ophiuchi, and 48 Librae. The average distance to the subgroup is 145 light years, which places Omega-1 pretty much in the middle. Its mass of a dozen Suns is enough to someday send it over the edge. An iron core derived from advanced nuclear fusion will collapse to a neutron star, blowing the outer envelope away in a grand supernova, the event creating all the elements of the periodic table, which will mix with the gases of interstellar space from which new generations of stars will be born.

Nowhere near as dramatic, Omega-2 is an ordinary helium fusing giant. Including some infrared radiation from its 5300 Kelvin surface, the star radiates at a rate of 141 Suns, which gives a radius 14.1 times solar and a mass of 3 Suns. There is little if any intervening interstellar dust. An interferometer measure of radius in the infrared gives 15.6 times solar, which is decent agreement given that such large stars do not have sharply defined surfaces. Instead of exploding, Omega-2 will slough off its outer envelope. The inner portion will turn into a planetary nebula illuminated by the exposed core, which in turn will cool forever as a white dwarf of around 0.7 solar masses.
Written byJim Kaler 09/09/16. Return to STARS.