KAPPA SCO (Kappa Scorpii). The curves of stars that make the body of Scorpius, the Scorpion, is one of the most dramatic and recognizable figures of the nightly sky. At the southeastern end, just short of the two-star "stinger" (made of Shaula and Lesath) lies bright, second magnitude (2.41) Kappa Scorpii, which if in most other constellations would bear a proper name, but here tends to be lost among the host of other bright stars. And too bad, because this class B (B1.5) giant has a double surprise: it's both binary, with a lesser companion, at the same time that the dominant star is a subtle Beta Cephei- type variable. At a rather large measured distance of 465 light years (enough for about a ten percent dimming due to interstellar dust), the two together radiate a hefty energy of 15,300 Suns. Spectroscopic analysis and orbit (known from the shifts in the wavelengths of the stars' light) gives individual temperatures of 23,400 and 18,800 Kelvin for the Kappa Sco A and Kappa Sco B (making the latter a mid-class B star), plus radii of 6.8 and 5.8 solar and an estimate for individual luminosities that lead to 11,700 Suns for the primary star and 3550 for the lesser, from which in turn we derive masses of 10.5 and 7 solar (published masses are even higher). With a measured orbital period of 195.65 days (0.536 years), the two must be separated by 1.7 Astronomical Units, a bit farther than Mars is from the Sun. A rather high orbital eccentricity takes them from 2.5 AU apart to only 0.87 AU, less than the Earth's orbital size. The more massive, Kappa Sco A, falls nicely into the realm of the Beta Cephei class of stars, oscillating over only three hundredths of a magnitude with multiple periods of 0.200, 0.205, 7.3. 0.19, and 2.59 days, some parts of the star moving outward, while others move inward. Kappa Sco's metal content is (typical of class B stars) 65 percent that of the Sun, while it spins (the two unrelated) with a typical equatorial velocity of at least 130 kilometers per second (giving a rotation period of under 2.5 days). The star is assumed to be part of the huge Scorpius-Centaurus association of hot blue stars, though its membership is not confirmed. Both are at the ends of their hydrogen-fusing lives. The larger of the two may be big enough to explode as a supernova, while the lesser will become a massive white dwarf. Such an explosion may even eject Kappa Sco B from the system, creating a "runaway star" like Zeta Ophiuchi.
Written by Jim Kaler. Return to STARS.