AL NIYAT (Sigma Scorpii). Double stars are common. So are stars with two names. But here is the reverse, a rare case in which one name applies to two stars! Al Niyat is the upper of the two stars that flank Antares, the bright star in the heart of Scorpius (and whose Arabic name means just that), the Scorpion. The name, which comes directly from an Arabic word that means "the arteries" (to the Arabic view of Antares as the heart) applies to both Sigma Scorpii and to the lower of the two stars, Tau Scorpii, and can be used for either. Rather well down the Greek alphabet, Al Niyat, a mid-third magnitude star (2.89), ranks eighth in the constellation. In line with its "double name" Al Niyat (Sigma) is not only double, but is at least quadruple! The system is dominated by a brilliant double star (seen as one with eye) that is too close to separate in any way other than with the spectrograph. The two hot components, one a class O (O9) hydrogen- fusing dwarf, the other a class B (B2) giant, go around each other in a mere 33 days, and are separated by only about the distance of Venus from the Sun. Still close, but still resolvable (only 0.4 seconds of arc away) lies another, what is probably a mid-class B star two magnitudes fainter, and farther out yet is a telescopic (ninth magnitude) companion of cooler class B (B9 or so). The distance, and consequently the luminosities, of the stars are not well known. Direct measures give 735 light years, but with a large error. Sigma is, however, a member of a large group or association of hot stars called "Upper Scorpius," whose average distance is 470 light years. If the two bright components of Sigma are the same brightness, which is reasonable, then the two distances give respective individual luminosities for each (accounting for ultraviolet radiation from a 30,000 Kelvin surface) of 65,000 and 27,000 Suns (and probably closer to the latter). One of the stars, probably the cooler class B giant, is a subtle "Beta Cephei" variable that changes brightness by around 10 percent with multiple periods that range from a quarter of a day to years (in much the same way as Mirzam, Beta Canis Majoris). Al Niyat (Sigma) is involved with a large mass of interstellar gas, which it ionizes and makes to glow as a diffuse nebula easily visible on photographs of the constellation. Rather unusual among bright naked eye stars, Sigma's light is dimmed by interstellar dust by over a magnitude, which must be corrected for. Absorption of starlight by the dust also "reddens" the star, making a naturally blue-white star look a rather dulled yellow-white. Whatever the distance, the two stars are very young (only a few million years old) and have large masses that lie between perhaps 12 and 20 solar masses each. To be a giant now, one that has already started its death process, the class B star must once have been the more massive of the pair. Unless the distance is still overestimated, each of the stars will eventually blow up as supernovae, the B star first, the O star next. Much later, the other two more-distant companions will die as massive white dwarfs like Sirius B.
Written by Jim Kaler. Return to STARS.