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SHAULA

In temperate northern summers, Scorpius glides above the southern horizon, its lower curved tail almost out of sight, while in the temperate southern winter, the constellation passes high overhead. At the end of the tail lies a pair of stars that represent the scorpion's "stinger," once called Shaula, from Arabic meaning exactly that. In more modern times the name moved to the brighter of the pair, the fainter now called Lesath. Even though Bayer gave Shaula the lowly Lambda (the 11th letter of the Greek alphabet) designation (probably because of its far southern position) the star is the second brightest in the constellation, following Antares. At bright second magnitude, it is the 24th brightest star in the sky. Of second magnitude stars, only Castor in Gemini is brighter. Though the stinger stars appear close together, only half a degree apart, they are not a real couple, Shaula lying at a distance of 700 light years, Lesath closer at 520. However both stars and several others in southern Scorpius do belong to the huge nearby "Scorpius OB1 association," an expanding disintegrating group of hot stars that were all born about the same time. Shaula is a hot class B star with a temperature around 25,000 Kelvin, over four times hotter than the Sun. Its distance, apparent brightness, and temperature (from which we find the amount of invisible ultraviolet radiation), shows it to radiate some 35,000 times more energy than our Sun. Shaula, however, is a close double made of roughly similar hot stars with an orbital period of 5.9 days, so the luminosity of the brighter component shines some 18,000 times solar. There seems to be a third, more distant companion as well, about which nothing is known (showing how little we understand some stars). Shaula is an unusually strong source of low-energy X-rays, which also suggests a third component, possibly a white dwarf. Shaula is classed as a "subgiant," telling us that hydrogen fusion in the core of the main component is shutting down or has shut down altogether. With a mass around 11 times that of the Sun, the star may explode or more likely turn into a rare, heavy white dwarf, possibly one with a neon-oxygen core. Like Mirzam, Shaula is a subtle variable of the "Beta Cephei" type, changing its brightness by less than a tenth of a magnitude with two periods of 0.21 and 0.11 days going on at the same time, the variation the result of internal structural changes that no one understands very well. Thanks to S. Kabir for suggesting this star.