SHAULA (Lambda Scorpii). 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 (1.63), it is tied with Gacrux (Gamma Crucis) for 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 365 light years, Lesath farther 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 complex triple system that is not completely understood. The distance has been a problem. While the Hipparcos satellite gave 700 light years, more recent observations give only half that (the above 365 light years). The principal star, Shaula A, is a hot (25,000 Kelvin) class B (B1.5) subgiant with a radius 6.2 times that of the Sun and a mass (from temperature and luminosity) of 11 solar. The star seems more to be a hydrogen fusing dwarf rather than a subgiant. It is also a subtle pulsating variable of the "Beta Cephei" class, changing its brightness by less than a tenth of a magnitude with at least two periods of 0.2137 and 0.1069 days going on at the same time. The pulsations are caused by subsurface ionizing metals that act as a heat valve. Number 2 in the system, Shaula B, is a somewhat lesser class B2 star that orbits Shaula A every 2.96 years at (from the stellar masses and period) an average separation of 5.7 Astronomical units, just over Jupiter's distance from the Sun. A modest eccentricity carries them as close as 4.4 AU and as far as 7.0 AU. Shaula B, with a temperature of 21,000 Kelvin, radiates 5000 solar luminosities, from which we find a radius of 5.4 solar and a mass of 8 solar. Orbiting Shaula A with a period of just 5.9525 days is a far lesser star called "Shaula Ab" (rendering the principal star "Shaula Aa") that is hypothesized to be the origin of Shaula's highly unusual X-ray radiation. With an estimated mass of 1.8 solar, Shaula AB would orbit at a distance from Shaula Aa of only 0.15 AU, less than half Mercury's distance from the Sun. One would expect a small companion this close to have a circular orbit. Instead, Shaula Ab winds its way from as far as 0.19 AU from Shaula Aa to as close as 0.11 AU. Shaula Ab might be a neutron star created in a supernova blast from a much more massive progenitor, a massive white dwarf that is the result of mass transfer, or -- which seems to be the most likely -- a star that is still in the act of forming, a "T Tauri" star (such stars vigorous X-ray sources), which is consistent with a system age of less than 15 million years. Shaula Aa will probably explode as a supernova (expansion as a supergiant probably killing off Shaula a), though creation of a massive neon-oxygen white dwarf may still be possible. Shaula B, on the other hand, if not absorbed or destroyed, will most likely pursue white dwarfhood. Thanks to S. Kabir for suggesting this star, to Ken Croswell for correspondence and a data summary, and to Bill Hartkopf for clarifying multiple star nomenclature.
Written by Jim Kaler. Return to STARS.