Al NIYAT (Tau Scorpii). Many stars in the sky are referred to by the same, or similar, names. "Deneb," for example, means "tail" in Arabic, and is used for first magnitude Deneb at the tail of Cygnus the Swan and for the tails of the whale (Deneb Kaitos), the tail of Capricornus (Deneb Algedi), and others. But there are few names that actually refer to a pair of stars as a set. The stars that flank Antares in Scorpius are together known as Al Niyat, the "arteries" of the heart of the Scorpion, the upper one Bayer's Sigma, the lower one his Tau. Mid-third magnitude (2.82), Al Niyat (Tau) is just a hair brighter than Al Niyat (Sigma), and ranks seventh in the constellation. An archetypal class B (B0) hydrogen-fusing dwarf 430 light years away, Al Niyat (Tau) is closer to us than Sigma. It is also not as luminous, radiating "only" 18,000 Suns-worth of energy (including a lot of ultraviolet) into space. Sigma should appear much brighter than Tau, but it is more heavily obscured by interstellar dust. From its high temperature of 30,700 Kelvin and its luminosity, we calculate a large (for a dwarf) radius of 5 times that of the Sun. Like Sigma, Tau is also a member of the "Upper Scorpius Association" (an "association" a loose group of hot stars that were born more or less at the same time) whose average distance agrees nicely with the star's individual distance. Al Niyat (Tau) is among the most-observed stars of the sky. In the past half-century, it has been part of well over 400 scientific papers. Its popularity is due to its hot- star status, its luminosity, the clarity of its spectrum caused by slow rotation, and its singularity. Unlike many stars of its kind, Tau seems distinctly single, with no evidence at all of any companion. Among hot class B stars it is most unusual in having a rotation speed less than 5 kilometers per second, just 2.5 times that of the Sun (many class B stars whip around with speeds in the low hundreds). Rotation of such stars is determined by measuring rotation velocities by means of the Doppler effect. We suspect that Tau may in fact be a faster rotator, but that its rotation axis is pointed directly at us, so that rotation is not noticed. The star is blowing a wind and radiates X-rays. Detailed composition studies show Al Niyat (Tau) to be deficient in several elements, particularly oxygen and iron, relative to the Sun. The feeling has been growing that it is not the local stars that are deficient, but that the Sun is slightly metal-rich for its surroundings. Why, we do not know, since the Sun is much older than any class B star, and should, if anything, have a lower metal abundance. Al Niyat's luminosity and temperature show it to have a mass of a dozen times that of the Sun and to be near the lower end of stars whose fates are to explode as supernovae.
Written by Jim Kaler. Return to STARS.