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.