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.