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.