SIGMA ORI (Sigma Orionis). Double stars are among the amateur's
favorite targets, Albireo, Mizar always on the list. Multiples are
better yet, and there are few more attractive than Sigma Orionis
(which has no proper name), where you see a quartet of stars, the
brightest of which is also a close double. Indeed, Sigma Ori,
whose five stars together shine in Orion
at bright fourth magnitude (3.66) just south of Alnitak in Orion's belt, is really at the
pinnacle of a small star cluster that lies a somewhat-uncertain
1150 light years away. In turn, the stars and the cluster are a
part of the Orion OB1 association, which includes many of the other
stars in the constellation. Sigma's main component, "AB,"
dominates, the two a mere 0.25 seconds of arc apart shining at
magnitudes 4.2 and 5.1. Both very young hydrogen-fusing dwarfs
only a few million years old, the brighter is a magnificent blue
class O (09.5) star, while the lesser is class B (B0.5). The pair
orbit every 170 years at a distance of about 90 Astronomical Units.
After correction for ultraviolet light from very hot (32,000 and
29,600 Kelvin) surfaces, they respectively radiate at a rate of
35,000 and 30,000 Suns. Temperature and luminosity give masses of
18 and 13.5 times that of the Sun, the sum
of nearly 32 solar masses making the close AB pair among the most
massive of visual binaries. Together they illuminate their
surroundings, causing interstellar gas to glow. The next brightest
stars in the system are Sigma Ori "D" and "E," bright seventh
magnitude class B (B2) dwarf stars that at magnitudes 6.62 and 6.65
are nearly identical in brightness and have masses around 7 times
that of the Sun. The similarity stops there. "E" is the prototype
of the weird "helium-rich" stars that have strangely elevate
abundances of helium. Even odder, the helium in "E" seems to be
concentrated toward particular patches that involve a combination
of the rotational and magnetic field axes. They may be related to
cooler magnetic stars such as Cor Caroli, but no one really
understands them. The last of Sigma's stars, "C," appears to be a
normal ninth magnitude class A dwarf. In projection on the sky,
"C" is the closest to the AB pair, and is at least 3900 AU away,
while "D" and "E" lie at least 4600 and 15,000 AU distant. While
the orbit of the AB pair is stable, the orbits of the other three
are not, and long before they die they will probably be
gravitationally sped up and forced out. "A" will explode first and
may even kick "B" (which will explode next) out of the system. The
other stars, wherever they wind up, will die as white dwarfs. The
cluster seems also to contain numerous low-mass stars, brown
dwarfs, that have masses only a few times that of Jupiter.