Sigma Orionis

Sigma Orionis is a quintuple system in the constellation Orion, whose five stars shine together at magnitude 3.66 just south of Alnitak in Orion's belt. σ Ori is the pinnacle of a small star cluster that lies about 1150 light years away. In turn, the cluster is a part of the Orion OB1 association, which includes many of the other stars in the constellation.

Components

There are few multiple stars more attractive than σ Orionis. Amateur telescopes show a quartet of stars, the brightest of which is also a close double, σ Ori AB. Its two components are merely 0.25" apart. The brighter star, σ Ori A, is a blue class O9.5 star with an apparent visual magnitude of +4.2. σ Ori B is a class B0.5 star with an apparent visual magnitude of +5.1. The pair orbit each other every 170 years at a distance of about 90 AU. σ Ori A and B are very hot stars, around 32,000 and 29,600 K, with luminosities of 35,000 and 30,000 suns. The stars' temperatures and luminosities give masses of 18 and 14 solar masses, making the close AB pair among the most massive visual binaries known.

The next brightest stars in the system are σ Ori D and E, which lie about 4,600 and 15,000 AU from the AB pair, respectively. Both are seven solar mass B-type dwarf stars with magnitudes of 6.62 and 6.66. Sigma Orionis E is a prototype of the strange "helium-rich" stars, which have large amounts 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. The faintest star in the system is σ Ori C, a class A main sequence star. σ Ori C is closest to the AB pair, about 3,900 AU away.

While the orbit of the AB pair is stable, the orbits C, D, and E are not. In time these three stars will be gravitationally sped up ejected from the system. Component A will explode, and may even kick B (which will explode next) out of the system. The other three stars, wherever they wind up, will die as white dwarfs.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]