Sigma Orionis

Sigma Orionis A/B/C/D/E

Sigma Orionis is the bright star in the lower right corner, the brighter stars are Alnitak and Alnilam.
Observation data Epoch J2000      Equinox J2000
Constellation	Orion
Right ascension	05h 38m 44.80s
Declination	−2° 36' 00.0"
Apparent magnitude (V)	4.2/5.1/8.79/6.62/6.66 (total 3.66)
Characteristics
Spectral type	O9V/B0.5V/A2V/B2V/B2Vp
U−B color index	?/?/−0.25/−0.87/−0.86
B−V color index	?/?/−0.02/−0.20/−0.18
Variable type	No/No/No/No/SX Ari
Other designations
Sigma Orionis, Sigma Ori, σ Orionis, σ Ori, 48 Orionis, 48 Ori, BD-02°1326, HD 37468, HR 1931, HIP 26549, SAO 132406
Database references
SIMBAD	data

Sigma Orionis or Sigma Ori (σ Orionis, σ Ori) is a five-star system in the constellation Orion, just to the southwest of Alnitak. It is approximately 1,150 light years from Earth.^[1]

The primary component is the binary Sigma Orionis AB, with the two stars being a mere 0.25 arcseconds apart. Both stars are hydrogen-fusing dwarfs only a few million years old. The brighter one, Sigma Orionis A, is a blue O-type star with an apparent visual magnitude of +4.2. Sigma Orionis B is a B-type star with an apparent visual magnitude of +5.1. The pair orbit each other every 170 years at a distance of about 90 AU. A and B have very hot surfaces, around 32,000 and 29,600 kelvins, radiating at about 35,000 and 30,000 solar luminosities, respectively. Temperature and luminosity give masses of 18 and 13.5 solar masses, making the close AB pair among the most massive of visual binaries.

The next brightest stars in the system are Sigma Orionis D and E, which lie about 4,600 and 15,000 AU respectively from the AB pair. 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 significantly large amounts of helium.

The last star in the system is Sigma Orionis C, an A-type dwarf star. C is the closest to the AB pair, about 3,900 AU away.

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 of the system.

References