Sigma Orionis

Sigma Orionis

Starbox begin
name=Sigma Orionis A/B/C/D/E
Starbox observe
epoch=J2000
constell=Orion
ra=05h 38m 44.80s
dec=−2° 36' 00.0"
appmag_v=4.2/5.1/8.79/6.62/6.66
Starbox character
class=O9V/B0.5V/A2V/B2V/B2Vp
b-v=?/?/−0.02/−0.20/−0.18
u-b=?/?/−0.25/−0.87/−0.86
variable=No/No/No/No/Yes
Starbox catalog
names=Sigma Orionis, BD-02°1326, HD 37468, HR 1931, HIP 26549, SAO 132406
Starbox reference
Simbad=HD+37468

Sigma Orionis or Sigma Ori (σ Orionis / σ Ori) is a five star system in the constellation Orion. It is approximately 1,150 light years from Earth.

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 and is one of the most luminous stars known. 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, a 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.


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