Sigma Orionis
Encyclopedia
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. 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.
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
. It is approximately 1,150 light years from Earth.
The primary component is the binary
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
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
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
star with an apparent visual magnitude of +4.2. Sigma Orionis B is a B-type
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
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
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
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
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
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.