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Eta Orionis
Star in the constellation Orion From Wikipedia, the free encyclopedia
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Eta Orionis is a multiple star in the constellation Orion. It lies a little to the west of Orion's Belt between Delta Orionis and Rigel, being closer to Delta Orionis than to Rigel. It lies at a distance of around 1,000 light-years from Earth and is part of the Orion OB1 association.
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Nomenclature
Eta Orionis, Latinized from η Orionis, is the star's Bayer designation. It has the traditional Arabic name Saif al Jabbar, the Sword of the Giant, but this name is now used for another star, Saiph (Kappa Orionis).[10] It has also sometimes been called by the Latin name Ensis,[10] and Algjebbah.[11]
System
Eta Orionis is listed in multiple star catalogues as having two companions: a bright component B less than 2″ away; and a faint component C nearly 2′ away.[12] The two are estimated to orbit every 1,800 years.[8]
The primary star, Eta Orionis A, is itself a spectroscopic triple star, known from multiple spectral lines with varying radial velocities.[7] The most distant component Ac, has been resolved using speckle interferometry, at a separation of about 0.04″. It orbits the other two in 9.4 years.[3] The two closest stars, Aa and Ab, are separated by only about a tenth of an astronomical unit and orbit in just under eight days.[7]
The system lies within the Orion OB1 association, a group of massive stars that includes most of the bright stars of Orion.[13] It is assigned to the oldest and closest part of the association, known as OB1a.[14]
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Variability
Eta Orionis drops in brightness every four days from a combined apparent magnitude of 3.31 to about magnitude 3.6. This is due to eclipses between the two closest components, Aa and Ab.[4] The primary and secondary eclipses are very similar, 0.24 and 0.23 magnitudes deep, respectively.[16]
It has also been suggested that component Ab is intrinsically variable with a period of 0.3 days and a very small amplitude. This star has unusual variable spectral lines and lies with the β Cephei variable instability strip.[7] However, it is now thought that the variable component is either B and Ac, possibly due to an unseen companion or rotational modulation. The actual period is 0.432 days and the 0.3-day period was an alias.[16]
See also
References
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