Eta Orionis
Star in the constellation Orion
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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). It has also sometimes been called by the Latin name Ensis, and Algjebbah.
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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. The two are estimated to orbit every 1,800 years.
The primary star, Eta Orionis A, is itself a spectroscopic triple star, known from multiple spectral lines with varying radial velocities. 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. 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.
The system lies within the Orion OB1 association, a group of massive stars that includes most of the bright stars of Orion. It is assigned to the oldest and closest part of the association, known as OB1a.
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. The primary and secondary eclipses are very similar, 0.24 and 0.23 magnitudes deep, respectively.
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. 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.