Editing Eta Carinae (section)

===Orbit===
[[File:Eta carinae orbit.png|upright=1.6|left|thumb|alt=η&nbsp;Carinae B orbits in large ellipse, and η&nbsp;Carinae A in a smaller elliptical orbit.|Eta Carinae orbit]]
The binary nature of η&nbsp;Carinae is clearly established, although the components have not been directly observed and cannot even be clearly resolved spectroscopically due to scattering and re-excitation in the surrounding nebulosity. Periodic photometric and spectroscopic variations prompted the search for a companion, and modelling of the colliding winds and partial "eclipses" of some spectroscopic features have constrained the possible orbits.<ref name="madura">{{cite journal|bibcode=2012MNRAS.420.2064M|title=Constraining the absolute orientation of η Carinae's binary orbit: A 3D dynamical model for the broad [Fe III] emission|journal=Monthly Notices of the Royal Astronomical Society|volume=420 |issue=3|pages=2064|last1=Madura|first1=T. I.|last2=Gull|first2=T. R.|last3=Owocki|first3=S. P.|last4=Groh|first4=J. H.|last5=Okazaki|first5=A. T.|last6=Russell|first6=C. M. P.|year=2012|doi=10.1111/j.1365-2966.2011.20165.x|doi-access=free |arxiv = 1111.2226|s2cid=119279180}}</ref>

The period of the orbit is accurately known at 5.539&nbsp;years, although this has changed over time due to mass loss and accretion. Between the Great Eruption and the smaller 1890&nbsp;eruption, the orbital period was apparently 5.52&nbsp;years, while before the Great Eruption it may have been lower still, possibly between 4.8 and 5.4&nbsp;years.<ref name=KashiSoker/> The orbital separation is only known approximately, with a [[semi-major axis]] of {{nobr|15–16 {{sc|AU}}.}} The orbit is highly [[orbital eccentricity|eccentric]], {{nobr|{{math|&hairsp; ''[[orbital eccentricity|e]]'' {{=}} 0.9}} .}} This means that the separation of the stars varies from around {{nobr|1.6 [[astronomical unit|{{sc|AU}}]],}} similar to the distance of [[Mars (planet)|Mars]] from the Sun, to 30&nbsp;AU, similar to the distance of Neptune.<ref name=madura/>

Perhaps the most valuable use of an accurate orbit for a binary star system is to directly calculate the masses of the stars. This requires the dimensions and inclination of the orbit to be accurately known. The dimensions of η&nbsp;Carinae's orbit are only known approximately as the stars cannot be directly and separately observed. The inclination has been modelled at 130–145 degrees, but the orbit is still not known accurately enough to provide the masses of the two components.<ref name=madura/>