Editing Eta Carinae (section)

==Properties==
[[File:EtaCarinaeStarSystem-3Views-XRayOpticalIR-20140826.jpg|thumb|upright=1.6|alt=η&nbsp;Carinae star system, 3&nbsp;views side by side|X-ray, optical and infrared images of η&nbsp;Carinae (26 August 2014)]]
The η&nbsp;Carinae star system is currently one of the [[List of most massive stars|most massive stars]] that can be studied in great detail. Until recently η&nbsp;Carinae was thought to be the most massive single star, but the system's binary nature was proposed by the Brazilian astronomer Augusto Damineli in 1996<ref name="damineli"/> and confirmed in 2005.<ref name="massive">{{cite journal|bibcode=2005AAS...20717506I|title=Searching for radial velocity variations in Eta Carinae |journal=American Astronomical Society Meeting 207|volume=207|pages=1445|last1=Iping|first1=R. C.|last2=Sonneborn|first2=G.|last3=Gull|first3=T. R. |last4=Ivarsson|first4=S.|last5=Nielsen|first5=K.|year=2005}}</ref> Both component stars are largely obscured by circumstellar material ejected from η&nbsp;Carinae A, and basic properties such as their temperatures and luminosities can only be inferred. Rapid changes to the stellar wind in the 21st century suggest that the star itself may be revealed when dust from the great eruption finally clears.<ref name="mehner2012">{{cite journal |bibcode=2012ApJ...751...73M |title=Secular changes in Eta Carinae's wind 1998–2011 |journal=[[The Astrophysical Journal]] |volume=751 |issue=1 |page=73 |last1=Mehner|first1=Andrea |last2=Davidson|first2=Kris|last3=Humphreys|first3=Roberta M.| last4=Ishibashi|first4=Kazunori |last5=Martin|first5=John C.|last6=Ruiz|first6=María Teresa |last7=Walter |first7=Frederick M. |year=2012 |doi=10.1088/0004-637X/751/1/73 |arxiv = 1112.4338|s2cid=119271857 }}</ref>

===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/>

===Classification===
η&nbsp;Carinae&nbsp;A is classified as a [[luminous blue variable]] (LBV) due to the distinctive spectral and brightness variations. This type of [[variable star]] is characterised by irregular changes from a high temperature quiescent state to a low temperature outburst state at roughly constant luminosity. LBVs in the quiescent state lie on a narrow {{nowrap|[[S Doradus]]}} instability strip, with more luminous stars being hotter. In outburst all LBVs have about the same temperature, which is near 8,000&nbsp;K. LBVs in a normal outburst are visually brighter than when quiescent although the [[wikt:bolometric magnitude|bolometric]] luminosity is unchanged.

An event similar to η&nbsp;Carinae&nbsp;A's Great Eruption has been observed in only one other star in the [[Milky Way]] — {{nobr|[[P Cygni]]}} — and in a handful of other possible LBVs in other galaxies. None of them seem to be quite as violent as η&nbsp;Carinae's. It is unclear if this is something that only a very few of the most massive LBVs undergo, something that is caused by a close companion star, or a very brief but common phase for massive stars. Some similar events in external galaxies have been mistaken for supernovae and have been called [[supernova impostor]]s, although this grouping may also include other types of non-terminal transients that approach the brightness of a supernova.<ref name="impostors">{{cite book |last1=Davidson |first1=Kris |last2=Humphreys |first2=Roberta M. |year=2012 |title=Eta Carinae and the Supernova Impostors |volume=384 |series=Astrophysics and Space Science Library |publisher=Springer Science & Business Media |location=New York, NY |isbn=978-1-4614-2274-7 |pages=26–27 |url=https://cds.cern.ch/record/1453273 |doi=10.1007/978-1-4614-2275-4|bibcode=2012ASSL..384.....D }}</ref>

η&nbsp;Carinae&nbsp;A is not a typical LBV. It is more luminous than any other LBV in the Milky Way although possibly comparable to other supernova impostors detected in external galaxies. It does not currently lie on the S&nbsp;Doradus instability strip, although it is unclear what the temperature or spectral type of the underlying star actually is, and during its Great Eruption it was much cooler than a typical LBV outburst, with a middle-G&nbsp;spectral type. The 1890 eruption may have been fairly typical of LBV eruptions, with an early F&nbsp;spectral type, and it has been estimated that the star may currently have an opaque stellar wind, forming a pseudo-photosphere with a temperature of 9,000–{{Val|10000|fmt=commas|ul=K}}.<ref name="hillier"/><ref name="groh"/><ref name="davidson1997"/>

η&nbsp;Carinae&nbsp;B is a massive luminous hot star, about which little else is known. From certain high excitation spectral lines that ought not to be produced by the primary, η&nbsp;Carinae&nbsp;B is thought to be a young [[O-type star]]. Most authors suggest it is a somewhat evolved star such as a supergiant or giant, although a [[Wolf–Rayet star]] cannot be ruled out.<ref name="massive"/>

===Mass===
The masses of stars are difficult to measure except by determination of a binary orbit. η&nbsp;Carinae is a binary system, but certain key information about the orbit is not known accurately. The mass can be strongly constrained to be greater than {{solar mass|90}}, due to the high luminosity.<ref name="hillier"/> Standard models of the system assume masses of {{solar mass|100–120}}<ref name="KashiSoker"/><ref name="mehner2015">{{cite journal |last1=Mehner |first1=A. |last2=Davidson |first2=K. |last3=Humphreys |first3=R.M. |last4=Walter |first4=F.M. |last5=Baade |first5=D. |last6=de Wit |first6=W.J. |last7=Martin |first7=J. |last8=Ishibashi |first8=K. |last9=Rivinius |first9=T. |last10=Martayan |first10=C. |last11=Ruiz |first11=M.T. |last12=Weis |first12=K. |display-authors=6 |year=2015 |title=Eta Carinae's 2014.6 spectroscopic event: Clues to the long-term recovery from its Great Eruption |journal=Astronomy & Astrophysics |volume=578 |page=A122 |doi=10.1051/0004-6361/201425522 |bibcode=2015A&A...578A.122M |arxiv=1504.04940|s2cid=53131136}}</ref><ref name="clementel">{{cite journal|bibcode=2015MNRAS.447.2445C|arxiv=1412.7569|title=3D radiative transfer simulations of Eta Carinae's inner colliding winds – I. Ionization structure of helium at apastron|journal=Monthly Notices of the Royal Astronomical Society|volume=447|issue=3|pages=2445|last1=Clementel|first1=N. |last2=Madura|first2=T. I.|last3=Kruip|first3=C. J. H.|last4=Paardekooper|first4=J.-P.|last5=Gull|first5=T. R.|year=2015|doi=10.1093/mnras/stu2614|doi-access=free |s2cid=118405692}}</ref> and {{solar mass|30–60}}<ref name="KashiSoker"/> for the primary and secondary, respectively. Higher masses have been suggested, to model the energy output and mass transfer of the Great Eruption, with a combined system mass of over {{solar mass|250}} before the Great Eruption.<ref name="KashiSoker"/> η&nbsp;Carinae&nbsp;A has clearly lost a great deal of mass since it formed, and it is thought that it was initially {{solar mass|150–250}}, although it may have formed through binary merger.<ref name="smith2015">{{cite journal |last1=Smith |first1=Nathan |last2=Tombleson |first2=Ryan |year=2015 |title=Luminous blue variables are antisocial: Their isolation implies that they are kicked mass gainers in binary evolution |journal=Monthly Notices of the Royal Astronomical Society |volume=447 |issue=1 |pages=598–617 |doi=10.1093/mnras/stu2430 |doi-access=free |bibcode=2015MNRAS.447..598S |arxiv=1406.7431|s2cid=119284620}}</ref><ref name="nature">{{cite journal |last1=Smith |first1=Nathan |year=2008 |title=A blast wave from the 1843 eruption of η&nbsp;Carinae |journal=Nature |volume=455 |issue=7210 |pages=201–203 |arxiv=0809.1678 |bibcode=2008Natur.455..201S |doi=10.1038/nature07269 |pmid=18784719|s2cid=4312220}}</ref> Masses of {{solar mass|200}} for the primary and {{solar mass|90}} for the secondary best-fit one-mass-transfer model of the Great Eruption event.<ref name="KashiSoker"/>

===Mass loss===
[[File:ESO - The Carina Nebula (by).jpg|thumb|left|upright=1.4|alt=Carina Nebula|The Carina Nebula. η&nbsp;Carinae is the brightest star, on the left side.]]
Mass loss is one of the most intensively studied aspects of massive star research. Put simply, calculated mass loss rates in the best models of stellar evolution do not reproduce the observed properties of evolved massive stars such as Wolf–Rayets, the number and types of [[Supernova#Core collapse|core collapse supernovae]], or their progenitors. To match those observations, the models require much higher mass loss rates. η&nbsp;Carinae A has one of the highest known mass loss rates, currently around {{solar mass|10<sup>−3</sup>}}/year, and is an obvious candidate for study.<ref name="kashi">{{cite journal |last1=Kashi |first1=A. |last2=Soker |first2=N. |title=Possible implications of mass accretion in Eta Carinae |journal=New Astronomy |volume=14 |issue=1 |pages=11–24 |year=2009 |arxiv=0802.0167 |bibcode=2009NewA...14...11K |s2cid=11665477 |doi=10.1016/j.newast.2008.04.003}}</ref>

η&nbsp;Carinae A is losing a lot of mass due to its extreme luminosity and relatively low surface gravity. Its stellar wind is entirely opaque and appears as a pseudo-photosphere; this optically dense surface hides any true physical surface of the star that may be present. (At extreme rates of radiative mass loss, the density gradient of lofted material may become continuous enough that a meaningfully discrete physical surface may not exist.) During the Great Eruption the mass loss rate was a thousand times higher, around {{solar mass|1}}/year sustained for ten years or more. The total mass loss during the eruption was at least {{solar mass|10–20}} with much of it now forming the Homunculus Nebula. The smaller 1890 eruption produced the Little Homunculus Nebula, much smaller and only about {{solar mass|0.1}}.<ref name="gull"/> The bulk of the mass loss occurs in a wind with a terminal velocity of about 420&nbsp;km/s, but some material is seen at higher velocities, up to 3,200&nbsp;km/s, possibly material blown from the accretion disk by the secondary star.<ref name="noam">{{cite journal |last=Soker |first=Noam |title=Why a Single-Star Model Cannot Explain the Bipolar Nebula of η Carinae |journal=The Astrophysical Journal |volume=612 |issue=2 |pages=1060–1064 |year=2004 |bibcode=2004ApJ...612.1060S |arxiv=astro-ph/0403674 |s2cid=5965082 |doi=10.1086/422599}}</ref>

η&nbsp;Carinae B is presumably also losing mass via a thin, fast stellar wind, but this cannot be detected directly. Models of the radiation observed from interactions between the winds of the two stars show a mass loss rate of the order of {{solar mass|10<sup>−5</sup>}}/year at speeds of 3,000&nbsp;km/s, typical of a hot O-class star.<ref name="pittard"/> For a portion of the highly [[Orbital eccentricity|eccentric]] orbit, it may actually gain material from the primary via an [[accretion disk]]. During the Great Eruption of the primary, the secondary could have accreted {{solar mass|several}}, producing strong jets which formed the bipolar shape of the Homunculus Nebula.<ref name="kashi"/>

===Luminosity===
The stars of the η&nbsp;Carinae system are completely obscured by dust and opaque stellar winds, with much of the ultraviolet and visual radiation shifted to infrared. The total electromagnetic radiation across all wavelengths for both stars combined is several million solar luminosities ({{solar luminosity}}).<ref name="groh">{{cite journal |last1=Groh |first1=Jose H. |last2=Hillier |first2=D. John |last3=Madura |first3=Thomas I. |last4=Weigelt |first4=Gerd |bibcode=2012MNRAS.423.1623G|title=On the influence of the companion star in Eta Carinae: 2D radiative transfer modelling of the ultraviolet and optical spectra|journal=Monthly Notices of the Royal Astronomical Society|volume=423|issue=2|pages=1623|year=2012|doi=10.1111/j.1365-2966.2012.20984.x|doi-access=free |arxiv = 1204.1963 |s2cid=119205238}}</ref> The best estimate for the luminosity of the primary is {{solar luminosity|5 million}} making it one of the most luminous stars in the Milky Way. The luminosity of η&nbsp;Carinae B is particularly uncertain, probably {{solar luminosity|several hundred thousand}} and almost certainly no more than {{solar luminosity|1 million}}.

The most notable feature of η&nbsp;Carinae is its giant eruption or [[supernova impostor]] event, which originated in the primary star and was observed around 1843. In a few years, it produced almost as much visible light as a faint supernova explosion, but the star survived. It is estimated that at peak brightness the luminosity was as high as {{solar luminosity|50 million}}.<ref name="impostors"/> Other supernova impostors have been seen in other galaxies, for example the possible false supernova [[SN 1961V]] in [[NGC 1058]]<ref name="Stockdale_2001">{{cite journal |last1=Stockdale |first1=Christopher J. |last2=Rupen |first2=Michael P. |last3=Cowan |first3=John J. |last4=Chu |first4=You-Hua |last5=Jones |first5=Steven S. |title=The fading radio emission from SN 1961v: evidence for a Type II peculiar supernova? |journal=The Astronomical Journal |volume=122 |issue=1 |page=283 |date=2001 |bibcode=2001AJ....122..283S |arxiv= astro-ph/0104235 |s2cid=16159958 |doi=10.1086/321136}}</ref> and [[SN 2006jc]]'s pre-explosion outburst in [[UGC 4904]].<ref name="pastorello">{{cite journal |last1=Pastorello |first1=A. |last2=Smartt |first2=S.J. |last3=Mattila |first3=S. |last4=Eldridge |first4=J.J. |last5=Young |first5=D. |last6=Itagaki |first6=K. |last7=Yamaoka |first7=H. |last8=Navasardyan |first8=H. |last9=Valenti |first9=S. |last10=Patat |first10=F. |last11=Agnoletto |first11=I. |last12=Augusteijn |first12=T. |last13=Benetti |first13=S. |last14=Cappellaro |first14=E. |last15=Boles |first15=T. |last16=Bonnet-Bidaud |first16=J.-M. |last17=Botticella |first17=M.T. |last18=Bufano |first18=F. |last19=Cao |first19=C. |last20=Deng |first20=J. |last21=Dennefeld |first21=M. |last22=Elias-Rosa |first22=N. |last23=Harutyunyan |first23=A. |last24=Keenan |first24=F.P. |last25=Iijima |first25=T. |last26=Lorenzi |first26=V. |last27=Mazzali |first27=P.A. |last28=Meng |first28=X. |last29=Nakano |first29=S. |last30=Nielsen |first30=T.B. |display-authors=6 |year=2007 |title=A giant outburst two years before the core-collapse of a massive star |journal=Nature |volume=447 |issue=7146 |pages=829–832 |bibcode=2007Natur.447..829P |pmid=17568740 |arxiv=astro-ph/0703663 |s2cid=4409319 |doi=10.1038/nature05825}}</ref>

Following the Great Eruption, η&nbsp;Carinae became self-obscured by the ejected material, resulting in dramatic reddening. This has been estimated at four magnitudes at visual wavelengths, meaning the post-eruption luminosity was comparable to the luminosity when first identified.<ref name="smith2011a">{{cite journal|bibcode=2011MNRAS.415..773S|title=Luminous blue variable eruptions and related transients: Diversity of progenitors and outburst properties|journal=Monthly Notices of the Royal Astronomical Society|volume=415|issue=1|pages=773–810 |last1=Smith|first1=Nathan|last2=Li|first2=Weidong|last3=Silverman|first3=Jeffrey M.|last4=Ganeshalingam|first4=Mohan|last5=Filippenko|first5=Alexei V.|year=2011 |doi=10.1111/j.1365-2966.2011.18763.x|doi-access=free |arxiv=1010.3718 |s2cid=85440811}}</ref> η&nbsp;Carinae is still much brighter at infrared wavelengths, despite the presumed hot stars behind the nebulosity. The recent visual brightening is considered to be largely caused by a decrease in the extinction, due to thinning dust or a reduction in mass loss, rather than an underlying change in the luminosity.<ref name="mehner2012"/>

===Temperature===
[[File:Eta Car, Hubble-ESO.jpg|thumb|left|upright=1.2|alt=The Homunculus Nebula on the left, and a zoomed-in infrared image on the right|[[Hubble Space Telescope|Hubble]] image of the Homunculus Nebula; inset is a [[Very Large Telescope|VLT]] NACO infrared image of η&nbsp;Carinae.]]
Until late in the 20th century, the temperature of η&nbsp;Carinae was assumed to be over 30,000&nbsp;K because of the presence of high-excitation spectral lines, but other aspects of the spectrum suggested much lower temperatures and complex models were created to account for this.<ref name="davidson1971">{{cite journal|doi= 10.1093/mnras/154.4.415|title= On the Nature of Eta Carinae|journal= Monthly Notices of the Royal Astronomical Society|volume= 154|issue= 4|pages= 415–427|year= 1971|last1= Davidson|first1= K.|bibcode= 1971MNRAS.154..415D|doi-access= free}}</ref> It is now known that the Eta Carinae system consists of at least two stars, both with strong stellar winds and a shocked colliding wind (wind-wind collision or WWC) zone, embedded within a dusty nebula that reprocesses 90% of the electromagnetic radiation into the mid and far infrared. All of these features have different temperatures.

The powerful stellar winds from the two stars collide in a roughly conical WWC zone and produce temperatures as high as {{val|100|ul=MK}} at the apex between the two stars. This zone is the source of the hard X-rays and gamma rays close to the stars. Near periastron, as the secondary ploughs through ever denser regions of the primary wind, the colliding wind zone becomes distorted into a spiral trailing behind η&nbsp;Carinae B.<ref name="3d">{{cite journal|bibcode=2013MNRAS.436.3820M |title=Constraints on decreases in η Carinae's mass-loss from 3D hydrodynamic simulations of its binary colliding winds |journal=Monthly Notices of the Royal Astronomical Society |volume=436 |issue=4 |pages=3820 |last1=Madura |first1=T. I. |last2=Gull |first2=T. R. |last3=Okazaki |first3=A. T. |last4=Russell |first4=C. M. P. |last5=Owocki |first5=S. P. |last6=Groh |first6=J. H. |last7=Corcoran |first7=M. F. |last8=Hamaguchi |first8=K. |last9=Teodoro |first9=M. |display-authors=6 |year=2013 |doi=10.1093/mnras/stt1871 |doi-access=free |arxiv = 1310.0487|s2cid=118407295}}</ref>

The wind-wind collision cone separates the winds of the two stars. For 55–75° behind the secondary, there is a thin hot wind typical of O or Wolf–Rayet stars. This allows some radiation from η&nbsp;Carinae B to be detected and its temperature can be estimated with some accuracy due to spectral lines that are unlikely to be produced by any other source. Although the secondary star has never been directly observed, there is widespread agreement on models where it has a temperature between 37,000&nbsp;K and 41,000&nbsp;K.<ref name="mehner"/>

In all other directions on the other side of the wind-wind collision zone, there is the wind from η&nbsp;Carinae A, cooler and around 100 times denser than η&nbsp;Carinae B's wind. It is also optically dense, completely obscuring anything resembling a true photosphere and rendering any definition of its temperature moot. The observable radiation originates from a pseudo-photosphere where the [[optical density]] of the wind drops to near zero, typically measured at a particular [[Opacity (optics)|Rossland opacity]] value such as {{frac|2|3}}. This pseudo-photosphere is observed to be elongated and hotter along the presumed axis of rotation.<ref name=boekel>{{cite journal |last1=van Boekel|first1=R. |last2=Kervella |first2=P. |last3=SchöLler |first3=M. |last4=Herbst |first4=T. |last5=Brandner |first5=W. |last6=de Koter |first6=A. |last7=Waters|first7=L.B.F.M. |last8=Hillier |first8=D.J. |last9=Paresce |first9=F. |last10=Lenzen |first10=R. |last11=Lagrange |first11=A.-M. |display-authors=6 |year=2003 |title=Direct measurement of the size and shape of the present-day stellar wind of η&nbsp;Carinae |journal=Astronomy and Astrophysics |volume=410 |issue=3 |pages=L37 |bibcode=2003A&A...410L..37V |arxiv = astro-ph/0310399 |doi=10.1051/0004-6361:20031500 |s2cid=18163131}}</ref>

η&nbsp;Carinae A is likely to have appeared as an early B&nbsp;[[hypergiant]] with a temperature of between 20,000&nbsp;K and 25,000&nbsp;K at the time of its discovery by Halley. An [[effective temperature]] determined for the surface of a spherical optically thick wind at {{solar radius|several hundred}} would be 9,400–15,000&nbsp;K, while the temperature of a theoretical {{solar radius|60}} hydrostatic "core" at [[Optical depth (astrophysics)|optical depth]] 150 would be 35,200&nbsp;K.<ref name="groh"/><ref name="martin2014"/><ref name="mehner2012"/><ref name="martin">{{cite journal|bibcode=2010AJ....139.2056M |title=Mid-cycle changes in Eta Carinae |journal=The Astronomical Journal|volume=139 |issue=5|pages=2056|last1=Martin|first1=John C.|last2=Davidson|first2=Kris|last3=Humphreys|first3=Roberta M.|last4=Mehner|first4=Andrea|year=2010 |doi=10.1088/0004-6256/139/5/2056|arxiv = 0908.1627 |s2cid=118880932}}</ref> The effective temperature of the visible outer edge of the opaque primary wind is generally treated as being 15,000–25,000&nbsp;K on the basis of visual and ultraviolet spectral features assumed to be directly from the wind or reflected via the Weigelt Blobs.<ref name="impostors"/><ref name="gull"/> During the great eruption, η&nbsp;Carinae A was much cooler at around 5,000&nbsp;K.<ref name="rest"/>

The Homunculus contains dust at temperatures varying from 150&nbsp;K to 400&nbsp;K. This is the source of almost all the infrared radiation that makes η&nbsp;Carinae such a bright object at those wavelengths.<ref name="impostors"/>

Further out, expanding gases from the Great Eruption collide with interstellar material and are heated to around {{val|5|u=MK}}, producing less energetic X-rays seen in a horseshoe or ring shape.<ref name="corcoran">{{cite journal|bibcode=1997Natur.390..587C|title=Increasing X-ray emissions and periodic outbursts from the massive star Eta Carinae|journal=Nature|volume=390|issue=6660|pages=587|last1=Corcoran|first1=Michael F.|last2=Ishibashi|first2=Kazunori|last3=Davidson|first3=Kris|last4=Swank|first4=Jean H.|last5=Petre|first5=Robert|last6=Schmitt|first6=Jurgen H. M. M.|year=1997|doi=10.1038/37558|s2cid=4431077}}</ref><ref name="chlebowski">{{cite journal|bibcode=1984ApJ...281..665C|title=X-rays from Eta Carinae|journal=The Astrophysical Journal|volume=281|pages=665|last1=Chlebowski|first1=T.|last2=Seward|first2=F. D.|last3=Swank|first3=J.|last4=Szymkowiak|first4=A. |year=1984|doi=10.1086/162143}}</ref>

===Size===
The size of the two main stars in the η&nbsp;Carinae system is difficult to determine precisely, for neither star can be seen directly. η&nbsp;Carinae B is likely to have a well-defined photosphere, and its radius can be estimated from the assumed type of star. An O&nbsp;supergiant of {{solar luminosity|933,000}} with a temperature of 37,200&nbsp;K has an effective radius of {{solar radius|23.6}}.<ref name="verner"/>

The size of η&nbsp;Carinae A is not even well defined. It has an optically dense stellar wind, so the typical definition of a star's surface being approximately where it becomes opaque gives a very different result to where a more traditional definition of a surface might be. One study calculated a radius of {{solar radius|60}} for a hot "core" of 35,000&nbsp;K at [[optical depth]] 150, near the sonic point or very approximately what might be called a physical surface. At optical depth 0.67 the radius would be {{solar radius|over 800}}, indicating an extended optically thick stellar wind.<ref name="hillier">{{cite journal |last1=Hillier |first1=D. John |last2=Davidson |first2=K. |last3=Ishibashi |first3=K. |last4=Gull |first4=T. |title=On the Nature of the Central Source in η Carinae |journal=The Astrophysical Journal |volume=553 |issue=837 |pages=837 |date=June 2001 |bibcode=2001ApJ...553..837H |doi=10.1086/320948 |doi-access=free}}</ref> At the peak of the Great Eruption the radius, so far as such a thing is meaningful during such a violent expulsion of material, would have been around {{solar radius|1,400}}, comparable to the [[List of largest known stars|largest-known]] [[red supergiant]]s, including [[VY Canis Majoris]].<ref name="smith2011b">{{cite journal |last=Smith |first=Nathan |title=Explosions triggered by violent binary-star collisions: Application to Eta Carinae and other eruptive transients |journal=Monthly Notices of the Royal Astronomical Society |volume=415 |issue=3 |pages=2020–2024 |year=2011 |bibcode=2011MNRAS.415.2020S |arxiv=1010.3770 |s2cid=119202050 |doi=10.1111/j.1365-2966.2011.18607.x|doi-access=free }}</ref>

The stellar sizes should be compared with their orbital separation, which is only around {{solar radius|250}} at periastron. The accretion radius of the secondary is around {{solar radius|60}}, suggesting strong accretion near periastron leading to a collapse of the secondary wind.<ref name="KashiSoker"/> It has been proposed that the initial brightening from 4th magnitude to 1st at relatively constant bolometric luminosity was a normal LBV outburst, albeit from an extreme example of the class. Then the companion star passing through the expanded photosphere of the primary at periastron triggered the further brightening, increase in luminosity, and extreme mass loss of the Great Eruption.<ref name="smith2011b"/>

===Rotation===
Rotation rates of massive stars have a critical influence on their evolution and eventual death. The rotation rate of the η&nbsp;Carinae stars cannot be measured directly because their surfaces cannot be seen. Single massive stars spin down quickly due to braking from their strong winds, but there are hints that both η&nbsp;Carinae A and B are fast rotators, up to 90% of critical velocity. One or both could have been spun up by binary interaction, for example accretion onto the secondary and orbital dragging on the primary.<ref name="groh2010">{{cite journal|bibcode=2010ApJ...716L.223G |title=Is Eta Carinae a fast rotator, and how much does the companion influence the inner wind structure? |journal=The Astrophysical Journal Letters|volume=716 |issue=2 |page=L223 |last1=Groh|first1=J.H. |last2=Madura|first2=T.I. |last3=Owocki |first3=S.P. |last4=Hillier |first4=D.J. |last5=Weigelt |first5=G.|year=2010|doi=10.1088/2041-8205/716/2/L223|arxiv=1006.4816|s2cid=119188874}}</ref>