Editing Eta Carinae (section)

===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>