Editing Eta Carinae (section)

===Visual spectrum===
[[File:Hubble composite of Eta Carinae.jpg|thumb|left|upright=1.4|alt=Hubble composite of η&nbsp;Carinae, montage showing a spectrum against an actual image of the Homunculus Nebula|[[Hubble Space Telescope]] composite of η&nbsp;Carinae showing the unusual emission spectrum (near-IR image spectrum from the Hubble [[Space Telescope Imaging Spectrograph]] CCD)]]
The [[Equivalent width|strength]] and [[spectral line shape|profile]] of the [[spectral line|lines]] in the η&nbsp;Carinae [[stellar spectrum|spectrum]] are highly variable, but there are a number of consistent distinctive features. The spectrum is dominated by [[emission line]]s, usually broad although the higher excitation lines are overlaid by a narrow central component from dense [[ionisation|ionised]] nebulosity, especially the [[Weigelt Blobs]]. Most lines show a [[P Cygni#P Cygni Profile|P&nbsp;Cygni profile]] but with the absorption wing much weaker than the emission. The broad P&nbsp;Cygni lines are typical of strong [[stellar wind]]s, with very weak [[absorption spectrum|absorption]] in this case because the central star is so heavily obscured. [[Thomson scattering|Electron scattering wings]] are present but relatively weak, indicating a clumpy wind. [[Hydrogen spectral series|Hydrogen lines]] are present and strong, showing that η&nbsp;Carinae still retains much of its [[hydrogen]] envelope.

[[Ion notation|He<sup>I</sup>]]{{efn|The roman numerals are [[ion notation]], where "I" indicates neutral elements, "II" singly ionized elements, etc. See [[Spectral line#nomencature anchor|Spectral line]].}} lines are much weaker than the hydrogen lines, and the absence of He<sup>II</sup> lines provides an upper limit to the possible temperature of the primary star. N<sup>II</sup> lines can be identified but are not strong, while carbon lines cannot be detected and oxygen lines are at best very weak, indicating [[Stellar evolution#Hydrogen fusion|core hydrogen burning]] via the [[CNO cycle]] with some mixing to the surface. Perhaps the most striking feature is the rich Fe<sup>II</sup> emission in both [[forbidden mechanism|permitted and forbidden lines]], with the forbidden lines arising from excitation of low density nebulosity around the star.<ref name="hillier"/><ref name="hillier1992">{{cite journal |last1=Hillier |first1=D.J. |last2=Allen |first2=D.A. |year=1992 |title=A spectroscopic investigation of Eta Carinae and the Homunculus Nebula. I – Overview of the spectra |journal=Astronomy and Astrophysics |volume=262 |pages=153 |issn=0004-6361 |bibcode=1992A&A...262..153H}}</ref>

The earliest analyses of the star's spectrum are descriptions of visual observations from 1869, of prominent emission lines "[[Fraunhofer lines|C, D, b, F]] and the principal green nitrogen line". Absorption lines are explicitly described as not being visible.<ref name="lesueur1869">{{cite journal |last1=Le Sueur |first1=A. |author-link=Albert Le Sueur |year=1869 |title=On the nebulae of Argo and Orion, and on the spectrum of Jupiter |journal=[[Proceedings of the Royal Society of London]] |volume=18 |issue=114–122 |page=245 |bibcode=1869RSPS...18..245L |doi=10.1098/rspl.1869.0057 |s2cid=122853758 }}</ref> The letters refer to [[Fraunhofer lines|Fraunhofer's spectral notation]] and correspond to [[H-alpha|H<sub>α</sub>]], He<sup>I</sup>,{{efn|
[[Fraunhofer lines|Fraunhofer]] "D" usually refers to the sodium doublet; "d" or "D<sub>3</sub>" was used for the nearby helium line.
}} Fe<sup>II</sup>, and H<sub>β</sub>.

It is assumed that the final line is from Fe<sup>II</sup> very close to the green [[nebulium]] line now known to be from O<sup>III</sup>.<ref name="walborn">{{cite journal |last1=Walborn |first1=N.R. |author1-link=Nolan R. Walborn |last2=Liller |first2=M.H. |year=1977 |title=The earliest spectroscopic observations of Eta&nbsp;Carinae and its interaction with the Carina Nebula |journal=[[The Astrophysical Journal]] |volume=211 |pages=181 |doi=10.1086/154917 |bibcode=1977ApJ...211..181W}}</ref>

Photographic spectra from 1893 were described as similar to an [[F-type star|F5 star]], but with a few weak emission lines. Analysis to modern spectral standards suggests an early F&nbsp;[[spectral type]]. By 1895 the spectrum again consisted mostly of strong emission lines, with the absorption lines present but largely obscured by emission. This spectral transition from F&nbsp;[[supergiant]] to strong emission is characteristic of [[nova]]e, where ejected material initially radiates like a pseudo-[[photosphere]] and then the emission spectrum develops as it expands and thins.<ref name="walborn"/>

The emission line spectrum associated with dense stellar winds has persisted ever since the late 19th&nbsp;century. Individual lines show widely varying [[Spectral line#Line broadening and shift|widths, profiles and Doppler shifts]], often multiple velocity components within the same line. The spectral lines also show variation over time, most strongly with a 5.5-year period but also less dramatic changes over shorter and longer periods, as well as ongoing secular development of the entire spectrum.<ref name="baxendall">{{cite journal |last1=Baxandall |first1=F.E. |year=1919 |title=Note on apparent changes in the spectrum of η&nbsp;Carinæ |journal=Monthly Notices of the Royal Astronomical Society |volume=79 |issue=9 |page=619 |doi=10.1093/mnras/79.9.619 |bibcode=1919MNRAS..79..619B|doi-access=free }}</ref><ref name="gaviola">{{cite journal |last1=Gaviola |first1=E. |year=1953 |title=Eta &nbsp;Carinae. II. &nbsp;The Spectrum |journal=The Astrophysical Journal |volume=118 |page=23 |bibcode=1953ApJ...118..234G |doi=10.1086/145746|doi-access=free }}</ref> The spectrum of light reflected from the [[Homunculus Nebula#Weigelt Blobs|Weigelt Blobs]], and assumed to originate mainly with the primary, is similar to the [[P Cygni#P Cygni Profile|extreme P&nbsp;Cygni-type]] star {{nowrap|[[HDE 316285]]}} which has a spectral type of B0Ieq.<ref name="gull"/>

[[File:Eta Carinae light echo.gif|thumb|right|Animation showing the expanding [[light echo]] caused by the η&nbsp;Carinae eruption in the Carina Nebula]]
Direct spectral observations did not begin until after the Great Eruption, but [[light echo]]es from the eruption reflected from other parts of the Carina Nebula were detected using the U.S. [[National Optical Astronomy Observatory]]'s [[Víctor M. Blanco Telescope|Blanco 4-meter telescope]] at the [[Cerro Tololo Inter-American Observatory]]. Analysis of the reflected spectra indicated the light was emitted when η&nbsp;Carinae had the appearance of a {{Val|5000|fmt=commas|ul=K}} G2-to-G5 supergiant, some 2,000&nbsp;K cooler than expected from other [[supernova impostor]] events.<ref name="rest">{{cite journal |last1=Rest |first1=A. |last2=Prieto |first2=J.L. |last3=Walborn |first3=N.R. |last4=Smith |first4=N. |last5=Bianco |first5=F.B. |last6=Chornock |first6=R. |last7=Welch |first7=D.L. |last8=Howell |first8=D.A. |last9=Huber |first9=M.E. |last10=Foley |first10=R.J. |last11=Fong |first11=W. |last12=Sinnott |first12=B. |last13=Bond |first13=H.E. |last14=Smith |first14=R.C. |last15=Toledo |first15=I. |last16=Minniti |first16=D. |last17=Mandel |first17=K. |display-authors=6 |year=2012 |title=Light echoes reveal an unexpectedly cool η&nbsp;Carinae during its nineteenth-century Great Eruption |journal=Nature |volume=482 |issue=7385 |pages=375–378 |pmid=22337057 |doi=10.1038/nature10775 |bibcode=2012Natur.482..375R |arxiv=1112.2210|s2cid=205227548}}</ref> Further light echo observations show that following the peak brightness of the Great Eruption the spectrum developed prominent P&nbsp;Cygni profiles and [[Cyano radical|CN]] molecular bands, although this is likely from the material being ejected which may have been colliding with [[Circumstellar dust|circumstellar material]] in a similar way to a type IIn [[supernova]].<ref name="prieto">{{cite journal |last1=Prieto |first1=J.L. |last2=Rest |first2=A. |last3=Bianco |first3=F.B. |last4=Matheson |first4=T. |last5=Smith |first5=N. |last6=Walborn |first6=N.R. |last7=Hsiao |first7=E.Y. |last8=Chornock |first8=R. |last9=Paredes Álvarez |first9=L. |last10=Campillay |first10=A. |last11=Contreras |first11=C. |last12=González |first12=C. |last13=James |first13=D.|last14=Knapp |first14=G.R. |last15=Kunder |first15=A. |last16=Margheim |first16=S. |last17=Morrell |first17=N. |last18=Phillips|first18=M.M. |last19=Smith |first19=R.C. |last20=Welch |first20=D.L. |last21=Zenteno |first21=A. |display-authors=6 |year=2014 |title=Light echoes from η&nbsp;Carinae's Great Eruption: Spectrophotometric evolution and the rapid formation of nitrogen-rich molecules |journal=The Astrophysical Journal Letters |volume=787 |issue=1 |page=L8 |bibcode=2014ApJ...787L...8P |doi=10.1088/2041-8205/787/1/L8 |arxiv=1403.7202|s2cid=119208968}}</ref>

In the second half of the 20th&nbsp;century, much higher-resolution visual spectra became available. The spectrum continued to show complex and baffling features, with much of the energy from the central star being recycled into the infrared by surrounding dust, some reflection of light from the star from dense localised objects in the circumstellar material, but with obvious high-ionisation features indicative of very high temperatures. The line profiles are complex and variable, indicating a number of absorption and emission features at various [[Doppler effect|velocities relative]] to the central star.<ref name="davidson1986">{{cite journal |last1=Davidson |first1=K. |last2=Dufour |first2=R.J. |last3=Walborn |first3=N.R. |last4=Gull |first4=T.R. |year=1986 |title=Ultraviolet and visual wavelength spectroscopy of gas around Eta&nbsp;Carinae |journal=The Astrophysical Journal |volume=305 |pages=867 |doi=10.1086/164301 |bibcode=1986ApJ...305..867D}}</ref><ref name="davidson1995">{{cite journal |last1=Davidson |first1=Kris |last2=Ebbets |first2=Dennis |last3=Weigelt |first3=Gerd |last4=Humphreys |first4=Roberta M. |last5=Hajian |first5=Arsen R. |last6=Walborn |first6=Nolan R. |last7=Rosa |first7=Michael |year=1995 |title=HST/FOS spectroscopy of Eta&nbsp;Carinae: The star itself, and ejecta within 0.3&nbsp;arcsec |journal=Astronomical Journal |volume=109 |page=1784 |issn=0004-6256 |bibcode=1995AJ....109.1784D |doi=10.1086/117408}}</ref>

The 5.5-year orbital cycle produces strong spectral changes at periastron that are known as spectroscopic events. Certain wavelengths of radiation suffer eclipses, either due to actual [[occultation]] by one of the stars or due to passage within opaque portions of the complex stellar winds. Despite being ascribed to orbital rotation, these events vary significantly from cycle to cycle. These changes have become stronger since 2003 and it is generally believed that long-term secular changes in the stellar winds or previously ejected material may be the culmination of a return to the state of the star before its Great Eruption.<ref name="landes">{{cite journal |last1=Landes |first1=H. |last2=Fitzgerald |first2=M. |year=2010 |title=Photometric observations of the η&nbsp;Carinae 2009.0 spectroscopic event |journal=Publications of the Astronomical Society of Australia |volume=27 |issue=3 |pages=374–377 |doi=10.1071/AS09036 |bibcode=2010PASA...27..374L |arxiv=0912.2557|s2cid=118568091}}</ref><ref name="martin2014">{{cite journal |last1=Martin |first1=John C. |last2=Mehner |first2=A. |last3=Ishibashi |first3=K. |last4=Davidson |first4=K. |last5=Humphreys |first5=R.M. |year=2014 |title=Eta&nbsp;Carinae's change of state: First new HST/NUV data since 2010, and the first new FUV since 2004 |journal=American Astronomical Society |volume=223 |issue=151 |page=09 |bibcode=2014AAS...22315109M }}</ref><ref name="davidson2014">{{cite journal |last1=Davidson |first1=Kris |last2=Mehner |first2=Andrea |last3=Humphreys |first3=Roberta |last4=Martin |first4=John C. |last5=Ishibashi |first5=Kazunori |year=2014 |title=Eta &nbsp;Carinae's 2014.6 spectroscopic event: The extraordinary He&nbsp;II and N&nbsp;II features |journal=The Astrophysical Journal |volume=1411 |issue=1 |page=695 |bibcode=2015ApJ...801L..15D |doi=10.1088/2041-8205/801/1/L15 |arxiv=1411.0695|s2cid=119187363}}</ref>