Editing Eta Carinae (section)

==Observational history==
η&nbsp;Carinae was first recorded as a fourth-magnitude star in the 16th or 17th&nbsp;century. It became the second-brightest star in the sky in the mid-19th&nbsp;century, before fading below naked-eye visibility. During the second half of the 20th&nbsp;century, it slowly brightened to again become visible to the naked eye, and by 2014 was again a fourth-magnitude star.

===Discovery and naming===
There is no reliable evidence of η&nbsp;Carinae being observed or recorded before the 17th&nbsp;century, although Dutch navigator [[Pieter Dirkszoon Keyser|Pieter Keyser]] described a fourth-magnitude star at approximately the correct position around 1595–1596, which was copied onto the celestial globes of [[Petrus Plancius]] and [[Jodocus Hondius]] and the 1603 ''[[Uranometria]]'' of [[Johann Bayer]]. [[Frederick de Houtman]]'s independent star catalogue from 1603 does not include η&nbsp;Carinae among the other 4th&nbsp;magnitude stars in the region. The earliest firm record was made by [[Edmond Halley]] in 1677 when he recorded the star simply as ''Sequens'' (i.e. "following" relative to another star) within a new constellation [[Robur Carolinum]]. His ''Catalogus Stellarum Australium'' was published in 1679.<ref name="Halley 1679">{{cite book |last=Halley |first=Edmund |author-link=Edmond Halley |year=1679 |title=Catalogus stellarum australium; sive, Supplementum catalogi Tychenici, exhibens longitudines et latitudines stellarum fixarum, quae, prope polum Antarcticum sitae, in horizonte Uraniburgico Tychoni inconspicuae fuere, accurato calculo ex distantiis supputatas, & ad annum 1677 completum correctas ... Accedit appendicula de rebus quibusdam astronomicis |url=http://babel.hathitrust.org/cgi/pt?id=mdp.39015007000170;view=1up;seq=24 |url-status=dead |publisher=T. James |location=London, UK |page=13 |archive-url=https://archive.today/20151106091831/http://babel.hathitrust.org/cgi/pt?id=mdp.39015007000170;view=1up;seq=24 |archive-date=6 November 2015}}</ref> The star was also known by the [[Bayer designation]]s η&nbsp;Roboris Caroli, [[Eta|η]]&nbsp;Argus, or η Navis.<ref name=frew>{{cite journal |last=Frew |first=David J. |year=2004 |title=The historical record of η&nbsp;Carinae. I.&nbsp;The visual light curve, 1595–2000 |journal=The Journal of Astronomical Data |volume=10 |issue=6 |pages=1–76 |bibcode=2004JAD....10....6F }}</ref> In 1751 [[Nicolas-Louis de Lacaille]] gave the stars of [[Argo Navis]] and [[Robur Carolinum]] a single set of Greek letter Bayer designations within his constellation Argo, and designated three areas within Argo for the purposes of using Latin letter designations three times over. The letter [[Eta|η]] fell within the keel portion of the ship which was later to become the constellation [[Carina (constellation)|Carina]].<ref name=warner>{{cite journal |last=Warner |first=Brian |year=2002 |title=Lacaille 250&nbsp;years on |journal=Astronomy and Geophysics |volume=43 |issue=2 |pages=2.25–2.26 |issn=1366-8781 |bibcode=2002A&G....43b..25W |doi=10.1046/j.1468-4004.2002.43225.x |doi-access=free}}</ref> It was not generally known as η&nbsp;Carinae until 1879, when the stars of Argo Navis were finally given the epithets of the daughter constellations in the ''Uranometria Argentina'' of [[Benjamin Apthorp Gould|Gould]].<ref name=wagman>{{cite book |last=Wagman |first=Morton |title=Lost Stars: Lost, missing and troublesome stars from the catalogues of Johannes Bayer, Nicholas Louis de&nbsp;Lacaille, John Flamsteed, and sundry others |publisher=The McDonald & Woodward Publishing Company |location=Blacksburg, VA |year=2003 |pages=7–8, 82–85 |isbn=978-0-939923-78-6 }}</ref>

η&nbsp;Carinae is too far south to be part of the [[Twenty-Eight Mansions|mansion-based]] [[Chinese astronomy|traditional Chinese astronomy]], but it was mapped when the [[Chinese constellations#The Southern Asterisms (近南極星區)|Southern Asterisms]] were created at the start of the 17th&nbsp;century. Together with [[HD 90853|s Carinae]], [[λ Centauri]] and [[λ Muscae]], η&nbsp;Carinae forms the [[Asterism (astronomy)|asterism]] {{lang|zh|海山}} ([[Carina (Chinese astronomy)|Sea and Mountain]]).<ref name="Chen Jiu Jin 2005">{{cite book |script-title=zh:中國星座神 |title=Chinese horoscope mythology |author=陳久金 (Chen Jiu Jin) |publisher=台灣書房出版有限公司 (Taiwan Book House Publishing Co., Ltd.) |date=2005 |isbn=978-986-7332-25-7 |lang=zh}}</ref> η&nbsp;Carinae has the names Tseen She (from the Chinese 天社 [Mandarin: ''tiānshè''] "Heaven's altar") and Foramen. It is also known as {{lang|zh|海山二}} ({{lang|zh-Latn|Hǎi Shān èr}}, {{langx|en|the Second Star of Sea and Mountain}}).<ref name="Chen Huihua 2006">{{cite web |script-title=zh:天文教育資訊網 |title=Activities of Exhibition and Education in Astronomy |editor=陳輝樺 (Chen Huihua) |url=http://aeea.nmns.edu.tw/2006/0607/ap060728.html |website=nmns.edu.tw |date=28 July 2006 |access-date=30 December 2012 |language=zh |archive-date=13 May 2013 |archive-url=https://web.archive.org/web/20130513091029/http://aeea.nmns.edu.tw/2006/0607/ap060728.html |url-status=dead }}</ref>

Halley gave an approximate [[apparent magnitude]]&nbsp;4 at the time of discovery, which has been calculated as magnitude&nbsp;3.3 on the modern scale. The handful of possible earlier sightings suggest that η&nbsp;Carinae was not significantly brighter than this for much of the 17th&nbsp;century.<ref name=frew/> Further sporadic observations over the next 70&nbsp;years show that η&nbsp;Carinae was probably around 3rd&nbsp;magnitude or fainter, until Lacaille reliably recorded it at 2nd magnitude in 1751.<ref name=frew/> It is unclear whether η&nbsp;Carinae varied significantly in brightness over the next 50&nbsp;years; there are occasional observations such as [[William Burchell]]'s at 4th&nbsp;magnitude in 1815, but it is uncertain whether these are just re-recordings of earlier observations.<ref name=frew/>

===Great Eruption===
[[File:Historical visual lightcurve for Eta Carinae (1686 - 2014).png|upright=1.4|thumb|left|alt=Historical visual lightcurve for η&nbsp;Carinae from 1686 to 2015|The lightcurve of η&nbsp;Carinae from some of the earliest observations to the current day]]
In 1827, Burchell specifically noted η&nbsp;Carinae's unusual brightness at 1st&nbsp;magnitude, and was the first to suspect that it varied in brightness.<ref name="frew"/> [[John Herschel]], who was in South Africa at the time, made a detailed series of accurate measurements in the 1830s showing that η&nbsp;Carinae consistently shone around magnitude&nbsp;1.4 until November 1837. On the evening of 16 December 1837, Herschel was astonished to see that it had brightened to slightly outshine [[Rigel]].<ref name="herschel"/> This event marked the beginning of a roughly 18-year period known as the Great Eruption.<ref name="frew"/>

η&nbsp;Carinae was brighter still on 27&nbsp;January 1838, equivalent to [[Alpha Centauri]], before fading slightly over the following three months. Herschel did not observe the star after this, but received correspondence from the Reverend W.S. Mackay in Calcutta, who wrote in 1843, "To my great surprise I observed this March last (1843), that the star η&nbsp;Argus had become a star of the first magnitude fully as bright as [[Canopus]], and in colour and size very like [[Arcturus]]." Observations at the [[Cape of Good Hope]] indicated it peaked in brightness, surpassing Canopus, from 11 to 14 March 1843, then began to fade, then brightened to between the brightness of Alpha Centauri and Canopus between 24 and 28 March before fading once again.<ref name="herschel">{{cite book |last=Herschel |first=John Frederick William |author-link=John Herschel |title=Results of astronomical observations made during the years 1834, 5, 6, 7, 8, at the Cape of Good Hope: being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825 |url=https://books.google.com/books?id=8WBNAAAAcAAJ&q=Canopus |publisher=[[Smith, Elder and Co.]] |location=London |volume=1 |year=1847 |pages=33–35 |bibcode=1847raom.book.....H}}</ref> For much of 1844 the brightness was midway between [[Alpha Centauri]] and [[Beta Centauri]], around magnitude +0.2, before brightening again at the end of the year. At its brightest in 1843 it likely reached an apparent magnitude of −0.8, then −1.0 in 1845.<ref name="smithfrew">{{cite journal |last1=Smith |first1=Nathan |last2=Frew |first2=David J. |title=A revised historical light curve of η&nbsp;Carinae and the timing of close periastron encounters |journal=Monthly Notices of the Royal Astronomical Society |volume=415 |issue=3 |pages=2009–2019 |year=2011 |arxiv=1010.3719 |s2cid=118614725 |bibcode=2011MNRAS.415.2009S |doi=10.1111/j.1365-2966.2011.18993.x|doi-access=free }}</ref> The peaks in 1827, 1838 and 1843 are likely to have occurred at the [[periastron]] passage—the point the two stars are closest together—of the [[Binary star|binary]] orbit.<ref name="damineli">{{cite journal |last=Damineli |first=A. |title=The 5.52&nbsp;year cycle of Eta Carinae |journal=[[Astrophysical Journal Letters]] |volume=460 |pages=L49 |year=1996 |bibcode=1996ApJ...460L..49D |doi=10.1086/309961 |doi-access=free }}</ref> From 1845 to 1856, the brightness decreased by around 0.1&nbsp;magnitudes per year, but with possible rapid and large fluctuations.<ref name="smithfrew"/>

In their oral traditions, the Boorong clan of the [[Wergaia]] people of [[Lake Tyrrell]], north-western Victoria, Australia, told of a reddish star they knew as ''Collowgullouric War''<!-- In Table 3, Hamacher & Frew spell it "Collowgulloric War", without the 2nd 'u', and the same a few lines down with "Collowgulloric Warepil". However, the Wotjobaluk form is given as "Collow-collouric", and in the photocopy of Stanbridge (1858:140) in sect. 4, it's clearly "Collowgullouric War". The translation is "female crow",, only the identification is "wife of War". --> {{IPAc-en|'|k|Q|l|@|g|V|l|@|r|ɪ|k|_|'|w|ɑr}} "Old Woman Crow", the wife of ''War'' "Crow" ([[Canopus]]).<ref name="GSSCZ">Or more accurately ''gala-gala gurrk waa'', with the onset of ''gurrk'' "woman" lost in Stanbridge. {{cite book |last=Reid |first=Julie |year=2008 |title=Wergaia Community Grammar and Dictionary |url=https://books.google.com/books?id=haxkygAACAAJ |via=Google }}</ref> In 2010, astronomers Duane Hamacher and David Frew from [[Macquarie University]] in Sydney showed that this was η&nbsp;Carinae during its Great Eruption in the 1840s.<ref name=boorong>{{cite journal |last1=Hamacher |first1=Duane W. |last2=Frew |first2=David J. |year=2010 |title=An aboriginal Australian record of the great eruption of Eta Carinae |journal=Journal of Astronomical History and Heritage |volume=13 |issue=3 |pages=220–234 |arxiv=1010.4610 |bibcode=2010JAHH...13..220H |s2cid=118454721 |doi=10.3724/SP.J.1440-2807.2010.03.06}}</ref> From 1857, the brightness decreased rapidly until it faded below [[naked-eye]] visibility by 1886. This has been calculated to be due to the condensation of [[Cosmic dust|dust]] in the ejected material surrounding the star, rather than to an intrinsic change in luminosity.<ref name="davidson1997">{{cite journal |last1=Davidson |first1=Kris |last2=Humphreys |first2=Roberta M. |author2-link=Roberta M. Humphreys |title=Eta Carinae and its environment |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=35 |pages=1–32 |year=1997 |bibcode=1997ARA&A..35....1D |s2cid=122193829 |doi=10.1146/annurev.astro.35.1.1}}</ref>

===Lesser Eruption===
A new brightening started in 1887, peaked at about magnitude&nbsp;6.2 in 1892, then at the end of March 1895 faded rapidly to about magnitude&nbsp;7.5.<ref name="frew"/> Although there are only visual records of the 1890 eruption, it has been calculated that η&nbsp;Carinae was suffering 4.3&nbsp;magnitudes of visual extinction due to the gas and dust ejected in the Great Eruption. An unobscured brightness would have been magnitude 1.5–1.9, significantly brighter than the historical magnitude. Despite this, it was similar to the first one, even almost matching its brightness, but not the amount of material expelled.<ref name=humphreys1999>{{cite journal |last1=Humphreys |first1=Roberta M. |author1-link=Roberta M. Humphreys |last2=Davidson |first2=Kris |last3=Smith |first3=Nathan |title=η Carinae's second eruption and the light curves of the η&nbsp;Carinae variables |journal=The Publications of the Astronomical Society of the Pacific |volume=111 |issue=763 |pages=1124–1131 |year=1999 |bibcode=1999PASP..111.1124H |doi=10.1086/316420 |doi-access=free}}</ref><ref name="smith">{{cite journal |last=Smith |first=Nathan |year=2004 |title= The systemic velocity of Eta Carinae| journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=351 |issue=1 |pages=L15–L18 |bibcode= 2004MNRAS.351L..15S |doi=10.1111/j.1365-2966.2004.07943.x |doi-access= free| arxiv = astro-ph/0406523 |s2cid= 17051247}}</ref><ref name="homunculus">{{cite journal |last1=Ishibashi |first1=Kazunori |last2=Gull |first2=Theodore R. |last3= Davidson |first3= Kris |last4= Smith |first4=Nathan |last5= Lanz |first5= Thierry |last6=Lindler |first6=Don |last7=Feggans|first7=Keith |last8=Verner|first8=Ekaterina |last9=Woodgate|first9=Bruce E. |last10=Kimble |first10=Randy A. |last11= Bowers |first11=Charles W. |last12=Kraemer |first12=Steven |last13=Heap |first13=Sarah R. |last14=Danks |first14=Anthony C. |last15=Maran |first15=Stephen P. |last16=Joseph |first16=Charles L. |last17=Kaiser |first17=Mary Elizabeth|last18=Linsky|first18=Jeffrey L. |last19=Roesler |first19=Fred |last20=Weistrop |first20=Donna |display-authors=6 |year=2003 |title=Discovery of a little Homunculus within the Homunculus nebula of η&nbsp;Carinae |journal=[[The Astronomical Journal]] |volume= 125 |issue=6 |page=3222 | bibcode=2003AJ....125.3222I |doi=10.1086/375306 |doi-access=free}}</ref>

===Twentieth century===
Between 1900 and at least 1940, η&nbsp;Carinae appeared to have settled at a constant brightness of around magnitude&nbsp;7.6,<ref name="frew"/> but in 1953 it was noted to have brightened again to magnitude&nbsp;6.5.<ref name="thackeray">{{cite journal|bibcode=1953MNRAS.113..237T|title=Note on the brightening of Eta Carinae |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=113|issue=2|pages=237–238|last1=Thackeray |first1=A.D. |year=1953 |doi=10.1093/mnras/113.2.237 |doi-access=free}}</ref> The brightening continued steadily, but with fairly regular variations of a few tenths of a magnitude.<ref name="damineli"/>

[[File:Eta Carinae light curve.png|thumb|right|upright=1.2|Light curve for η&nbsp;Carinae between 1972 and 2019]]
In 1996, the variations were first identified as having a 5.52&nbsp;year period,<ref name="damineli"/> later measured more accurately at 5.54&nbsp;years, leading to the idea of a binary system. The binary theory was confirmed by observations of radio, optical and [[near-infrared]] radial velocity and line profile changes, referred to collectively as a ''spectroscopic event'', at the predicted time of periastron passage in late 1997 and early 1998.<ref name="damineli2000">{{cite journal|bibcode= 2000ApJ...528L.101D |title= η&nbsp;Carinae: Binarity confirmed |journal=[[The Astrophysical Journal]] |volume= 528 |issue= 2 |pages= L101–L104 |last1= Damineli |first1= Augusto |last2= Kaufer |first2= Andreas |last3= Wolf |first3= Bernhard |last4= Stahl |first4= Otmar |last5= Lopes |first5= Dalton F. |last6= de Araújo |first6= Francisco X. |year= 2000 |doi= 10.1086/312441 |pmid= 10600628 |arxiv = astro-ph/9912387 |s2cid= 9385537}}</ref> At the same time there was a complete collapse of the X-ray emission presumed to originate in a [[Colliding-wind binary|colliding wind zone]].<ref name="ishibashi1999">{{cite journal|bibcode=1999ApJ...524..983I|title=Recurrent X-Ray Emission Variations of η&nbsp;Carinae and the Binary Hypothesis|journal=The Astrophysical Journal|volume=524|issue=2|pages=983|last1=Ishibashi|first1=K.|last2=Corcoran |first2=M.F. |last3=Davidson|first3=K.|last4=Swank |first4=J.H. |last5=Petre |first5=R. |last6=Drake |first6=S.A. |last7=Damineli|first7=A.|last8=White|first8=S. |display-authors=6 |year=1999|doi=10.1086/307859|doi-access=free}}</ref> The confirmation of a luminous binary companion greatly modified the understanding of the physical properties of the η&nbsp;Carinae system and its variability.<ref name="mehner"/>

A sudden doubling of brightness was observed in 1998–99 bringing it back to naked-eye visibility. During the 2014 spectroscopic event, the apparent visual magnitude became brighter than magnitude 4.5.<ref name="telegram">{{cite journal|bibcode=2014ATel.6368....1H|title=Eta Carinae – caught in transition to the photometric minimum|journal=The Astronomer's Telegram|volume=6368|pages=1|last1=Humphreys|first1=R.M.|last2=Martin|first2=J.C.|last3=Mehner|first3=A.|last4=Ishibashi|first4=K.|last5=Davidson|first5=K. |year=2014 }}</ref> The brightness does not always vary consistently at different wavelengths, and does not always exactly follow the 5.5&nbsp;year cycle.<ref name="mehnerir">{{cite journal|bibcode=2014A&A...564A..14M|doi=10.1051/0004-6361/201322729|arxiv=1401.4999|title=Near-infrared evidence for a sudden temperature increase in Eta Carinae|last1=Mehner|first1=Andrea|last2=Ishibashi|first2=Kazunori|last3=Whitelock|first3=Patricia|last4=Nagayama|first4=Takahiro|last5=Feast|first5=Michael|last6=Van Wyk|first6=Francois|last7=De Wit|first7=Willem-Jan|journal=[[Astronomy & Astrophysics]]|volume=564|year=2014|pages=A14|s2cid=119228664}}</ref><ref name="landes"/> Radio, infrared and space-based observations have expanded coverage of η&nbsp;Carinae across all wavelengths and revealed ongoing changes in the [[spectral energy distribution]].<ref name="martin2014"/>

In July 2018, η&nbsp;Carinae was reported to have the strongest [[Colliding-wind binary|colliding wind shock]] in the solar neighbourhood. Observations with the [[NuSTAR]] satellite gave much higher resolution data than the earlier [[Fermi Gamma-ray Space Telescope]]. Using direct focussing observations of the non-thermal source in the extremely hard X-ray band that is spatially coincident with the star, they showed that the source of non-thermal X-rays varies with the orbital phase of the binary star system and that the photon index of the emission is similar to that derived through analysis of the γ-ray (gamma) spectrum.<ref name=2018-07-02_NAL>{{cite journal |last1=Hamaguchi |first1=Kenji|last2=Corcoran |first2=Michael F. |last3=Pittard |first3=Julian M. |last4=Sharma |first4=Neetika |last5=Takahashi |first5=Hiromitsu |last6=Russell |first6=Christopher M.P. |last7=Grefenstette |first7=Brian W. |last8=Wik|first8=Daniel R. |last9=Gull |first9=Theodore R. |last10=Richardson |first10=Noel D. |last11=Madura |first11=Thomas I. |display-authors=6 |year=2018|title=Non-thermal X-rays from colliding wind shock acceleration in the massive binary Eta Carinae |journal=[[Nature Astronomy]] |volume=2|issue=9|pages=731–736|bibcode=2018NatAs...2..731H|doi=10.1038/s41550-018-0505-1| arxiv=1904.09219|s2cid=126188024|first12=Anthony F. J|last12=Moffat}} [http://eprints.whiterose.ac.uk/133419/ Alt URL]</ref><ref name="NASA vis">{{cite web|url=https://svs.gsfc.nasa.gov/vis/a010000/a011700/a011725/R1_XY_Density.gif |title=GIF of a computer simulation of the stellar winds of Eta Carinae |publisher=[[NASA]] |access-date=2018-08-02}}</ref>