Editing Eta Carinae (section)

===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>