Editing Eta Carinae (section)

===Radio emission===
[[radio astronomy|Radio]] emissions have been observed from η&nbsp;Carinae across the [[microwave]] band. It has been detected in the [[hydrogen line|21 cm]] H<sup>I</sup> line, but has been particularly closely studied in the [[millimetre wave|millimetre]] and [[Super high frequency|centimetre bands]]. [[Maser|Masing]] hydrogen [[recombination line]]s (from the combining of an electron and proton to form a hydrogen atom) have been detected in this range. The emission is concentrated in a small non-point source less than 4 [[arcsecond]]s across and appears to be mainly free-free emission (thermal [[bremsstrahlung]]) from ionised gas, consistent with a compact H<sup>II</sup> region at around 10,000&nbsp;K.<ref name="radio">{{cite journal|bibcode=2005A&A...437..977A|title=Millimeter-wave emission during the 2003 low excitation phase of η Carinae|journal=Astronomy and Astrophysics|volume=437|issue=3|pages=977|last1=Abraham|first1=Z.|last2=Falceta-Gonçalves|first2=D.|last3=Dominici |first3=T.P. |last4=Nyman|first4=L.-Å. |last5=Durouchoux|first5=P.|last6=McAuliffe |first6=F.|last7=Caproni|first7=A.|last8=Jatenco-Pereira|first8=V. |display-authors=6 |year=2005 |doi=10.1051/0004-6361:20041604|arxiv = astro-ph/0504180|s2cid=8057181}}</ref> High resolution imaging shows the radio frequencies originating from a disk a few arcseconds in diameter, 10,000 [[astronomical unit]]s ({{sc|AU}}) wide at the distance of η&nbsp;Carinae.<ref name=KashiSoker2007/>

The radio emission from η&nbsp;Carinae shows continuous variation in strength and distribution over a 5.5&nbsp;year cycle. The H<sup>II</sup> and recombination lines vary very strongly, with continuum emission (electromagnetic radiation across a broad band of wavelengths) less affected. This shows a dramatic reduction in the ionisation level of the hydrogen for a short period in each cycle, coinciding with the spectroscopic events at other wavelengths.<ref name=KashiSoker2007>{{cite journal |last1=Kashi |first1=Amit |last2=Soker |first2=Noam |year=2007 |title=Modelling the radio light curve of η&nbsp;Carinae |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=378 |issue=4 |pages=1609–1618 |arxiv=astro-ph/0702389 |doi=10.1111/j.1365-2966.2007.11908.x |doi-access=free |bibcode=2007MNRAS.378.1609K |s2cid=119334960 }}</ref><ref name=white>{{cite conference |last1=White |first1=S.M. |last2=Duncan |first2=R.A. |last3=Chapman |first3=J.M. |last4=Koribalski |first4=B. |year=2005 |title=The radio cycle of Eta Carinae |book-title=The Fate of the Most Massive Stars |volume=332 |pages=126 |url=https://aspbooks.org/custom/publications/paper/332-0126.html |bibcode=2005ASPC..332..126W }}</ref>