Editing Eta Carinae (section)

===Ultraviolet===
[[File:Cosmic Fireworks in Ultraviolet Eta Carinae Nebula.tif|thumb|Ultraviolet image of [[Homunculus Nebula]] taken by ESA / [[Hubble Space Telescope|Hubble]] ]]
The [[ultraviolet]] spectrum of the η&nbsp;Carinae system shows many emission lines of ionised metals such as Fe<sup>II</sup> and Cr<sup>II</sup>, as well as [[Lyman series|Lyman]]<sub>α</sub> (Ly<sub>α</sub>) and a continuum from a hot central source. The ionisation levels and continuum require the existence of a source with a temperature at least 37,000&nbsp;K.<ref name="uv">{{cite journal|doi=10.1086/509785|title=Eta Carinae across the 2003.5 Minimum: Deciphering the Spectrum toward Weigelt D|journal=The Astrophysical Journal Supplement Series|volume=168|issue=2|pages=289|year=2007|last1=Nielsen|first1=K. E.|last2=Ivarsson|first2=S.|last3=Gull|first3=T. R.|bibcode=2007ApJS..168..289N|doi-access=free}}</ref>

Certain Fe<sup>II</sup> UV lines are unusually strong. These originate in the Weigelt Blobs and are caused by a [[gain (laser)|low-gain]] [[Laser|lasing]] effect. Ionised hydrogen between a blob and the central star generates intense Ly<sub>α</sub> emission which penetrates the blob. The blob contains [[Hydrogen atom|atomic hydrogen]] with a small admixture of other elements, including iron [[photo-ionisation|photo-ionised]] by radiation from the central stars. An accidental [[Resonance#Optical|resonance]] (where emission coincidentally has a suitable energy to [[optical pumping|pump]] the excited state) allows the Ly<sub>α</sub> emission to pump the Fe<sup>+</sup> [[ion]]s to certain [[metastability|pseudo-metastable states]],<ref name=letokhov2008>{{cite book |first1=Vladilen |last1=Letokhov |first2=Sveneric |last2=Johansson |date=June 2008 |title=Astrophysical Lasers |publisher=Oxford University Press |place=Oxford, UK |isbn=978-0-19-156335-5 |page=39 }}</ref> creating a [[population inversion]] that allows the [[stimulated emission]] to take place.<ref name=johansson77>{{cite conference |last1=Johansson |first1=S. |last2=Zethson |first2=T. |year=1999 |title=Atomic physics aspects on previously and newly identified iron lines in the HST spectrum of η&nbsp;Carinae |editor1-last=Morse |editor1-first=J.A. |editor2-last=Humphreys |editor2-first=R.M. |editor3-last=Damineli |editor3-first=A. |book-title=Eta Carinae at the Millennium |series=ASP Conference Series |volume=179 |pages=171 |publisher=[[Astronomical Society of the Pacific]] |bibcode=1999ASPC..179..171J |url=https://ui.adsabs.harvard.edu/link_gateway/1999ASPC..179..171J/ADS_PDF |via=NASA / [[Astrophysics Data System]] }}</ref> This effect is similar to the [[astrophysical maser|maser]] emission from dense pockets surrounding many cool supergiant stars, but the latter effect is much weaker at optical and UV wavelengths and η&nbsp;Carinae is the only clear instance detected of an ultraviolet [[astrophysical laser]]. A similar effect from pumping of metastable O<sup>I</sup> states by Ly<sub>β</sub> emission has also been confirmed as an astrophysical UV laser.<ref name="johansson">{{cite journal|bibcode=2005MNRAS.364..731J|title=Astrophysical laser operating in the O I 8446-Å line in the Weigelt blobs of η Carinae |journal=Monthly Notices of the Royal Astronomical Society|volume=364|issue=2|pages=731|last1=Johansson|first1=S.|last2=Letokhov|first2=V. S.|year=2005|doi=10.1111/j.1365-2966.2005.09605.x|doi-access=free}}</ref>