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In [[astronomy]] and [[atomic physics]], '''doubly ionized oxygen''' is the [[ion]] [[oxygen|O]]<sup>2+</sup> ('''O III''' in [[spectroscopic notation]]). Its emission [[forbidden line]]s in the [[visible spectrum]] fall primarily at the wavelength 500.7 nm, and secondarily at 495.9 nm. Before spectra of oxygen ions became known, these lines once led to a spurious identification of the substance as a new [[chemical element]]. Concentrated levels of O III are found in [[diffuse nebula|diffuse]] and [[planetary nebula|planetary]] nebulae. Consequently, narrow [[Dichroic filter|band-pass filter]]s that isolate the 500.7 nm and 495.9 nm wavelengths of light, that correspond to [[green]]-[[turquoise (color)|turquoise]]-[[cyan]] [[spectral colors]], are useful in observing these objects, causing them to appear at higher contrast against the filtered and consequently blacker background of space (and possibly [[light pollution|light-polluted]] terrestrial atmosphere) where the frequencies of [O III] are much less pronounced.
These [[emission line]]s were first discovered in the spectra of planetary nebulae in the 1860s. At that time, they were thought to be due to a new element which was named ''[[nebulium]]''. In 1927, [[Ira Sprague Bowen]] published the current explanation identifying their source as doubly ionized oxygen.<ref>{{cite journal | doi = 10.1038/120473a0 | title = The Origin of the Nebulium Spectrum | year = 1927 | author = Bowen, I. S. | journal = Nature | volume = 120 | issue = 3022 | pages = 473|bibcode = 1927Natur.120..473B | doi-access = free }}</ref>
Other transitions include the forbidden 88.4 μm and 51.8 μm transitions in the far [[infrared]] region.<ref name="Osterbrock">{{cite book |last1=Osterbrock |first1=Donald E. |title=Astrophysics of gaseous nebulae and active galactic nuclei |date=1989 |publisher=University Science Books |___location=Mill Valley, Calif. |isbn=0935702229 |page=73}}</ref>
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