[[File:Oiii-linesp.svg|thumb|right|EmissionA lines[[Grotrian diagram]] of doubly ionized oxygen: The forbidden linestransitions are shown in green.]]
In [[astronomy]] and [[atomic physics]], '''Doublydoubly ionized oxygen''' (also known as '''<nowiki>[O III]</nowiki>''') is a [[forbidden line]] of the [[ion]] [[oxygen|O]]<sup>2+</sup>. ItIts isemission significant[[forbidden in that it emits lightline]]s in the green part of the[[visible spectrum]], primarily at the wavelength 500.7 nanometres ( nm), and secondarily at 495.9 nm, are known in [[astronomical spectroscopy]] as '''[O III]'''. 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 <nowiki>[O III]</nowiki> are found in [[diffuse nebula|diffuse]] and [[planetary nebula|planetary]] nebulae. Consequently, narrow [[Dichroic filter|band-pass filter]]s that isolate the 501 nm and 496 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 <nowiki>[O III]</nowiki> are much less pronounced.
These [[emission line]]s were first discovered in the spectrumsspectra 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]] came up with the current explanation of them being due to 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 }}</ref>
Permitted lines of O III lie in the Middle [[Ultraviolet]] band and are hence inaccessible to terrestrial astronomy.
In the Gas Pillars of the ''[[Eagle Nebula]]''. Doubly-ionized oxygen atoms emit blue light.
