Doubly ionized oxygen

Doubly ionized oxygen (also known as [O III]) is a forbidden line of the ion O²⁺. It is significant in that it emits light in the green part of the spectrum primarily at the frequency 500.7 nanometres (nm) and secondarily at 495.9 nm. Concentrated levels of [O III] are found in diffuse and planetary nebulae. Consequently, narrow band-pass filters that isolate the 501 nm and 496 nm wavelengths of light are useful in observing these objects, causing them appear at higher contrast against the filtered and consequently blacker background of space where the frequencies of [O III] are much less pronounced.

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These emission lines were first discovered in the spectrums 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.