Doubly ionized oxygen (also known as ) 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 wavelength 500.7 nanometres (nm) and secondarily at 495.9 nm. Concentrated levels of 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 to appear at higher contrast against the filtered and consequently blacker background of space (and possibly light polluted terrestrial atmosphere) where the frequencies of are much less pronounced.
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.
In the Gas Pillars of the Eagle Nebula. Doubly-ionized oxygen atoms emit blue light.
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