There exist excited states in some of the ions which may last for hours before the ion drops to a lower state and emits a photon. Only if the ion is left undisturbed by collisions (which could change the state of the ion) will the transition actually occur. In earth-bound laboratories we are not able to reduce the density low enough to see these transitions, but at the low densities of the nebulae they do occur and in fact these "forbidden" spectra may contribute significantly to the light observed from a nebula. An example of this is a level in doubly ionized oxygen, which after several hours will decay to a lower level with the emission of green lines. When these green lines were first seen, they did not seem to be associated with any elements know on earth and a new element "nebulium" was proposed.
Text by Sam Lightner
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