An atom is in a neutral state when the electron and proton are associated. When the electron and proton are separated, the atom is said to be ionized. An HII region, for example, is a nebula composed of ionized hydrogen atoms. Ionized atoms normally revert back into neutral atoms, releasing radiation in the process. Thus an ionized region would eventually disappear; that is, unless it is rejuvenated by an agent such as ultraviolet light from a hot star. The UV light is absorbed by neutral atoms, causing a re-separation of electrons from protons. This is a recurring process in many nebulae, so radiation is continually emitted at various wavelengths.

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This page last updated on April 5, 1999