Forbidden Line

Spectral line seen in emission nebulaeAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive but not seen in laboratory experiments, because under laboratory conditions, collisions kick the electron in question into some other state before emission can occur.