Unlike stars which show a continuous spectrum, the optical spectrum of PN is dominated by emission lines. Line emission occurs when atoms or ions make a transition from one bound electronic state to another bound state at a lower energy. Such transitions, usually by means of spontaneous emission, are referred to as bound-bound (b-b) transitions. In the interior of stars, electrons in an atom are distributed over many energy levels because of the high particle and radiation densities. The bound electrons are excited either by free electrons colliding with the atom, or by the absorption of a photon. However, in the interstellar medium, both the particle and radiation densities are low, and the population distribution of the bound electrons can be far from the thermodynamical equilibrium condition given by the Boltzmann equation [Eq. (2.23)].

The typical energy separations between the electronic states of atoms are of the order of 1 eV, corresponding to photons in the visible or UV parts of the spectrum. The only available visible or UV background in the interstellar medium is from diluted starlight, which is generally not strong enough for excitation by stimulated absorption to be significant. Therefore the only way that a bound electron can be found in an excited state is by collisional excitation from a lower state, or as a consequence of recombination between a free electron and a proton.