Planetary Nebulae - Primary Mechanism

In general terms, an atom or ion may be ionized by very energetic photons, a process referred to as photo-ionization. Photons of the far ultraviolet region have sufficient energy to ionize an atom that is in the ground state. After being photo-ionized from the ground level, the ion recaptures an electron in any one of its various excited levels. After this recombination, as it is called, the electron cascades down to the lower levels, emitting photons of different frequencies. The origin of the permitted lines of hydrogen and helium are explained in this manner. This also applies to the ionic permitted lines of carbon, nitrogen, oxygen, and neon observed in the ordinary optical region. These lines are weaker, however, than those of H and He, and this is due to their much lower abundance in the nebula.