A so-called ‘forbidden line’ is an atomic transition that can only occur under very special conditions present in places such as nebulae, the solar corona, and active galactic nuclei. A forbidden line is produced when an electron falls from a metastable state (temporarily stable) to a lower energy state. Electrons end up in such metastable states through decay from higher energy levels. Under normal conditions, the particle densities would be too high to allow any electron to stay in a metastable state for any significant period of time. In this case, any electron in a metastable state would be knocked out due to collisions and not emit a photon. However, in low density environments (such as in a nebula), collision times become much longer and electrons can remain in metastable states undisturbed before spontaneously decaying to lower energy states. The strongest forbidden lines observed are from doubly ionized oxygen [O III] at 4959 and 5007 Angstroms in the green part of the spectrum (clearly seen in the image of the planetary nebula NGC 6210 (below). For a historical perspective on forbidden lines, be sure to look at Eddington (1927) and Bowen (1936).