An excited energy level in an atom, ion, or molecule, from which transitions to lower energy levels are said to be forbidden. A forbidden transition is one of such low probability that it is not observed under laboratory conditions. However, in the low-density conditions of interstellar space and nebulae, electrons cannot escape from the metastable state by upward excitation to a higher level by collisions. The metastable state can then become highly overpopulated, and emission lines from the forbidden transitions can be the strongest lines in the spectrum of a nebula. See Forbidden Line.