spectral lines, not normally observed under laboratory conditions because they have an intrinsically low probability of occurrence, or because they result from a transition between a metastable excited state and the ground state. Under typical conditions, an atom in a metastable state will lose energy through a collision before it is able to decay to the ground state. Under astrophysical conditions, however, where there are huge numbers of atoms and densities may be very low, it is possible for such forbidden transitions to occur and to produce strong spectral lines.