Fr.: raie nébulaire
A → forbidden line that is found in the spectra of → interstellar → ionized gas. The nebular lines are emitted by several atomic species (e.g. O, O+, O++, N+, S++) and correspond to the → transition from the electronic → metastable state 1D to the → ground state 3P. Examples are the doubly ionized oxygen lines [O III] at 4959 and 5007 Å (→ [O III] doublet) and the ionized nitrogen doublet [N II] at 6548 and 6583 Å. See also → auroral line; → transauroral line.