An Etymological Dictionary of Astronomy and Astrophysics
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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.

→ nebular; → line.