In the 1860s, the term Nebula reffered to a fuzzy light object. Many nebulaes such as the Adromeda Nebula had a spectra similar to stellar spectra, they were in fact galaxies.

In 1864, Sir William Huggins obesrved the spectra of the Cat's Eye Nebula and noticed that it didn't have a continous spectra but a spectra made of a few strong lines. First called Nebulium (a new unknown chemical element), these lines were then identified as corresponding to Oxygen (Bowen 1928).

The Oxygen in Nebula exist in a special state as the vaccum is so intense and those lines are the forbidden transitions in the O II, O III, and N II spectra.

Cat's eye nebula NGC6543

Emission nebula - No continuum + H, He lines + Forbidden lines O II, O III, and N II spectra.