Nebulae
In the early days of telescopic astronomy, the word nebula was used to describe any fuzzy patch of light that didn't look like a star. Many of these, such as the Andromeda Nebula, had spectra that looked in many ways a lot like stellar spectra, and these turned out to be galaxies. Others, such as the Cat's Eye Nebula, had very different spectra. When William Huggins looked at the Cat's Eye, he found no continuous spectrum like that seen in the Sun, but just a few strong emission lines. These lines did not correspond to any known elements on earth, and so just as helium had been identified in the Sun, astronomers suggested that the lines were due to a new element, nebulium (occasionally nebulum or nephelium). The hypothetical nebulium that was invoked to account for certain bright lines in gaseous nebulae were shown by Ira Sprague Bowen in 1927 as due to doubly ionized oxygen at extremely low density. As Henry Norris Russell put it, "Nebulium has vanished into thin air." But nebulae are typically extremely rarefied, much less dense than the hardest vacuum ever produced on earth. In these conditions, atoms behave quite differently and lines can form which are suppressed at normal densities. These lines are known as forbidden lines, and are the strongest lines in most nebular spectra.
Read more about this topic: Astronomical Spectroscopy