Wiktionary
n. (context astronomy physics English) A supposed chemical element proposed as a result of spectral analysis of light from a nebula; the emission lines concerned are now known to be due to doubly-ionized oxygen.
Wikipedia
Nebulium was a proposed element found in a nebula by William Huggins in 1864. The strong green emission lines of the Cat's Eye Nebula, discovered using spectroscopy, led to the postulation that an as yet unknown element was responsible for this emission. In 1927, Ira Sprague Bowen showed that the lines are emitted by doubly ionized oxygen (O), and no new element was necessary to explain them.
Usage examples of "nebulium".
And what miracles are there in coronium and nebulium which, as the child of nebula and sun, we inherit?
Angry green luminescence that had once been called nebulium edged these stormy, denser regions.