Nebulium

Nebulium was a hypothetical chemical element proposed to explain bright green spectral lines in nebulae. The lines were later identified as forbidden transitions in highly ionized oxygen.

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William Huggins: An English astronomer who pioneered astronomical spectroscopy and established the gaseous nature of some nebulae. His early nebular spectra helped make the unexplained emission lines a central problem.
Forbidden transition: An atomic transition with a very low probability under ordinary conditions, disallowed by the dominant spectroscopic selection rules. The nebular lines arise from transitions that are too slow to observe in dense laboratory samples.
Green nebular line at 500.7 nm: A strong green emission line produced by a forbidden transition in doubly ionized oxygen. This prominent line was among the features that made nebulium seem plausible.
Chemical element: A substance whose atoms all have the same number of protons in their nuclei. Nebulium was proposed as an element, but no distinct atomic species exists behind its lines.
Nebular hypothesis: A theory that stars and planetary systems form from diffuse clouds of gas and dust. Nineteenth-century nebular research supplied the astronomical setting for the proposed element.
Oxygen III: Doubly ionized oxygen, an oxygen atom that has lost two electrons. Its forbidden transitions produce prominent green lines once attributed to nebulium.
Orion Nebula: A nearby star-forming region in the constellation Orion, visible to the unaided eye as a diffuse patch. Its bright emission spectrum exemplifies the nebular lines once assigned to nebulium.
Oxygen: A chemical element with atomic number eight, abundant in Earth's crust and in the universe. The supposed element's spectral signature belongs to ionized oxygen instead.
Spectroscopy: The study of how matter interacts with light across different wavelengths. Spectroscopy revealed the lines that nebulium was invented to explain.
Oxygen II: Singly ionized oxygen, an oxygen atom that has lost one electron. Its emission lines account for other features in nebular spectra associated with the old element.