Spectral emission lines that are highly improbable under normal laboratory conditions but commonly observed in the low-density environments of space. These lines arise from electron transitions that violate usual quantum mechanical selection rules.
The term 'forbidden' was adopted from quantum mechanics in the 1920s, referring to transitions that have very low probability. Initially mysterious when first observed in nebular spectra, they were explained when scientists realized space conditions are very different from Earth laboratories.
Forbidden lines aren't actually forbidden - they're just incredibly shy! These spectral lines only appear when atoms have lots of time and space to themselves, which is why they're common in the near-vacuum of space but almost never seen in dense laboratory gases where atoms are constantly bumping into each other.
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