In astronomy, collisional excitation gives rise to spectral lines in the spectra of astronomical objects such as planetary nebulae and H II regions.
In these objects, most atoms are ionised by photons from hot stars embedded within the nebular gas, stripping away electrons.
CELs are only seen in gases at very low densities (typically less than a few thousand particles per cm³) for forbidden transitions.
For this reason, when CELs were first observed by William Huggins in the spectrum of the Cat's Eye Nebula, he did not know what they were, and attributed them to a hypothetical new element called nebulium.
[1] Molecular ions are accelerated to high kinetic energy and then collide with neutral gas molecules (e.g. helium, nitrogen or argon).