Editing Collisional excitation (section)

==Astronomy==
In astronomy, collisional excitation gives rise to [[spectral line]]s in the [[electromagnetic spectrum|spectra]] of [[astronomical object]]s such as [[planetary nebula]]e and [[H II region]]s.

In these objects, most [[atom]]s are [[ionisation|ionised]] by [[photon]]s from hot [[star]]s embedded within the nebular [[gas]], stripping away [[electrons]]. The emitted electrons, (called [[photoelectron]]s), may collide with atoms or ions within the gas, and [[Excited state|excite]] them.  When these excited atoms or ions revert to their [[ground state]], they will emit a photon.  The spectral lines formed by these photons are called ''collisionally excited lines'' (often abbreviated to CELs).

CELs are only seen in gases at very low densities (typically less than a few thousand particles per cm³) for forbidden transitions. For allowed transitions, the gas density can be substantially higher. At higher densities, the reverse process of '''collisional de-excitation''' suppresses the lines.  Even the hardest [[vacuum]] produced on earth is still too dense for CELs to be observed.  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]].  However, the lines he observed were later found to be emitted by extremely rarefied [[oxygen]].

CELs are very important in the study of gaseous nebulae, because they can be used to determine the density and temperature of the gas.