Editing Interstellar medium (section)

=== Cooling mechanisms ===

; Fine structure cooling: The process of fine structure cooling is dominant in most regions of the Interstellar Medium, except regions of hot gas and regions deep in molecular clouds. It occurs most efficiently with abundant atoms having fine structure levels close to the fundamental level such as: C&nbsp;II and O&nbsp;I in the neutral medium and O&nbsp;II, O&nbsp;III, N&nbsp;II, N&nbsp;III, Ne&nbsp;II and Ne&nbsp;III in H&nbsp;II regions. Collisions will excite these atoms to higher levels, and they will eventually de-excite through photon emission, which will carry the energy out of the region.

; Cooling by permitted lines: At lower temperatures, more levels than fine structure levels can be populated via collisions. For example, collisional excitation of the ''n''&nbsp;=&nbsp;2 level of [[hydrogen]] will release a Ly-α photon upon de-excitation. In molecular clouds, excitation of rotational lines of [[carbon monoxide|CO]] is important. Once a [[molecule]] is excited, it eventually returns to a lower energy state, emitting a photon which can leave the region, cooling the cloud.