Photodissociation region
In astrophysics, photodissociation regions (or photon-dominated regions, PDRs) are predominantly neutral regions of the interstellar medium in which far ultraviolet photons strongly influence the gas chemistry and act as the most important source of heat. They constitute a sort of shell around sources of far-UV photons at a distance where the interstellar gas is dense enough, and the flux from the photon source is no longer strong enough, to strip electrons from the neutral constituent atoms.