| Astrophysics (Index) | About |
A photodissociation region (or photon-dominated region or PDR or photodissociation front) is a region where photodissociation of molecular hydrogen is taking place, resulting in atomic hydrogen, along a front where ultraviolet EMR from a nearby source is sufficiently intense to warm the molecules to the point where the balance favors atomic hydrogen. They form a border between an HII region and a molecular cloud. Other molecules of the molecular cloud can also photodissociate, providing some of the spectral signatures used to identify and analyze PDRs.