Photodissociation region

[2] 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.

[3] Despite being composed of denser gas, PDRs still have too low a column density to prevent the penetration of far-UV photons from distant, massive stars.

PDRs are also composed of a cold molecular zone that has the potential for star formation.

[4] They achieve this cooling by far-infrared fine line emissions of neutral oxygen and ionized carbon.

[5] It is theorized that PDRs are able to maintain their shape by trapped magnetic fields originating from the far-UV source.

The image shows the 4 primary zones of a photodissociation region: the molecular zone, the dissociation front, the ionization front, and the fully ionized flow of gas. [ 1 ]