Photodissociation region

Photodissociation regions (or photon-dominated regions, or 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.cite journal | author = Hollenbach, D.J. | coauthors = Tielens, A.G.G.M. | journal = Review of Modern Physics | year = 1999 | title = Photodissociation regions in the interstellar medium of galaxies | volume = 71 | pages = 173 | url = http://adsabs.harvard.edu/abs/1999RvMP...71..173H] They occur in any region of interstellar gas that is dense and cold enough to remain neutral, but that has too low a column density to prevent the penetration of far-UV photons from distant, massive stars. A typical and well-studied example is the gas at the boundary of a giant molecular cloud.

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