What is a photodissociation region in astronomy?
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As far as I can tell from the Wikipedia article, it is a region of the interstellar medium (the extremely thin gas which permeates space) which is cold and dense enough to remain mostly neutral (i.e. not a plasma), yet not so dense that it keeps out far UV radiation from distant massive stars. This radiation alters the chemistry of the medium, and its absorption is the main source of heat energy in the region.
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