This graphic depicts the stratified nature of a photodissociation region (PDR) such as the Orion Bar. Once thought to be homogenous areas of warm gas and dust, PDRs are now known to contain complex structure and four distinct zones. The box at the left shows a portion of the Orion Bar within the Orion Nebula. The box at the top right illustrates a massive star-forming region whose blasts of ultraviolet radiation are affecting a PDR. The box at the bottom right zooms in on a PDR to depict its four, distinct zones: 1) the molecular zone, a cold and dense region where the gas is in the form of molecules and where stars could form; 2) the dissociation front, where the molecules break apart into atoms as the temperature rises; 3) the ionization front, where the gas is stripped of electrons, becoming ionized, as the temperature increases dramatically; and 4) the fully ionized flow of gas into a region of atomic, ionized hydrogen. For the first time, Webb will be able to separate and study these different zones' physical conditions.
Credits
Illustration
NASA, ESA, CSA, Jason Champion (CNRS), Pam Jeffries (STScI), PDRs4ALL ERS Team
