Photodissociation regions (PDRs) are regions where the UV photons from massive stars heat the gas and drive the chemistry. They are found at the surfaces of protoplanetary disks, at the boundaries between HII regions and molecular clouds, in the shells of planetary nebulae or in the nuclei of star-forming galaxies. Understanding these objects requires to understand the detailed physics and chemistry at play in PDRs, and their connections with infrared signatures (e.g. PAH emission, emission in the main cooling lines ([CII], [OI], CO etc.) In this SOFIA teletalk, I will present some of the observational study we have been conducting in the last 10 years with Spitzer, Herschel and SOFIA on NGC 7023, a nearby reflection nebula which can be considerered as a test-bed to understand PDRs. I will describe the photochemical evolution of PAHs and fullerenes which we have unveiled, and the intense dynamical activity which we have discovered. These insights raise new (sometimes unexpected) questions, which we hope we will answer with the help of SOFIA.
Understanding the Physics and Chemistry of Photodissociation Regions: Insights from Spitzer, Herschel and SOFIA Observations of NGC 7023