Simulated optical (top) and long-wavelength (bottom) emission for an H ii region around an O star after 200 000 yr of evolution for MHD (left) and purely HD (right) simulations. The viewing angles are θ = 350° and φ = 350°, where θ is the polar angle, measured from the x-axis, and φ is the azimuthal angle, measured around the x-axis. Top: synthetic narrow-band optical emission-line images in the light of [N ii] 6584 Å (red), Hα 6563 Å (green) and O iii 5007 Å (blue). Bottom: synthetic images in the light of 6-cm radio free–free emission (blue), generic PAH (green) and molecular gas column density (red).