Mean intensity (top) and flux (bottom) of the continuum radiation as function of radius and wavelength for an H ii region ionized by UV radiation (model B4). The gas is transparent below 1 Ryd and can therefore ionize elements with ionization thresholds lower than that of hydrogen, such as carbon and sulfur, even outside the hydrogen Strömgren sphere. However, as the main heating source – photoionization of hydrogen – is not present in this outer region, the temperature is generally too low for significant collisionally excited line radiation to be generated there.