 Planetary-nebula spectral energy distribution

The spectral energy distribution of a planetary nebula combines hot central-star radiation, nebular free-free/free-bound/two-photon continua, recombination and collisionally excited lines, infrared dust emission and sometimes molecular emission. Ionizing-photon supply, gas thermal/ionization balance, density, composition, geometry and extinction determine their relative strengths. A line-dominated broad-band flux is not by itself a continuum temperature measurement.