= Ionization-bounded nebula
An ionization-bounded nebula contains enough absorbing material to trap the relevant ionizing photons, unlike a density-bounded region that ends before its ionization front. In dust-free hydrogen at steady <photoionization equilibrium>, the absorbed stellar photon rate equals the integrated <Case B recombination> rate. Escape, absorption by dust, and competition from other species must be accounted for before identifying the absorbed hydrogen rate with the entire stellar ionizing luminosity.
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