Sketch Illustrating the Stromgren Depth

Sketch illustrating how the Stromgren depth is defined. The total number of ionizing photons entering a cloud per unit area is given on the left, and in equilibrium this must equal the total number of recombinations on the right. Since the recombination rate is given as _H n_e n_p per cubic centimeter, then a tube of length d_S gives the total recombination rate. Notice here, _H gives the total recombination rate for Hydrogen (26 × 10^-14 cm³ s^-1).