Observed and computed H₂ excitation diagram. N is the column density (cm⁻²) of the emitting rovibrational level, (v, J), and g= (2J+ 1)(2I+ 1) is its degeneracy (the nuclear spin quantum number I= 1 for ortho‐H₂, and I= 0 for para‐H₂). Observed values of ln N/g are plotted against the excitation energy of the emitting level, together with the values computed at the four dynamical ages indicated, and in the limit of steady state. See the text, Section 3.3.