Measured line hydrogen line ratios, normalized to Pa α for the five regions in the Horsehead. Over plotted in colored lines are the theoretical ratios for a range of temperatures from Prozesky & Smits (2018). All the theoretical ratios are computed for a density of 1000 cm⁻³. Observed series are, left to right, Paschen, Brackett, Pfund, and Humphreys. Except for the H II region, 4–8 and 4–6 in the Brackett are plotted as light gray symbols as they are confused with nearby H₂ lines.