Diagnostic diagram, adapted from Veilleux & Osterbrock (1987), plotting log [O III] λ5007/Hβ vs. log [N II] λ6583/Hα. The symbols have the following meanings: filled circles, disk H II regions; open circles, H II nuclei; asterisks, starburst nuclei. The dashed curve denotes the theoretical models of disk H II regions by McCall et al. (1985). The calculations assume that the effective temperature of the ionizing stars decreases with increasing oxygen abundance, ranging from T_eff = 47,000 to 38,500 K as (O/H)/(O/H)_☉ varies from 0.5 to 3.5 (top to bottom). Also shown are the photoionization models of Shields & Kennicutt (1995), which incorporate the effects of dust (dotted curves); the top curve was calculated for T_eff = 45,000 K and the bottom one for T_eff = 38,000 K. The models assume a constant density and ionization parameter, with abundances varying between 0.1 and 5 times the solar values from the upper left to the bottom right of each curve. The unusual object IC 2574 (see § 4.2) is labeled.