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The BPT diagrams are three types of diagram that plot the relative strengths of emission lines with the aim of determining the state of ionization of a plasma, specifically for classifying active galactic nuclei (AGNs). Each of the three diagrams plots an [O III] line's ratio with H-β against another such ratio:
The plots are log-log. BPT is for Baldwin, Phillips & Terlevich, the three authors who introduced the diagrams.