We propose the bolometric temperature T bol as a measure of the circumstellar obscuration and evolutionary development of a young stellar object (YSO). T bol is the temperature of a blackbody having the same mean frequency as the observed continuum spectrum. A log-log plot of bolometric luminosity L bol versus T bol has the same main sequence as the Hertzsprung-Russell diagram, but for YSOs T bol can have much lower values (∼30 K) than can the photospheric temperature T eff (∼2800 K)