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tokens: that all star formation within Upper Sco has ceased. According to de Geus ( 1992 ) and Preibisch Zinnecker ( 1999 ), star formation in Upper Sco may have been triggered by a supernova-driven shock wave from a massive star in the parent Sco-Cen OB Association (see also Preibisch Zinnecker 2007 ; Slesnick et al. 2008 ). In this context, smooth star formation could occur over a long interval, followed by a supernova that triggers a burst, and subsequent quenching of star formation. 4.1. Comparing Synthetic Populations to the Known Upper Sco Members Our simulations of the star formation history of a cluster correspond to smooth star formation with no trigger (the Smooth-Quench model) and smooth star formation followed by a supernova trigger (Smooth-Burst model). Figure 11 compares the results of these simulations to the stellar population in Upper Sco for the Smooth-Burst model with parameters , , and A = 60, and for the Smooth-Quench model with and . Figure 11. The inferred age vs. temperature of stars in Upper Sco (black, standard deviation in gray dotted lines), compared to results from the Smooth-Quench (top) model, with and , and the Smooth-Burst model (bottom) with A = 60, , and . The shaded regions are the standard deviation from 1000 simulations. The dashed line shows the same results, but when all binaries are resolved. The Smooth-Quench model exhibits a temperature-dependent age measurement up to ∼6 Myr between KM stars and F stars, thereby accounting for half of the ∼12 Myr discrepancy (see Table 2 ). This simplest model demonstrates that an age spread of more than a few Myr in a young ( Myr) population will lead to a significant temperature dependence in age estimates. This qualitative result is required by the evolution of intermediate-mass stars from the Hayashi track to the Henyey track. Cool stars are young and have a median age that changes slowly. For stars with , the median age increases rapidly. At temperatures higher than , stars tend to be older and have median ages that change slowly. These simulation results are consistent with the age spreads of 4–5 Myr over a relatively large temperature range of cool stars (e.g., Herczeg Hillenbrand 2015 ), and with the ∼3 Myr age spread for G type and F type stars (Pecaut et al. 2012 ). Figure 11 shows that the Smooth-Burst model may explain entirely the empirical temperature-age relationship of Upper Sco. The burst increases the population of cool stars at the end of the star formation process, thereby maximizing the possible differences in ages between cool and hot stars. Some of the age difference between K-M type and F type stars may be attributed to the incomplete physics in the calculations of pre-main sequence evolution of low-mass stars (e.g., Feiden 2016 ), although these explanations may introduce problems into other clusters. However, the age difference between G type stars (9 ± 3 Myr) and F type stars (13 ± 1 Myr), as calculated by Pecaut et al. ( 2012 ), are not easily