Range of observationally derived O VI to 1/4 keV intensity ratios (dash‐dotted line to dashed line) compared with the predicted ratio from an evolving SNR (solid line) and the remnant’s average ratio (dotted line). The SNR’s O VI to soft X‐ray intensity ratio reaches 1.1 (dash‐dotted line) when the remnant is about 40,000 yr old and reaches 4.7 (dashed line) when the remnant is about 70,000 yr old. The observationally determined ratio (1.1–4.7) is less than the remnant’s lifetime averaged intensity‐weighted O VI to 1/4 keV ratio (8.9, dotted line). The time period when the remnant evolves from the adiabatic (Sedov) phase to the radiative phase is also plotted on the graph. This phase is bracketed by vertical lines on the plot.