Delay-time distribution ($\Psi (\tau )$)

The event rate per unit initial stellar mass and per unit delay after star formation. An extended star formation history produces a rate $R(t)=\int \mathrm{SFR}(t-\tau )\Psi (\tau )d\tau$. Measuring rates versus age in old populations constrains progenitor channels only after accounting for stellar lifetimes, age spreads and selection.