Dust opacity index β as a function of evolutionary time of star formation. β describes the frequency dependence of the dust opacity κ_{ν, dust} in the Rayleigh-Jeans regime according to the power law κ_{ν, dust} ∝ ν^β. The cartoons are from McCoughrean/NASA/JWST. The major contributor to the dust emission at the various stages is indicated below the icons and the nature of the objects above them. The importance of a given contributor to the far-IR/submm emission is expressed in upper and lower case letters, respectively. The common nomenclature of stellar formation, labelled Class −1 through Class III, is shown above the data points with their 1σ errors. The line through the data points is an analytical fit of the form β(t) = a₁ exp(−γ₁t/τ) + a₂ exp(−γ₂t/τ), where τ = 5 × 10⁴ yr is a characteristic time scale, after which increased grain growth occurs in discs.