Evolution of hydrogen lines for the standard model (black) compared with the observations of SN 2011dh (red). Spectra from eight logarithmically spaced epochs between 10 and 100 days are shown; the model and observed spectra at each epoch have been shifted by the same, but otherwise arbitrary, amount. Here, as well as in Figs. 7–10, we also show the velocity extent of the helium envelope (red lines) and the carbon–oxygen core (blue lines).