Expanding photosphere. The blue diamond symbols are the estimated velocities from Johansson et al. (2021). The red square shows the estimated velocity by the same authors at the time of maximum luminosity. The solid line is a model with the standard exponent of 0.22, v_exp ∝ t^−0.22 (see text). The small inset plot in the top right shows the radius of the photosphere for this model of the velocity expansion.