Model solutions for the evolution of the outer boundary of a spherically-symmetric PWN in a homogeneous ISM. A basis parameter set is chosen as L₀ = 10³⁹ erg s^-1, τ = 3000 yr, M_ej = 12   M_⊙, and ρ_ism = 10^-25 gcm^-3, with the solution represented by a short-dashed line in panels a)–d). The other solutions were obtained by using the basis parameters, while varying the parameters listed in the legends of the figures. The ◇ symbol represents the time when the reverse shock reaches R_pwn. Also shown in panels a) and b) is the evolution of the forward shock of the SNR.