Evolution of the ejecta kinetic and radiative energies in a sample of stellar-collision simulations. We show radiation energy E_rad,ej (top) and kinetic energy E_kin,ej (bottom) for modelsets #1 (left, 1 M_⊙+0.9 M_⊙) and #5 (right, 10 M_⊙+1 M_⊙). The legend indicates the initial relative velocity (e.g., vrel5 means V_rel = 5000 km s⁻¹) and impact parameter (e.g., b0p5 means b = 0.5). The diagonal black lines in the top panels indicate the power law of t⁻¹, expected for an adiabatic expansion of a radiation-dominated plasma.