Relative collision probability in the life of a star

Cumulative collision probability (normalized to 1) integrated over the lifetime of three stellar models. The second star is assumed to be point-like. In each case, two limiting regimes are considered. At low relative velocities V_rel << V_ast, the collisional cross-section scales like R_star (solid lines); at very high velocities V_rel >> V_ast, we recover the geometrical cross section, ~ R_star^2 (dashed lines). Stellar evolution models are from Schaller et al. 1992 and Meynet et al. 1994. The evolution is only shown up to He-flash. The solid dot shows the end of the MS phase.