Sketch (not to scale) of a BS of a runaway star. The FS and the RS are separated by the CD, represented by a solid black line. The immediate post-shock external medium, the shocked medium ionised by the star, and the shocked stellar wind are shown in red, orange, and blue, respectively. The solid lines with arrows indicate the shocked gas streamlines, from the injection position up to the tail of the BS. Adapted from Martinez et al. (2022).