Top panel: evolution of our MESA star model in the HR diagram. The position of the progenitor star just before collapse is marked by a blue diamond. This progenitor star has a compact radius (see dashed black lines) and it is characterized by a rather high temperature (10⁵ K), therefore it belongs to the O stellar type. The phase when most of the H mass is lost owing to strong winds is shown by a thicker blue line. Bottom panel: density profile of our progenitor star model.