Schematic evolutionary paths of a massive binary system. Initially, if the two stars orbit close enough to each other, they may eventually merge, forming a single, more massive star (top path). Alternatively, the more massive star may explode as a supernova (SN), potentially ejecting the companion as a high-velocity runaway star or resulting in a system with a compact object. The companion may later undergo its own SN event, creating a double compact-object system that could eventually merge and emit gravitational waves (bottom paths).