Stellar-mass black holes form from the gravitational collapse of very massive stars at the end of their lives.

When a star roughly 8 times the mass of our Sun exhausts its nuclear fuel, its core can no longer withstand the inward pull of gravity. It collapses, often leading to a supernova explosion, leaving behind a dense remnant that can become a black hole if its mass exceeds a certain limit (the Tolman-Oppenheimer-Volkoff limit).