How Does a Black Hole Form?

A stellar black hole forms at the end of a massive star life (typically 20+ solar masses). When the star has fused all available fuel up to iron, fusion can no longer support the core against gravitational collapse. The core implodes in a fraction of a second, while the outer layers are blasted away in a supernova. If the remaining core exceeds about 2-3 solar masses (the Tolman-Oppenheimer-Volkoff limit), no known force can halt the collapse, and it compresses into a singularity — a point of theoretically infinite density surrounded by an event horizon. Supermassive black holes (millions to billions of solar masses) at galaxy centres formed through mergers and accretion over billions of years; their exact origin is still an active research topic.