gravitational collapse

As a massive star evolves, it forms onion-layered shells of fusing elements (a). When the inert iron core reaches an unsustainable mass, it collapses into neutrons (b–c). The descending matter rebounds (d–f), forming a Type II supernova

Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity. Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter, after sufficient accretion, may collapse to form pockets of higher density, such as stars or black holes.