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Gravitational collapse in astronomy is the inward fall of a massive body under the influence of the force of gravity. It occurs when all other forces fail to supply a sufficiently high pressure to counterbalance gravity and keep the massive body in hydrostatic equilibrium.
Gravitational collapse is at the heart of structure formation in the universe. An initial smooth distribution of matter will eventually collapse and cause the hierarchy of structures, such as clusters of galaxies, stellar groups, stars and planets. For example, a star is born through the gradual gravitational collapse of a cloud of interstellar matter. The compression caused by the collapse raises the temperature until nuclear fuel ignites in the center of the star and the collapse comes to a halt. The thermal pressure gradient (leading to expansion) compensates the gravity (leading to compression) and a star is in dynamical equilibrium between these two forces.
Gravitational collapse of a star occurs at the end of its life time, also called the death of the star. When all stellar energy sources are exhausted, the star will undergo a gravitational collapse. In this sense a star is in a "temporary" equilibrium state between a gravitational collapse at stellar birth and a further gravitational collapse at stellar death. The end states are called compact stars.
The types of compact stars are:
- White dwarfs, in which gravity is opposed by electron degeneracy pressure;
- Neutron stars, in which gravity is opposed by neutron degeneracy pressure and short-range repulsive neutron-neutron interactions mediated by the strong force;
- Black holes, in which the physics at the center is unknown.
The collapse to a white dwarf takes place over tens of thousands of years, while the star blows off its outer envelope to form a planetary nebula. If it has a companion star, a white dwarf-sized object can accrete matter from a companion star until it reaches the Chandrasekhar limit, at which point gravitational collapse takes over again. While it might seem that the white dwarf might collapse to the next stage (neutron star), they instead undergo runaway carbon fusion, blowing completely apart in a Type Ia supernova. Neutron stars are formed by gravitational collapse of larger stars, the remnant of other types of supernova.
Even more massive stars, above the Tolman-Oppenheimer-Volkoff limit cannot find a new dynamical equilibrium with any known force opposing gravity. Hence, the collapse continues with nothing to stop it. Once it collapses to within its Schwarzschild Radius, not even light can escape from the star, and hence it becomes a black hole. At some point later the collapsing object must reach the planck density (as there is nothing that can stop it), where the known laws of gravity cease to be valid. There are competing theories as to what occurs at this point, but it can no longer really be considered gravitational collapse at that stage.