http://en.wikipedia.org/wiki/Gravitational_collapse
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. In theory, a white dwarf-sized object could collapse to a neutron star by accreting matter from a companion star until it reaches the Chandrasekhar limit, but before that could happen a white dwarf will generally undergo runaway carbon fusion, blowing itself completely apart in a Type Ia supernova. Neutron stars are actually formed by gravitational collapse of larger stars, in the other types of supernova.
Very massive stars, above the Tolman-Oppenheimer-Volkoff limit cannot find a new dynamical equilibrium with any known force opposing gravity. It is clear that the collapse continues until a small ultra dense and highly compact object, within its Schwarzschild Radius, is created. Neglecting the effect of rotation, the Schwarzschild Radius is the location of the event horizon of a black hole, beyond which nothing can be observed. There are competing theories as to what happens inside the event horizon.
