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 thermonuclear fusion occurs at the center of the star, at which point the collapse gradually comes to a halt as the outward thermal pressure balances the gravitational forces. The star then exists in a state of dynamic equilibrium. Once all its energy sources are exhausted, a star will again collapse until it reaches a new equilibrium state.
An interstellar cloud of gas will remain in hydrostatic equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force. Mathematically this is expressed using the virial theorem, which states that, to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy.
If a pocket of gas is massive enough that the gas pressure is insufficient to support it, the cloud will undergo gravitational collapse. The mass above which a cloud will undergo such collapse is called the Jeans mass. This mass depends on the temperature and density of the cloud, but is typically thousands to tens of thousands of solar masses.
At what is called the death of the star (when a star has burned out its fuel supply), it will undergo a contraction that can be halted only if it reaches a new state of equilibrium. Depending on the mass during its lifetime, these stellar remnants can take one of three forms:
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 hole, in which there is no force strong enough to resist gravitational collapse.
The collapse of the stellar core 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 the companion star.
Before it reaches the Chandrasekhar limit (about one and a half times the mass of our Sun, at which point gravitational collapse would start again), the increasing density and temperature within a carbon–oxygen white dwarf initiate a new round of nuclear fusion, which is not regulated because the star’s weight is supported by degeneracy rather than thermal pressure, allowing the temperature to rise exponentially.
Neutron stars are formed by gravitational collapse of the cores of larger stars. Neutron stars are expected to have a skin or “atmosphere” of normal matter on the order of a milimeter thick, underneath which they are composed almost entirely of closely packed neutrons (popularly called “neutronium”) with a slight dusting of free electrons and protons mixed in. This degenerate neutron matter has a density of ~4×1017 kg/m3
According to Einstein’s theory, for even larger stars, above the Landau–Oppenheimer–Volkoff limit (roughly double the mass of our Sun) no known form of cold matter can provide the force needed to oppose gravity in a new dynamical equilibrium. Hence, the collapse continues with nothing to stop it.
Once a body collapses to within its Schwarzschild radius it forms what is called a black hole, meaning a spacetime region from which not even light can escape. It follows from general relativity and the theorem of Roger Penrose that the subsequent formation of some kind of singularity is inevitable.
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