gravitational collapse

1. The implosion of a star or other celestial body under the influence of its own gravity, resulting in a body that is many times smaller and denser than the original body.
2. The process by which stars, star clusters, and galaxies form from interstellar gas under the influence of gravity.

The stage in the evolution of a star in which the pressure in the star is insufficient to maintain the star at a stable size. The material in the star or in the core of the star then falls inward under its own gravitational attraction. Depending on the mass, composition, and spin of the star, the collapse may proceed to the formation of a neutron star or black hole, possibly accompanied by a supernova explosion. See also Stellar evolution; Supernova.

Stars similar to the Sun maintain a stable size by continually burning nuclear fuel. In most stars, as for the Sun, this burning involves the conversion of hydrogen to helium with a release of energy as the nuclear reactions take place. This energy supplies heat to the interior of the star, which in turn keeps the core of the star hot enough so that nuclear reactions can continue. Heat is also continually transferred from the interior to the exterior of the star, where it is lost, primarily in the form of radiation.

Suppose now that the nuclear burning is turned off, as is the case if the hydrogen in the core is used up and only helium remains. Then, no more heat is supplied to the core of the star, and its temperature drops as remaining heat is transported to the surface of the star and is lost. Likewise, since the pressure in the gas depends on the temperature, the pressure in the core also drops, and more importantly, the pressure gradient decreases throughout the star. The various chunks of gas are no longer in equilibrium, and they move inward. This compression of the gas then causes a rise in temperature and the temporary reestablishment of the required temperature gradient. However, as heat is transported outward, the temperature again drops, the gas is further compressed, and so on. In this stage, the star is undergoing contraction, the heating coming from the gravitational potential energy of the star rather than nuclear reactions.

Eventually, the star condenses, and the temperature rises to a sufficiently high value that helium burning can take place. Once again, the star is in stable equilibrium until the reservoir of fuel in the core of the star is used up, after which contraction takes place until conditions are right for nuclear burning of higher elements. The contraction discussed here refers to what is going on in the core of the star; the atmosphere may actually be expanding in the process.

When the star has a core which is composed of iron, no further nuclear burning can take place, since iron is the most stable element. Continued contraction must then take place. In fact, theoretical calculations indicate that under normal conditions the temperature does not rise high enough for nuclear burning to proceed beyond carbon, at which point the star effectively runs out of fuel, and continued contraction also takes place. If, after ejection of material by the star, the mass of the star is less than about 1.2–1.4 solar masses, the limiting value known as the Chandrasekhar limit, the contraction takes place to a white dwarf. Observations suggest that stars of up to 8 solar masses can eject sufficient matter to become white dwarfs. A white dwarf is stable without any nuclear burning taking place. The pressure gradient is produced by the same kind of quantum interactions among the electrons as those which make atoms stable. See also White dwarf star.

Theoretical studies indicate that a star of more than about 8 solar masses will not lose enough mass in its evolution to become a white dwarf. The contracting star at some point becomes unstable; that is, the heating as a result of contraction is insufficient to produce the required pressure gradient to support the gas against gravity. This instability occurs first in the core of the star, and the core starts to fall inward on itself. This is the stage of gravitational collapse. In essence, the core is freely falling inward under its own gravitational attraction.

A collapsing core will be too condensed to form a white dwarf, even if its mass is less than the maximum allowed mass. The only other known possibilities are collapse to a neutron star and collapse to a black hole. A neutron star is a highly condensed star in which the predominant constituent of matter is in the form of neutrons. The stability of such stars is a result of the quantum-mechanical interaction of neutrons, the same kind of interaction which, for electrons, leads to stability of white dwarfs. See also Neutron star.

Some collapsing stars are expected to end up with a core which has a mass larger than the maximum mass of a neutron star. In addition, neutron stars (and white dwarfs as well) which had been formed earlier could accrete enough matter to become more massive than the maximum mass of a neutron star. In these cases, the only known alternative is for such stars to collapse to a black hole. A black hole is a region in space in which gravity is so strong that even light cannot escape from its surface. Although black holes had been conjectured earlier, Karl Schwarzschild found the first black-hole solution of general relativity in 1916, although the significance of the solution as a black hole was not realized at the time. After suggestions that black holes might be an end point of stellar evolution, J. R. Oppenheimer and H. Snyder showed in 1939 that a black hole must result from spherically symmetric gravitational collapse if the mass of the collapsing body is large enough. At present, a black hole is believed to be the only result of a collapsing star that cannot lose enough matter to become a white dwarf or neutron star. Black holes are even more condensed than neutron stars. For example, a 1-solar-mass black hole would have a radius of about 2 mi (3 km). See also Black hole.

Because black holes emit no light, they are intrinsically dark and directly unobservable. However, they can still have an effect on nearby matter because their gravitational field is still present. In fact, there is strong evidence that at least one condensed body in a binary star system, Cygnus X-1, is a black hole. Its gravitational field results in matter heating up sufficiently to emit x-rays, which can only be done by a highly condensed body, and its mass is determined from the orbital motion to be larger than 5 solar masses. From theory, the only known body which has these properties is a black hole. Black holes have also been identified by similar evidence in several other x-ray binaries, including LMC X-3 and A0620-00. There is no proof that such black holes came from the gravitational collapse of a star, but that is the most reasonable hypothesis. See also Astrophysics, high-energy; Binary star; X-ray astronomy.

The noun has one meaning:

Meaning #1: the implosion of a star resulting from its own gravity; the result is a smaller and denser celestial object

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Gravitational collapse of a star

NGC 6745 produces material densities sufficiently extreme as to trigger star formation through gravitational collapse

Gravitational collapse in astronomy is the inward fall of a massive body under the influence 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. According to theories, at some point later the collapsing object will reach the maximum possible energy density for a certain volume of space or the Planck density (as there is nothing that can stop it), where the known laws of gravity cease to be valid^[1]. There are competing theories as to what occurs at this point, but it can no longer really be considered gravitational collapse at that stage.

See also

• Big Crunch

References

1. ^ [1]

External links

• Gravitational collapse on arxiv.org

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