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Gravitational collapse is the contraction of anastronomical object due to the influence of its owngravity, which tends to draw matter inward toward thecenter of gravity.[1] Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution ofmatter, after sufficientaccretion, may collapse to form pockets of higher density, such asstars orblack holes.
Star formation involves a gradual gravitational collapse ofinterstellar medium into clumps ofmolecular clouds and potentialprotostars. The compression caused by the collapse raises the temperature untilthermonuclear fusion occurs at the center of the star, at which point the collapse gradually comes to a halt as the outwardthermal pressure balances the gravitational forces. The star then exists in a state ofdynamic equilibrium. During the star's evolution a star might collapse again and reach several new states of equilibrium.
An interstellar cloud of gas will remain inhydrostatic equilibrium as long as thekinetic energy of the gaspressure is in balance with thepotential energy of the internalgravitational force. Mathematically this is expressed using thevirial theorem, which states that to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy.[2] If a pocket of gas is massive enough that the gas pressure is insufficient to support it, the cloud will undergo gravitational collapse. The critical mass above which a cloud will undergo such collapse is called theJeans mass. This mass depends on the temperature and density of the cloud but is typically thousands to tens of thousands ofsolar masses.[3]
At what is called the star's death (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, thesestellar remnants can take one of three forms:
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 aplanetary nebula. If it has acompanion star, a white dwarf-sized object canaccrete matter from the companion star. Before it reaches theChandrasekhar limit (about one and a half times the mass of the 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. The resultingrunawaycarbon detonation completely blows the star apart in atype Ia supernova.
Neutron stars are formed by the gravitational collapse of the cores of larger stars. They are the remnant of supernova typesIb,Ic, andII. Neutron stars are expected to have a skin or "atmosphere" of normal matter on the order of a millimeter thick, underneath which they are composed almost entirely of closely packed neutrons calledneutron matter[5] with a slight dusting of free electrons and protons mixed in. This degenerate neutron matter has a density of about6.65×1017 kg/m3.[6]
The appearance of stars composed ofexotic matter and their internal layered structure is unclear since any proposedequation of state ofdegenerate matter is highly speculative. Other forms of hypothetical degenerate matter may be possible, and the resultingquark stars,strange stars (a type of quark star), andpreon stars, if they exist, would, for the most part, be indistinguishable from aneutron star: In most cases, theexotic matter would be hidden under a crust of "ordinary" degenerate neutrons.[citation needed]
According to Einstein's theory, for even larger stars, above the Landau–Oppenheimer–Volkoff limit, also known as theTolman–Oppenheimer–Volkoff limit (roughly double the mass of the 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 itsSchwarzschild radius it forms what is called a black hole, meaning a spacetime region from which not even light can escape. It follows fromgeneral relativity and the theorem ofRoger Penrose[8] that the subsequent formation of some kind ofsingularity is inevitable. Nevertheless, according to Penrose'scosmic censorship hypothesis, the singularity will be confined within the event horizon bounding the black hole, so the spacetime region outside will still have a well-behaved geometry, with strong but finite curvature, that is expected[9] to evolve towards a rather simple form describable by the historicSchwarzschild metric in the spherical limit and by the more recently discoveredKerr metric if angular momentum is present. If the precursor has a magnetic field, it is dispelled during the collapse, as black holes are thought to have no magnetic field of their own.[10]
On the other hand, the nature of the kind of singularity to be expected inside a black hole remains rather controversial. According to theories based onquantum mechanics, at a later stage, the collapsing object will reach the maximum possible energy density for a certain volume of space or thePlanck density (as there is nothing that can stop it). This is the point at which it has been hypothesized that the known laws of gravity cease to be valid.[11] There are competing theories as to what occurs at this point. For exampleloop quantum gravity predicts that aPlanck star would form. Regardless, it is argued that gravitational collapse ceases at that stage and a singularity, therefore, does not form.[12]
The radii of larger mass neutron stars (about 2.8 solar mass)[13] are estimated to be about 12 km, or approximately 2 times their equivalent Schwarzschild radius.
It might be thought that a sufficiently massive neutron star could exist within its Schwarzschild radius (1.0 SR) and appear like a black hole without having all the mass compressed to a singularity at the center; however, this is probably incorrect. Within theevent horizon, the matter would have to move outward faster than the speed of light in order to remain stable and avoid collapsing to the center. No physical force, therefore, can prevent a star smaller than 1.0 SR from collapsing to a singularity (at least within the currently accepted framework ofgeneral relativity; this does not hold for the Einstein–Yang–Mills–Dirac system). A model for the nonspherical collapse in general relativity with the emission of matter andgravitational waves has been presented.[14]
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