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{{short description|Contraction of an astronomical object due to the influence of its gravity}}
{{misleading}}
{{Refimprove|date=June 2006}} {{more citations needed|date=October 2009}}
⚫ ]]]
{{Expert-subject|astronomy|date=November 2008}}
⚫ ]
'''Gravitational collapse''' in ] is the inward fall of a massive body under the influence of the force of ]. It occurs when all other forces fail to supply a sufficiently high pressure to counterbalance gravity and keep the massive body in ] equilibrium.


'''Gravitational collapse''' is the contraction of an ] due to the influence of its own ], which tends to draw matter inward toward the ].<ref>{{cite book|last1=Pilchin|first1=Lev Eppelbaum, Izzy Kutasov, Arkady|title=Applied geothermics|date=2013|publisher=Springer Berlin Heidelberg|location=Berlin, Heidelberg|isbn=9783642340239|page=2|edition=Aufl. 2014}}</ref> Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of ], after sufficient ], may collapse to form pockets of higher density, such as ]s or ]s.
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 ], stellar groups, ] 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 ] between these two forces.


] involves a gradual gravitational collapse of ] into clumps of ]s and potential ]s. The compression caused by the collapse raises the temperature until ] occurs at the center of the star, at which point the collapse gradually comes to a halt as the outward ] balances the gravitational forces. The star then exists in a state of ]. During the star's evolution a star might collapse again and reach several new states of equilibrium.
] occurs at the end of its life time, also called the ] 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 ]s.


==Star formation==
The types of compact stars are:
{{Main|Star formation}}
An interstellar cloud of gas will remain in ] as long as the ] of the gas ] is in balance with the ] of the internal ]. Mathematically this is expressed using the ], which states that to maintain equilibrium, the gravitational potential energy must equal twice the internal thermal energy.<ref>{{cite book
| first=Sun | last=Kwok | date=2006
| title=Physics and chemistry of the interstellar medium
| url=https://archive.org/details/physicschemistry0000kwok | url-access=registration | publisher=University Science Books
| isbn=1-891389-46-7 | pages=
}}</ref> 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 the ]. This mass depends on the temperature and density of the cloud but is typically thousands to tens of thousands of ]es.<ref>{{cite book
| first=Dina | last=Prialnik
| title=An Introduction to the Theory of Stellar Structure and Evolution
| publisher=] | date=2000
| isbn=0-521-65937-X | pages=198–199
}}</ref>


==Stellar remnants==
⚫ * ]s, in which gravity is opposed by ];
] produces material densities sufficiently extreme to trigger star formation through gravitational collapse]]
⚫ * ]s, in which gravity is opposed by ] and short-range repulsive neutron-neutron interactions mediated by the ];
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, these ] can take one of three forms:
* ]s, in which the physics at the center is unknown.

⚫ * ]s, in which gravity is opposed by ]<ref>And theoretically ]s – but: ''"...no black dwarfs are expected to exist in the universe yet"''</ref>
⚫ * ]s, in which gravity is opposed by ] and short-range repulsive neutron–neutron interactions mediated by the ]
* ], in which there is no force strong enough to resist gravitational collapse
===White dwarf===
⚫ 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 ]. If it has a ], a white dwarf-sized object can ] matter from a companion star until it reaches the ], 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 ] ], blowing completely apart in a ]. Neutron stars are formed by gravitational collapse of larger stars, the remnant of ].
{{Main|White dwarf}}
⚫ 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 ]. If it has a ], a white dwarf-sized object can ] matter from the companion star. Before it reaches the ] (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 resulting ] ] completely blows the star apart in a ].


===Neutron star===
Even more massive stars, above the ] 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 ], not even light can escape from the star, and hence it becomes a ]. At some point later the collapsing object must reach the ] (as there is nothing that can stop it), where the known laws of gravity cease to be valid{{Fact|date=January 2008}}. There are competing theories as to what occurs at this point, but it can no longer really be considered gravitational collapse at that stage.
{{Main|Neutron star}}
Neutron stars are formed by the gravitational collapse of the cores of larger stars. They are the remnant of supernova types ], ], and ]. 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 called ]<ref>{{Cite journal |last1=Gandolfi |first1=Stefano |last2=Gezerlis |first2=Alexandros |last3=Carlson |first3=J. |date=2015-10-19 |title=Neutron Matter from Low to High Density |url=https://www.annualreviews.org/doi/10.1146/annurev-nucl-102014-021957 |journal=Annual Review of Nuclear and Particle Science |language=en |volume=65 |issue=1 |pages=303–328 |doi=10.1146/annurev-nucl-102014-021957 |issn=0163-8998|arxiv=1501.05675 |bibcode=2015ARNPS..65..303G }}</ref> with a slight dusting of free electrons and protons mixed in. This degenerate neutron matter has a density of about {{val|6.65|e=17|u=kg/m3}}.{{sfn|Carroll|Ostlie|2017|p=578}}


The appearance of stars composed of ] and their internal layered structure is unclear since any proposed ] of ] is highly speculative. Other forms of hypothetical degenerate matter may be possible, and the resulting ]s, ]s (a type of quark star), and ]s, if they exist, would, for the most part, be indistinguishable from a ]: In most cases, the ] would be hidden under a crust of "ordinary" degenerate neutrons. {{Citation needed|date=September 2020}}
== External links ==
*


] ===Black holes===
{{Main|Black hole}}
]
According to Einstein's theory, for even larger stars, above the Landau–Oppenheimer–Volkoff limit, also known as the ] (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 its ] it forms what is called a black hole, meaning a spacetime region from which not even light can escape. It follows from ] and the theorem of ]<ref>{{cite journal | last=Penrose | first=Roger | title=Gravitational Collapse and Space–Time Singularities | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=14 | issue=3 | date=1965-01-18 | issn=0031-9007 | doi=10.1103/physrevlett.14.57 | pages=57–59| bibcode=1965PhRvL..14...57P | doi-access=free }}</ref> that the subsequent formation of some kind of ] is inevitable. Nevertheless, according to Penrose's ], 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<ref>{{cite journal | last=Carter | first=B. | title=Axisymmetric Black Hole Has Only Two Degrees of Freedom | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=26 | issue=6 | date=1971-02-08 | issn=0031-9007 | doi=10.1103/physrevlett.26.331 | pages=331–333| bibcode=1971PhRvL..26..331C }}</ref> to evolve towards a rather simple form describable by the historic ] in the spherical limit and by the more recently discovered ] 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.<ref>{{cite journal |last1=Baumgarte |first1=Thomas W. |last2=Shapiro |first2=Stuart L. |title=Collapse of a Magnetized Star to a Black Hole |journal=The Astrophysical Journal |date=10 March 2003 |volume=585 |issue=2 |pages=930–947 |doi=10.1086/346104 |arxiv=astro-ph/0211339 |bibcode=2003ApJ...585..930B |s2cid=15869680 }}</ref>


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 on ], at a later stage, the collapsing object will reach the maximum possible energy density for a certain volume of space or the ] (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.<ref>{{cite conference |last=Thorne |first=Kip S. |date=1966 |title=The general-relativistic theory of stellar structure and dynamics |url=https://www.its.caltech.edu/~kip/index.html/PubScans/II-12.pdf |conference=Proceedings of the International School of Physics “Enrico Fermi”, Course XXXV |editor=L. Gratton |location=Varenna, Italy |publisher=Academic Press, New York |pages=273 |language=en}}</ref> There are competing theories as to what occurs at this point. For example ] predicts that a ] would form. Regardless, it is argued that gravitational collapse ceases at that stage and a singularity, therefore, does not form.<ref>{{Cite journal |last1=Rovelli |first1=Carlo |last2=Vidotto |first2=Francesca |date=2014 |title=Planck stars |url=https://www.worldscientific.com/doi/abs/10.1142/S0218271814420267 |journal=] |language=en |volume=23 |issue=12 |pages=1442026 |doi=10.1142/S0218271814420267 |issn=0218-2718|arxiv=1401.6562 |bibcode=2014IJMPD..2342026R |s2cid=118917980 }}</ref>
]

]
====Theoretical minimum radius for a star====
]

]
The radii of larger mass neutron stars (about 2.8 solar mass)<ref>{{Cite web | url=https://ejje.weblio.jp/content/Bhatia+Hazarika+limit |title = Bhatia Hazarika limitの意味・使い方・読み方 &#124; Weblio英和辞書}}</ref> are estimated to be about 12&nbsp;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 the ], 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 of ]; 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 and ] has been presented.<ref>{{cite journal |last1=Bedran |first1=M. L. |last2=Calvão |first2=M. O. |last3=de Oliveira |first3=H. P. |last4=Damião |first4=I. |date=1996 |title=Model for nonspherical collapse and formation of black holes by the emission of neutrinos, strings and gravitational waves |journal=Physical Review D |volume=54 |issue=6 |pages=3826–3829 |doi=10.1103/PhysRevD.54.3826|pmid=10021057 |bibcode=1996PhRvD..54.3826B |url=https://cds.cern.ch/record/301262 }}</ref>
]

]
==See also==
]
*]
]
*]
]
*]
]
*]
]

]
==References==
]
{{reflist}}

===Bibliography===
* {{cite book |last1=Carroll |first1=B. W. |last2=Ostlie |first2=D. A. |title=An Introduction to Modern Astrophysics |edition=2nd |publisher=] |date=2017 |isbn=978-1-108-42216-1}}

{{Black holes}}
{{Portal bar|Physics|Astronomy|Stars|Spaceflight|Outer space|Solar System}}
{{Authority control}}

{{DEFAULTSORT:Gravitational Collapse}}
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Latest revision as of 17:31, 1 December 2024

Contraction of an astronomical object due to the influence of its gravity
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Gravitational collapse of a massive star, resulting in a Type II supernova

Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity. Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter, after sufficient accretion, may collapse to form pockets of higher density, such as stars or black holes.

Star formation involves a gradual gravitational collapse of interstellar medium into clumps of molecular clouds and potential protostars. 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. During the star's evolution a star might collapse again and reach several new states of equilibrium.

Star formation

Main article: Star formation

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 critical 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.

Stellar remnants

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

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, these stellar remnants can take one of three forms:

White dwarf

Main article: White dwarf

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 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 resulting runaway carbon detonation completely blows the star apart in a type Ia supernova.

Neutron star

Main article: Neutron star

Neutron stars are formed by the gravitational collapse of the cores of larger stars. They are the remnant of supernova types Ib, Ic, and II. 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 called neutron matter with a slight dusting of free electrons and protons mixed in. This degenerate neutron matter has a density of about 6.65×10 kg/m.

The appearance of stars composed of exotic matter and their internal layered structure is unclear since any proposed equation of state of degenerate matter is highly speculative. Other forms of hypothetical degenerate matter may be possible, and the resulting quark stars, strange stars (a type of quark star), and preon stars, if they exist, would, for the most part, be indistinguishable from a neutron star: In most cases, the exotic matter would be hidden under a crust of "ordinary" degenerate neutrons.

Black holes

Main article: Black hole
Logarithmic plot of mass against mean density (with solar values as origin) showing possible kinds of stellar equilibrium state. For a configuration in the shaded region, beyond the black hole limit line, no equilibrium is possible, so runaway collapse will be inevitable.

According to Einstein's theory, for even larger stars, above the Landau–Oppenheimer–Volkoff limit, also known as the Tolman–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.

Simulated view from outside black hole with thin accretion disc

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. Nevertheless, according to Penrose's cosmic 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 to evolve towards a rather simple form describable by the historic Schwarzschild metric in the spherical limit and by the more recently discovered Kerr 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.

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 on quantum mechanics, at a later stage, 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). This is the point at which it has been hypothesized that the known laws of gravity cease to be valid. There are competing theories as to what occurs at this point. For example loop quantum gravity predicts that a Planck star would form. Regardless, it is argued that gravitational collapse ceases at that stage and a singularity, therefore, does not form.

Theoretical minimum radius for a star

The radii of larger mass neutron stars (about 2.8 solar mass) 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 the event 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 of general 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 and gravitational waves has been presented.

See also

References

  1. Pilchin, Lev Eppelbaum, Izzy Kutasov, Arkady (2013). Applied geothermics (Aufl. 2014 ed.). Berlin, Heidelberg: Springer Berlin Heidelberg. p. 2. ISBN 9783642340239.{{cite book}}: CS1 maint: multiple names: authors list (link)
  2. Kwok, Sun (2006). Physics and chemistry of the interstellar medium. University Science Books. pp. 435–437. ISBN 1-891389-46-7.
  3. Prialnik, Dina (2000). An Introduction to the Theory of Stellar Structure and Evolution. Cambridge University Press. pp. 198–199. ISBN 0-521-65937-X.
  4. And theoretically Black dwarfs – but: "...no black dwarfs are expected to exist in the universe yet"
  5. Gandolfi, Stefano; Gezerlis, Alexandros; Carlson, J. (2015-10-19). "Neutron Matter from Low to High Density". Annual Review of Nuclear and Particle Science. 65 (1): 303–328. arXiv:1501.05675. Bibcode:2015ARNPS..65..303G. doi:10.1146/annurev-nucl-102014-021957. ISSN 0163-8998.
  6. Carroll & Ostlie 2017, p. 578.
  7. Marck, Jean-Alain (1996-03-01). "Short-cut method of solution of geodesic equations for Schwarzchild black hole". Classical and Quantum Gravity. 13 (3): 393–402. arXiv:gr-qc/9505010. Bibcode:1996CQGra..13..393M. doi:10.1088/0264-9381/13/3/007. ISSN 0264-9381. S2CID 119508131.
  8. Penrose, Roger (1965-01-18). "Gravitational Collapse and Space–Time Singularities". Physical Review Letters. 14 (3). American Physical Society (APS): 57–59. Bibcode:1965PhRvL..14...57P. doi:10.1103/physrevlett.14.57. ISSN 0031-9007.
  9. Carter, B. (1971-02-08). "Axisymmetric Black Hole Has Only Two Degrees of Freedom". Physical Review Letters. 26 (6). American Physical Society (APS): 331–333. Bibcode:1971PhRvL..26..331C. doi:10.1103/physrevlett.26.331. ISSN 0031-9007.
  10. Baumgarte, Thomas W.; Shapiro, Stuart L. (10 March 2003). "Collapse of a Magnetized Star to a Black Hole". The Astrophysical Journal. 585 (2): 930–947. arXiv:astro-ph/0211339. Bibcode:2003ApJ...585..930B. doi:10.1086/346104. S2CID 15869680.
  11. Thorne, Kip S. (1966). L. Gratton (ed.). The general-relativistic theory of stellar structure and dynamics (PDF). Proceedings of the International School of Physics “Enrico Fermi”, Course XXXV. Varenna, Italy: Academic Press, New York. p. 273.
  12. Rovelli, Carlo; Vidotto, Francesca (2014). "Planck stars". International Journal of Modern Physics D. 23 (12): 1442026. arXiv:1401.6562. Bibcode:2014IJMPD..2342026R. doi:10.1142/S0218271814420267. ISSN 0218-2718. S2CID 118917980.
  13. "Bhatia Hazarika limitの意味・使い方・読み方 | Weblio英和辞書".
  14. Bedran, M. L.; Calvão, M. O.; de Oliveira, H. P.; Damião, I. (1996). "Model for nonspherical collapse and formation of black holes by the emission of neutrinos, strings and gravitational waves". Physical Review D. 54 (6): 3826–3829. Bibcode:1996PhRvD..54.3826B. doi:10.1103/PhysRevD.54.3826. PMID 10021057.

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