Gravitational collapse

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Gravitational collapse of a massive star, resulting in a Type II supernova

Gravitational collapse is the contraction of an

center of gravity.[1] 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
.

As a clandestine form of gravitational collapse, the gradual gravitational collapse of

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

An interstellar cloud of gas will remain in

Jeans mass. This mass depends on the temperature and density of the cloud but is typically thousands to tens of thousands of solar masses.[3]

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

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

Neutron stars are formed by the gravitational collapse of the cores of larger stars. They are the remnant of supernova types

neutron matter[5] with a slight dusting of free electrons and protons mixed in. This degenerate neutron matter has a density of about 6.65×1017 kg/m3.[6]

The appearance of stars composed of

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.[citation needed
]

Black holes

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[7]

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[8] 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[9] 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.[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 on

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.[11] 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.[12]

Theoretical minimum radius for a star

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 the

gravitational waves has been presented.[14]

See also

References

  1. ISBN 9783642340239.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  2. .
  3. .
  4. ^ And theoretically Black dwarfs – but: "...no black dwarfs are expected to exist in the universe yet"
  5. ISSN 0163-8998
    .
  6. ^ Carroll & Ostlie 2017, p. 578.
  7. S2CID 119508131
    .
  8. .
  9. .
  10. .
  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. S2CID 118917980
    .
  13. ^ "Bhatia Hazarika limitの意味・使い方・読み方 | Weblio英和辞書".
  14. PMID 10021057
    .

Bibliography

External links