Gravitational collapse

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.[1] Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter will collapse to form pockets of higher density, typically creating a hierarchy of condensed structures such as clusters of galaxies, stellar groups, stars and planets.

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.

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

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.[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 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.[3]

Stellar remnants

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

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 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 our Sun, at which point gravitational collapse would take start again), the increasing density and temperature within a carbon-oxygen white dwarf initiates a new round of nuclear fusion, which is not regulated because the star's weight is supported by degeneracy rather than thermal pressure, allowing 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 gravitational collapse of the cores of larger stars, and are the remnant of other types of supernova. They are so compact that a Newtonian description is inadequate for an accurate treatment, which requires the use of Einstein's general relativity.

Black holes

MassDensi
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 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.

CNRSblackhole
Simulated view from outside black hole with thin accretion disc, by J. A. Marck[5]

Once a body collapses to within its Schwarzschild radius it forms what is called a black hole, meaning a space-time region from which not even light can escape. It follows from a theorem of Roger Penrose[6] 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 space-time region outside will still have a well behaved geometry, with strong but finite curvature, that is expected[7] 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.

On the other hand, the nature of the kind of singularity to be expected inside a black hole remains rather controversial. According to some theories, 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 when the known laws of gravity cease to be valid.[8] There are competing theories as to what occurs at this point, but it can no longer really be considered gravitational collapse at that stage.[9]

Theoretical minimum radius for a star

The radii of larger mass neutron stars (about 2.0 solar mass) are estimated to be about 12-km, or approximately 2.0 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, 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 nonspherical collapse in general relativity with emission of matter and gravitational waves has been presented.[10]

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.
  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. ^ Class. Quantum Grav. 13 (1996)p393
  6. ^ "Gravitational collapse and space-time singularities", R. Penrose, Phys. Rev. Let. 14 (1965) p 57
  7. ^ B. Carter, "Axisymmetric black hole has only two degrees of freedom", Phys. Rev. Let. 26 (1971) p331
  8. ^ "Black Holes - Planck Unit? WIP". Physics Forums. Archived from the original on 2008-08-02.
  9. ^ Brill, Dieter (19 January 2012). "Black Hole Horizons and How They Begin". Astronomical Review.
  10. ^ Bedran, ML et al.(1996)."Model for nonspherical collapse and formation of black holes by emission of neutrinos, strings and gravitational waves", Phys. Rev. D 54(6),3826.

External links

Black hole

A black hole is a region of spacetime exhibiting gravitational acceleration so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe.

Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Black holes were long considered a mathematical curiosity; it was during the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses (M☉) may form. There is general consensus that supermassive black holes exist in the centers of most galaxies.

The presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. Matter that falls onto a black hole can form an external accretion disk heated by friction, forming some of the brightest objects in the universe. If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location. Such observations can be used to exclude possible alternatives such as neutron stars. In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.

On 11 February 2016, the LIGO collaboration announced the first direct detection of gravitational waves, which also represented the first observation of a black hole merger. As of December 2018, eleven gravitational wave events have been observed that originated from ten merging black holes (along with one binary neutron star merger). On 10 April 2019, the first ever direct image of a black hole and its vicinity was published, following observations made by the Event Horizon Telescope in 2017 of the supermassive black hole in Messier 87's galactic centre.

Black star (semiclassical gravity)

A black star is a gravitational object composed of matter. It is a theoretical alternative to the black hole concept from general relativity. The theoretical construct was created through the use of semiclassical gravity theory. A similar structure should also exist for the Einstein–Maxwell–Dirac equations system, which is the (super)classical limit of quantum electrodynamics, and for the Einstein–Yang–Mills–Dirac system, which is the (super)classical limit of the standard model.

A black star doesn't need to have an event horizon, and may or may not be a transitional phase between a collapsing star and a singularity. A black star is created when matter compresses at a rate significantly less than the freefall velocity of a hypothetical particle falling to the center of its star, because quantum processes create vacuum polarization, which creates a form of degeneracy pressure, preventing spacetime (and the particles held within it) from occupying the same space at the same time. This vacuum energy is theoretically unlimited, and if built up quickly enough, will stop gravitational collapse from creating a singularity. This may entail an ever-decreasing rate of collapse, leading to an infinite collapse time, or asymptotically approaching a radius bigger than zero.

A black star with a radius slightly greater than the predicted event horizon for an equivalent-mass black hole will appear very dark, because almost all light produced will be drawn back to the star, and any escaping light will be severely gravitationally redshifted. It will appear almost exactly like a black hole. It will feature Hawking radiation, as virtual particle pairs created in its vicinity may still be split, with one particle escaping and the other being trapped. Additionally, it will create thermal Planckian radiation that will closely resemble the expected Hawking radiation of an equivalent black hole.

The predicted interior of a black star will be composed of this strange state of spacetime, with each length in depth heading inward appearing the same as a black star of equivalent mass and radius with the overlayment stripped off. Temperatures increase with depth towards the center.

Bonnor–Ebert mass

In astrophysics, the Bonnor–Ebert mass is the largest mass that an isothermal gas sphere embedded in a pressurized medium can have while still remaining in hydrostatic equilibrium. Clouds of gas with masses greater than the Bonnor–Ebert mass must inevitably undergo gravitational collapse to form much smaller and denser objects. As the gravitational collapse of an interstellar gas cloud is the first stage in the formation of a protostar, the Bonnor–Ebert mass is an important quantity in the study of star formation.

For a gas cloud embedded in a medium with a gas pressure , the Bonnor–Ebert mass is given by

where G is the gravitational constant and is the isothermal sound speed () with as the molecular mass. is a dimensionless constant which varies based on the density distribution of the cloud. For a uniform mass density and for a centrally peaked density .

Chandrasekhar limit

The Chandrasekhar limit () is the maximum mass of a stable white dwarf star. The currently accepted value of the Chandrasekhar limit is about 1.4 M☉ (2.765×1030 kg).White dwarfs resist gravitational collapse primarily through electron degeneracy pressure (compare main sequence stars, which resist collapse through thermal pressure). The Chandrasekhar limit is the mass above which electron degeneracy pressure in the star's core is insufficient to balance the star's own gravitational self-attraction. Consequently, a white dwarf with a mass greater than the limit is subject to further gravitational collapse, evolving into a different type of stellar remnant, such as a neutron star or black hole. Those with masses under the limit remain stable as white dwarfs.The limit was named after Subrahmanyan Chandrasekhar, an Indian astrophysicist who improved upon the accuracy of the calculation in 1930, at the age of 20, in India by calculating the limit for a polytrope model of a star in hydrostatic equilibrium, and comparing his limit to the earlier limit found by E. C. Stoner for a uniform density star. Importantly, the existence of a limit, based on the conceptual breakthrough of combining relativity with Fermi degeneracy, was indeed first established in separate papers published by Wilhelm Anderson and E. C. Stoner in 1929. The limit was initially ignored by the community of scientists because such a limit would logically require the existence of black holes, which were considered a scientific impossibility at the time. That the roles of Stoner and Anderson are often forgotten in the astronomy community has been noted.

Demetrios Christodoulou

Demetrios Christodoulou (Greek: Δημήτριος Χριστοδούλου; born October 19, 1951) is a Greek mathematician and physicist, who first became well known for his proof, together with Sergiu Klainerman, of the nonlinear stability of the Minkowski spacetime

of special relativity in the framework of general relativity.

Electron degeneracy pressure

Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli exclusion principle disallows two identical half-integer spin particles (electrons and all other fermions) from simultaneously occupying the same quantum state. The result is an emergent pressure against compression of matter into smaller volumes of space. Electron degeneracy pressure results from the same underlying mechanism that defines the electron orbital structure of elemental matter. For bulk matter with no net electric charge, the attraction between electrons and nuclei exceeds (at any scale) the mutual repulsion of electrons plus the mutual repulsion of nuclei; so absent electron degeneracy pressure, the matter would collapse into a single nucleus. In 1967, Freeman Dyson showed that solid matter is stabilized by quantum degeneracy pressure rather than electrostatic repulsion. Because of this, electron degeneracy creates a barrier to the gravitational collapse of dying stars and is responsible for the formation of white dwarfs.

Electroweak star

An electroweak star is a theoretical type of exotic star, whereby the gravitational collapse of the star is prevented by radiation pressure resulting from electroweak burning, that is, the energy released by conversion of quarks to leptons through the electroweak force. This process occurs in a volume at the star's core approximately the size of an apple, containing about two Earth masses.The stage of life of a star that produces an electroweak star is theorized to occur after a supernova collapse. Electroweak stars are denser than quark stars, and may form when quark degeneracy pressure is no longer able to withstand gravitational attraction, but may still be withstood by electroweak burning radiation pressure. This phase of a star's life may last upwards of 10 million years.

Exotic star

An exotic star is a hypothetical compact star composed of something other than electrons, protons, neutrons, or muons, and balanced against gravitational collapse by degeneracy pressure or other quantum properties. Exotic stars include quark stars (composed of quarks) and perhaps strange stars (composed of strange quark matter, a condensate of up, down and strange quarks), as well as speculative preon stars (composed of preons, which are hypothetical particles and "building blocks" of quarks, should quarks be decomposable into component sub-particles). Of the various types of exotic star proposed, the most well evidenced and understood is the quark star.

Exotic stars are largely theoretical – partly because it is difficult to test in detail how such forms of matter may behave, and partly because prior to the fledgling technology of gravitational-wave astronomy, there was no satisfactory means of detecting cosmic objects that do not radiate electromagnetically or through known particles. So it is not yet possible to verify novel cosmic objects of this nature by distinguishing them from known objects. Candidates for such objects are occasionally identified based on indirect evidence gained from observable properties.

Gravitational compression

Gravitational compression is a phenomenon in which gravity, acting on the mass of an object, compresses it, reducing its size and increases the object's density.

At the center of a planet or star, gravitational compression produces heat by the Kelvin–Helmholtz mechanism. This is the mechanism that explains how Jupiter continues to radiate heat produced by its gravitational compression.The most common reference to gravitational compression is stellar evolution. The Sun and other main-sequence stars are produced by the initial gravitational collapse of a molecular cloud. Assuming the mass of the material is large enough, gravitational compression reduces the size of the core, increasing its temperature until hydrogen fusion can begin. This hydrogen-to-helium fusion reaction releases energy that balances the inward gravitational pressure and the star becomes stable for millions of years. No further gravitational compression occurs until the hydrogen is nearly used up, reducing the thermal pressure of the fusion reaction. At the end of the Sun's life, gravitational compression will turn it into a white dwarf.At the other end of the scale are massive stars. These stars burn their fuel very quickly, ending their lives as supernovae, after which further gravitational compression will produce either a neutron star or a black hole from the remnants.

For planets and moons, equilibrium is reached when the compression is balanced by a pressure gradient. This is due to gravity. This pressure gradient is in the opposite direction due to the strength of the material, at which point gravitational compression ceases.

Gravitational singularity

A gravitational singularity, spacetime singularity or simply singularity is a location in spacetime where the gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system. The quantities used to measure gravitational field strength are the scalar invariant curvatures of spacetime, which includes a measure of the density of matter. Since such quantities become infinite within the singularity, the laws of normal spacetime cannot exist.Gravitational singularities are mainly considered in the context of general relativity, where density apparently becomes infinite at the center of a black hole, and within astrophysics and cosmology as the earliest state of the universe during the Big Bang. Physicists are undecided whether the prediction of singularities means that they actually exist (or existed at the start of the Big Bang), or that current knowledge is insufficient to describe what happens at such extreme densities.

General relativity predicts that any object collapsing beyond a certain point (for stars this is the Schwarzschild radius) would form a black hole, inside which a singularity (covered by an event horizon) would be formed. The Penrose–Hawking singularity theorems define a singularity to have geodesics that cannot be extended in a smooth manner. The termination of such a geodesic is considered to be the singularity.

The initial state of the universe, at the beginning of the Big Bang, is also predicted by modern theories to have been a singularity. In this case the universe did not collapse into a black hole, because currently-known calculations and density limits for gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not necessarily apply in the same way to rapidly expanding space such as the Big Bang. Neither general relativity nor quantum mechanics can currently describe the earliest moments of the Big Bang, but in general, quantum mechanics does not permit particles to inhabit a space smaller than their wavelengths.

Hartland Snyder

Hartland Sweet Snyder (1913, Salt Lake City – 1962) was an American physicist who along with Robert Oppenheimer calculated the gravitational collapse of a pressure-free homogenous fluid sphere, and found that it could not communicate with the rest of the universe.

In 1955, he bet against Maurice Goldhaber that antiprotons existed, and won.

Some publications he authored together with Ernest Courant laid the foundations for the field of accelerator physics. In particular, Hartland with Courant and Milton Stanley Livingston developed the principle of strong focusing that made modern particle accelerators possible.

Jeans instability

In stellar physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation, named after James Jeans. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of a region filled with matter. For stability, the cloud must be in hydrostatic equilibrium, which in case of a spherical cloud translates to:

,

where is the enclosed mass, is the pressure, is the density of the gas (at radius ), is the gravitational constant, and is the radius. The equilibrium is stable if small perturbations are damped and unstable if they are amplified. In general, the cloud is unstable if it is either very massive at a given temperature or very cool at a given mass; under these circumstances, the gas pressure cannot overcome gravity, and the cloud will collapse.

List of contributors to general relativity

This is a partial list of persons who have made major contributions to the development of standard mainstream general relativity. One simple rule of thumb for who belongs here is whether their contribution is recognized in the canon of standard general relativity textbooks. Some related lists are mentioned at the bottom of the page.

Magnetospheric eternally collapsing object

The Magnetospheric eternally collapsing object (MECO) is an alternative model for black holes proposed initially by Indian scientist Abhas Mitra in 1998, later on proposed by Darryl Leiter and Stanley Robertson a generalization of the eternally collapsing object (ECO) proposed by Abhas Mitra in 1998. A proposed observable difference between MECOs and black holes is that a MECO can produce its own intrinsic magnetic field. An uncharged black hole cannot produce its own magnetic field, though its accretion disc can.

NGC 1266

NGC 1266 is a lenticular galaxy in the constellation Eridanus. Although not currently starbursting, it has undergone a period of intense star formation in the recent past, ceasing only ≈500 Myr ago. The galaxy is host to an obscured AGN.

A massive molecular outflow, with 2.4 × 107 M ⊙ {\displaystyle _{\odot }} of hydrogen, is present from the nucleus of the galaxy, at a rate of 110 M yr−1. Less than 2% of the gas (2 M yr−1) is escaping the galaxy. Momentum coupling to the jet of the AGN is likely driving the outflow.

The current observed star-formation rate (SFR) of ~0.87 M yr−1 is significantly lower than expected for a galaxy of its properties, suppressed by a factor of 50 to 150. Authors have put forth several hypotheses to explain these observations. The most likely scenario is that the AGN-driven molecular outflow is injecting turbulence into the nuclear regions, preventing gravitational collapse of molecular clouds. NGC 1266 is the first known intermediate-mass galaxy to show AGN-driven suppression of star formation.

Two hypotheses exist to explain NGC 1266's nuclear activity and excessive far-IR emission. Either a heavily obscured ultracompact starburst is present in the nuclear regions, or a powerful buried AGN is present, beyond what has been inferred from other observations. Neither scenario is without problems. The black hole at the center of the galaxy is likely growing according to the M–sigma relation, and eventually the outflow will result in the removal of the majority of the gas from the nucleus.

Planetesimal

Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks.

A widely accepted theory of planet formation, the so-called planetesimal hypotheses, the Chamberlin–Moulton planetesimal hypothesis and that of Viktor Safronov, states that planets form out of cosmic dust grains that collide and stick to form larger and larger bodies. When the bodies reach sizes of approximately one kilometer, then they can attract each other directly through their mutual gravity, enormously aiding further growth into moon-sized protoplanets. This is how planetesimals are often defined. Bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. Alternatively, planetesimals may form in a very dense layer of dust grains that undergoes a collective gravitational instability in the mid-plane of a protoplanetary disk or via the concentration and gravitational collapse of swarms of larger particles in streaming instabilities. Many planetesimals eventually break apart during violent collisions, as may have happened to 4 Vesta and 90 Antiope, but a few of the largest planetesimals may survive such encounters and continue to grow into protoplanets and later planets.

It is generally thought that about 3.8 billion years ago, after a period known as the Late Heavy Bombardment, most of the planetesimals within the Solar System had either been ejected from the Solar System entirely, into distant eccentric orbits such as the Oort cloud, or had collided with larger objects due to the regular gravitational nudges from the giant planets (particularly Jupiter and Neptune). A few planetesimals may have been captured as moons, such as Phobos and Deimos (the moons of Mars) and many of the small high-inclination moons of the giant planets.

Planetesimals that have survived to the current day are valuable to science because they contain information about the formation of the Solar System. Although their exteriors are subjected to intense solar radiation that can alter their chemistry, their interiors contain pristine material essentially untouched since the planetesimal was formed. This makes each planetesimal a 'time capsule', and their composition might reveal the conditions in the Solar Nebula from which our planetary system was formed. The most primitive planetesimals visited by spacecraft are the contact binary 2014 MU69.

Primordial black hole

Primordial black holes are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and inhomogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming black holes. Yakov Borisovich Zel'dovich and Igor Dmitriyevich Novikov in 1966 first proposed the existence of such black holes. The theory behind their origins was first studied in depth by Stephen Hawking in 1971.

Since primordial black holes did not form from stellar gravitational collapse, their masses can be far below stellar mass (c. 2×1030 kg). Hawking calculated that primordial black holes could weigh as little as 10−8 kg, about the weight of a human ovum.

Quark star

A quark star is a type of compact exotic star, where extremely high temperature and pressure has forced nuclear particles to form quark matter, a continuous state of matter consisting of free quarks.

It is well known, both theoretically and observationally, that some massive stars collapse to form neutron stars at the end of their life cycle. Under the extreme temperatures and pressures inside neutron stars, the neutrons are normally kept apart by a degeneracy pressure, stabilizing the star and hindering further gravitational collapse. However, it is analyzed that under even more extreme temperature and pressure, the degeneracy pressure of the neutrons is overcome, and the neutrons are forced to merge and dissolve into their constituent quarks, creating an ultra-dense phase of quark matter based on densely packed quarks. In this state, a new equilibrium is supposed to emerge, as a new degeneracy pressure between the quarks, as well as repulsive electromagnetic forces, will occur and hinder gravitational collapse. If these ideas are correct, quark stars might occur, and be observable, somewhere in the universe. Theoretically, such a scenario is seen as scientifically plausible, but it has been impossible to prove both observationally and experimentally, because the very extreme conditions needed for stabilizing quark matter can not be created in any laboratory nor observed directly in nature. The stability of quark matter, and hence the existence of quark stars, is for these reasons among the unsolved problems in physics.

If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy - the point at which neutrons break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole.

If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or not have typical sizes, electromagnetic fields, or surface temperatures, compared to neutron stars.

The analysis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze. Their existence has not been confirmed. The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 1012 K) quark-gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 1012 K can not be recreated artificially, as there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.

White hole

In general relativity, a white hole is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it. In this sense, it is the reverse of a black hole which can only be entered from the outside and from which matter and light cannot escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein field equations has a white hole region in its past. However, this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed. Although information and evidence regarding white holes remains inconclusive, the 2006 GRB 060614 has been proposed as the first documented occurrence of a white hole.

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