Gravitational collapse isAstrophysicsUpper star orInterstellar matterThe process of collapsing inward under the gravitational force of its own material. The reason for this is that the star itself cannot provide enough pressure to balance its own gravity, so it cannot continue to maintain its originalHydrostatic balanceGravity pulls stellar matter closer to each other and collapses.
Gravitational collapse isAstrophysicsUpper star orInterstellar matterThe process of collapsing inward under the gravitational force of its own material. The reason for this is that the star itself cannot provide enough pressure to balance its own gravity, so it cannot continue to maintain its originalHydrostatic balanceGravity pulls stellar matter closer to each other and collapses.In astronomy, the process of star formation or decay will experience the corresponding gravitational collapse.In particular, gravitational collapse is considered to be the formation mechanism of type Ib and Ic supernovae and type II supernovae. The gravitational collapse of a massive star when it collapses into a black hole may also beGamma ray burstsOne of the formation mechanisms of.Up to now, people have not yet fully understood the theoretical basis of gravitational collapse, and many details have not yet been fully explained in theory.Since the gravitational collapse is likely to be accompanied by the release of gravitational waves, the computer numerical simulation of gravitational collapse to predict the shape of gravitational waves released is currentGravitational waveOne of the topics studied by the astronomical community.[1]
give an example
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For example, large masses with mass greater than 8-10 solar massfixed starAt the late stage of evolution, the central area has insufficient capacity or energy isneutrinoA large number of them are taken away, so that the radiation pressure is not enough to resist the action of the star's own gravity, and gravitational collapse occurs.Generally speaking, the gravitational collapse of a star results in the formation of a compact star, such asWhite dwarf、neutron star、black holeEtc.For stars with a mass less than 1.3 times the mass of the sun,Pauli exclusion principleCaused byElectronic degeneracy pressureIt will support its own weight and form a white dwarf star.Stars with mass between 1.3 and 3.2 times of solar mass, neutronsDegeneracy pressureWill support its own weight and form a neutron star.Stars whose mass is 3.2 times greater than that of the sun have no structure to support their own weight and will collapse into black holes.Some gravitational collapse is accompanied by a large amount of energy release and material ejection.For example,SupernovaDuring the explosion, the central part will collapse to form a compact star, while the outer part will be thrown into space to formSupernova remnantThe whole process releases a lot of energy.
During gravitational collapse, the central part of the star formsCompact starAnd may be accompanied by a large amount of energy release and material ejection.
formation
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fixed starThe core area passes throughoxygenAfter the nuclear reaction stage of combustion, if the mass is greater thanChandraseka limitAnd its equivalent polytropic index when it is composed of iron group nuclidesγClose to the critical value of 4/3 (see the balance and stability of the constant planet).At this time, the central temperature of the star is about 6 × 10nineK, it will undergo gravitational collapse.At this stage, the temperature of the star center is very high, and various neutrino production processes (such asPhotoneutrino process, electron pair annihilationneutrinoProcessneutrinoBremsstrahlungAnd so on) will cause neutrinos to rapidly take away the energy of the central part, so that the core area of the star cools quickly, so that the radiation pressure is not enough to resist the role of self gravity, thus forming gravitational collapse.
Star forming
Stars form in giant interstellar clouds of dust and gas. The particles in these nebulae usually move at a high speed and randomly, and their mutual gravitation is insufficient to compress them together.However, when the external conditions (such as the occurrence of a nearby supernova or other cataclysmic events) allow, these nebulae are compressed by strong enough pressure so that gravity can overcome the motion of these particles and make them close to each other.As a result, the nebula began the process of gravitational collapse, and its speed became faster and faster. Due to the restriction of angular momentum conservation, many small but denser nebulae were finally separated from the original huge nebula. This process is also often called gravitational condensation.These nebulae continue to collapse under their own gravity. At the same time, the collapsed energy is constantly converted into the internal energy of the nebula, generating an outward radiation pressure inside the nebula. This radiation pressure can gradually slow down and finally stop gravitational collapse by balancing the inward gravity.When the radiation pressure and gravity balance each other, the nebula collapses into a sphere with a certain density, which is called a protostar.The protostar is still surrounded by thick interstellar gas and dust.Astronomers have observed the process of partial gravitational condensation, but this process has not been fully understood [1].
A protostar with about 1/10 solar mass can have enough temperature and density to undergo hydrogen nuclear fusion, so that it can evolve into a main sequence star. The main source of star radiation pressure at the main sequence star stage is this hydrogen nuclear fusion.However, protostars with less than this mass can only form brown dwarfs or sub stellar objects, which cannot undergo hydrogen fusion, but some can undergo deuteron fusion;Smaller protostars can only become planets, just like the big planets in the solar system.
The star is dying
We mainly discuss the gravitational collapse process in the process of star decay in detail, which occurs in the final stage of star evolution.Since the radiation pressure supporting the star comes from the heat generated by the fusion of light elements to heavy elements inside the star, when the nuclear fuel of the star is exhausted, the temperature of the star will gradually cool, and the radiation pressure will gradually fail to balance the gravity of the star itself, resulting in collapse, and the radius of the star will gradually decrease.The basis of studying gravitational collapse physically is general relativity, so we consider the following star model.
reaction
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When the star centerdensityWhen it is large enough, the following reactions occur in gravitational collapse: e+(Z, A) → ve+(Z -1﹐A )。E is electron.(Z, A) YesProton numberIs ZNucleon numberFor ANucleus﹔vebyElectron neutrino。This process causes the neutron of matter.Under certain conditions (e.gγ≈4/3), strong shock wave will appear during gravitational collapse, which will causefixed starThe ejection of outer matter.However, under some conditions (such asγ>>4/3), the collapse process is not necessarily accompanied by mass ejection.Stars with different masses may form various types ofCompact star。[2]
Gravitational radiation
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Since the gravitational collapse of a supernova is not highly symmetric, this has already beenSN 1987AIt is confirmed that the supernova explosion is probably an importantGravitational waveSources can be divided into three categories according to different situations.
Formed after the gravitational collapse of a supernova beginsneutron starThe newborn neutron star is in a highly unstable stateconvectionAt the same time, it is also high temperature and non spherically symmetric, in a "boiling" state.This boiling can make the central hot nuclear material (~10twelveKelvin) rose to the surface of the neutron star and wasneutrinoFlow cooling.Theoretically, the asymmetric neutron star rotation in this process will produce quite weak and periodic gravitational radiation.It is speculated that this process may produce gravitational waves with a frequency of about 100 Hz and an intensity of about 10 cycles
R is the distance from the supernova to the earth.Due to the existence of hot neutrino stream, such events can be measured by correlation coincidence between neutrino detector and gravitational wave detector.
In the process of gravitational collapse of supernovae, rotation will gradually flatten the collapsed core, thus starting to generate gravitational radiation.If the kernel'sangular momentumSmall enough tocentrifugal forceIt is not enough to stop the collapse before the core reaches the density of the atomic nucleus, so the collapse, rebound and subsequent oscillation of the core are likely to be axisymmetric.Therefore, a burst signal (burst) of gravitational wave with short duration and no periodicity will be generated during this period, accompanied by electron capture andneutrinoThe process of transportation.However, it is difficult to predict the waveform and amplitude of gravitational radiation in theory, and now there is only a numerical simulation method.This burst signal may have a wide frequency band, with the center frequency at 1 kHz;Or it may be the periodicity of any frequency between 200 Hz and 10 kHzchirpSignal.It is theoretically estimated that if the energy emitted is greater than 0.01 times the solar mass, current ground detectors may observe thatVirgo Galaxy ClusterSuch events within.But in fact, the results of numerical simulation show that the energy of this part of gravitational radiation is very small, and it is generally believed that the radiation energy will not exceed the total mass of the supernova
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Is below the magnitude ofLIGOAnd VIRGO will not be able to detectLocal galaxy groupOther than.
If the angular momentum of the core in the collapse process is large enough that it can stop the collapse before the core reaches the density of the atomic nucleus, the dynamic instability generated in this process may destroy the axial symmetry of the core.The core may form a rotating rod structure, and may break into more massive pieces.The strength of gravitational waves formed in this process may be similar to that of double neutron starsPronationThe strength of gravitational waves is comparable to that of.The gravitational wave signal of this strength can be detected by LIGO and VIRGO to the Virgo Galaxy Cluster (the probability of supernova explosion is several times a year), and it is possible to extend to the range of tens of thousands of supernova explosions per year in the next generation of detectors.
