Theoretically, any cosmic body can turn into a black hole. For example, a planet like Earth would need to shrink to a radius of a few millimeters, which is, of course, unlikely in practice. In the new issue with the “Enlightener” award, T&P publishes an excerpt from the book by physicist Emil Akhmedov “On the Birth and Death of Black Holes,” which explains how celestial bodies turn into black holes and whether they can be seen in the starry sky.
How are black holes formed?
*If some force compresses a celestial body to the Schwarzschild radius corresponding to its mass, then it will bend space-time so much that even light will not be able to leave it. This means that the body will become a black hole.
For example, for a star with the mass of the Sun, the Schwarzschild radius is approximately three kilometers. Compare this value with the actual size of the Sun - 700,000 kilometers. At the same time, for a planet with the mass of the Earth, the Schwarzschild radius is equal to several millimeters.
[…]Only gravitational force is capable of compressing a celestial body to such small sizes as its Schwarzschild radius*, since only gravitational interaction leads exclusively to attraction, and actually increases unlimitedly with increasing mass. Electromagnetic interaction between elementary particles is many orders of magnitude stronger than gravitational interaction. However, any electric charge, as a rule, turns out to be compensated by a charge of the opposite sign. Nothing can shield the gravitational charge - the mass.
A planet like the Earth does not shrink under its own weight to the appropriate Schwarzschild dimensions because its mass is not enough to overcome the electromagnetic repulsion of the nuclei, atoms and molecules of which it consists. And a star like the Sun, being a much more massive object, does not contract due to strong gas-dynamic pressure due to the high temperature in its depths.
Note that for very massive stars, with a mass greater than one hundred Suns, compression does not occur mainly due to strong light pressure. For stars more massive than two hundred Suns, neither gas-dynamic nor light pressure is sufficient to prevent the catastrophic compression (collapse) of such a star into a black hole. However, below we will discuss the evolution of lighter stars.
The light and heat of stars are products of thermonuclear reactions. This reaction occurs because there is enough hydrogen in the interior of stars and the matter is highly compressed under the pressure of the entire mass of the star. Strong compression makes it possible to overcome the electromagnetic repulsion of identical charges of hydrogen nuclei, because a thermonuclear reaction is the fusion of hydrogen nuclei into a helium nucleus, accompanied by a large release of energy.
Sooner or later, the amount of thermonuclear fuel (hydrogen) will be greatly reduced, light pressure will weaken, and the temperature will drop. If the mass of the star is small enough, like the Sun, then it will go through the red giant phase and become a white dwarf.
If its mass is large, then the star will begin to shrink under its own weight. There will be a collapse, which we can see as a supernova explosion. This is a very complex process, consisting of many phases, and not all of its details are yet clear to scientists, but much is already clear. It is known, for example, that the further fate of a star depends on its mass at the moment before collapse. The result of such compression can be either a neutron star or a black hole, or a combination of several such objects and white dwarfs.
"Black holes are the result of the collapse of the heaviest stars"
Neutron stars and white dwarfs do not collapse into black holes because they do not have enough mass to overcome the pressure of the neutron or electron gas, respectively. These pressures are due to quantum effects that come into force after very strong compression. Discussion of the latter is not directly related to the physics of black holes and is beyond the scope of this book.
However, if, for example, a neutron star is located in a binary star system, then it can attract matter from a companion star. In this case, its mass will grow and, if it exceeds a certain critical value, collapse will occur again, this time with the formation of a black hole. The critical mass is determined from the condition that the neutron gas creates insufficient pressure to keep it from further compression.
*This is an estimate. The exact value of the limit is not yet known. - Approx. author.
So, black holes are the result of the collapse of the heaviest stars. In modern understanding, the mass of the star’s core after burning out thermonuclear fuel should be at least two and a half solar*. No state of matter known to us is capable of creating such a pressure that would keep such a large mass from being compressed into a black hole if all the thermonuclear fuel was burned out. We will discuss the facts that experimentally confirm the mentioned limitation on the mass of a star for the formation of a black hole a little later, when we talk about how astronomers discover black holes. […]
Rice. 7. Misconception of collapse from an outside observer's point of view as a slowing eternal fall instead of the formation of a black hole horizon
In connection with our discussion, it will be instructive to use an example to recall the interconnection of various ideas and concepts in science. This story may give the reader a sense of the potential depth of the issue being discussed.
It is known that Galileo came up with what is now called Newton's law of inertial reference frames in response to criticism of the Copernican system. The criticism was that the Earth cannot revolve around the Sun because otherwise we would not be able to stay on its surface.
In response, Galileo argued that the Earth revolves around the Sun by inertia. But we cannot distinguish inertial motion from rest, just as we do not feel the inertial motion of, for example, a ship. At the same time, he did not believe in gravitational forces between planets and stars, since he did not believe in action at a distance, and he could not even know about the existence of fields. And I would not have accepted such an abstract explanation at that time.
Galileo believed that inertial motion can only occur along an ideal curve, that is, the Earth can only move in a circle or in a circle, the center of which, in turn, rotates in a circle around the Sun. That is, there may be an overlap of different inertial motions. This last type of movement can be made more complex by adding even more circles to the composition. Such rotation is called movement along epicycles. It was invented to harmonize the Ptolemaic system with the observed positions of the planets.
By the way, at the time of its creation, the Copernican system described the observed phenomena much worse than the Ptolemaic system. Since Copernicus also believed only in motion in perfect circles, it turned out that the centers of the orbits of some planets were located outside the Sun. (The latter was one of the reasons for Copernicus’ delay in publishing his works. After all, he believed in his system based on aesthetic considerations, and the presence of strange displacements of orbital centers beyond the Sun did not fit into these considerations.)
It is instructive that, in principle, Ptolemy’s system could describe the observed data with any predetermined accuracy - it was only necessary to add the required number of epicycles. However, despite all the logical contradictions in the initial ideas of its creators, only the Copernican system could lead to a conceptual revolution in our views on nature - to the law of universal gravitation, which describes both the movement of the planets and the fall of an apple on Newton’s head, and later to the concept of field.
Therefore, Galileo denied Keplerian motion of planets along ellipses. He and Kepler exchanged letters, which were written in a rather irritable tone*. This is despite their full support of the same planetary system.
So, Galileo believed that the Earth moves around the Sun by inertia. From the point of view of Newtonian mechanics, this is a clear error, since the gravitational force acts on the Earth. However, from the point of view of the general theory of relativity, Galileo must be right: by virtue of this theory, bodies in a gravitational field move by inertia, at least when their own gravity can be neglected. This movement occurs along the so-called geodesic curve. In flat space, this is simply a straight world line, but in the case of a planet in the solar system, it is a geodesic world line that corresponds to an elliptical trajectory, and not necessarily a circular one. Unfortunately, Galileo could not know this.
However, from the general theory of relativity it is known that movement occurs along a geodesic only if one can neglect the curvature of space by the moving body itself (the planet) and assume that it is curved exclusively by the gravitating center (the Sun). A natural question arises: was Galileo right about the inertial motion of the Earth around the Sun? And although this is not such an important question, since we now know the reason why people do not fly off the Earth, it may have something to do with the geometric description of gravity.
How can you “see” a black hole?
[…] Let us now move on to a discussion of how black holes are observed in the starry sky. If a black hole has consumed all the matter that surrounded it, then it can only be seen through the distortion of light rays from distant stars. That is, if there were a black hole in such a pure form not far from us, then we would see approximately what is shown on the cover. But even having encountered such a phenomenon, one cannot be sure that this is a black hole, and not just a massive, non-luminous body. It takes some work to differentiate one from the other.
However, in reality, black holes are surrounded by clouds containing elementary particles, dust, gases, meteorites, planets and even stars. Therefore, astronomers observe something like the picture shown in Fig. 9. But how do they conclude that it is a black hole and not some kind of star?
Rice. 9. The reality is much more prosaic, and we have to observe black holes surrounded by various celestial bodies, gases and dust clouds
To begin, select a certain size area in the starry sky, usually in a binary star system or in an active galactic nucleus. The spectra of radiation emanating from it determine the mass and behavior of the substance in it. Next, it is recorded that radiation emanates from the object in question, as from particles falling in a gravitational field, and not just from thermonuclear reactions occurring in the bowels of stars. The radiation, which is, in particular, the result of mutual friction of matter falling on a celestial body, contains much more energetic gamma radiation than the result of a thermonuclear reaction.
“Black holes are surrounded by clouds containing elementary particles, dust, gases, meteorites, planets and even stars.”
If the observed region is small enough, is not a pulsar, and has a large mass concentrated in it, then it is concluded that it is a black hole. First, it is theoretically predicted that after the fusion fuel burns out, there is no state of matter that could create a pressure that could prevent the collapse of so much mass in so small a region.
Secondly, as just emphasized, the objects in question should not be pulsars. A pulsar is a neutron star that, unlike a black hole, has a surface and behaves like a large magnet, which is one of those subtler characteristics of the electromagnetic field than charge. Neutron stars, being the result of very strong compression of the original rotating stars, rotate even faster, because angular momentum must be conserved. This causes such stars to create magnetic fields that vary over time. The latter play a major role in the formation of characteristic pulsating radiation.
All pulsars found so far have a mass less than two and a half solar masses. Sources of characteristic energetic gamma radiation whose mass exceeds this limit are not pulsars. As can be seen, this mass limit coincides with theoretical predictions made based on the states of matter known to us.
All this, although not a direct observation, is a fairly convincing argument in favor of the fact that it is black holes that astronomers see and not anything else. Although what can be considered direct observation and what not is a big question. After all, you, the reader, do not see the book itself, but only the light scattered by it. And only the combination of tactile and visual sensations convinces you of the reality of its existence. In the same way, scientists draw a conclusion about the reality of the existence of this or that object based on the totality of the data they observe.
GRAVITATIONAL COLLAPSE, hydrodynamic compression of a space object under the influence of its own gravitational forces, leading to a significant reduction in its size. For the development of gravitational collapse, it is necessary that the pressure (repulsion) forces are absent altogether or, at least, are insufficient to counteract the gravitational forces. Gravitational collapse occurs at two extreme stages of stellar evolution. Firstly, the birth of a star begins with the gravitational collapse of a gas and dust cloud. Secondly, some stars end their evolution through gravitational collapse, their central part (core) passing into the final state of a neutron star or black hole. At the same time, the rarefied shell can be ejected by a strong shock wave, which leads to a supernova explosion. Gravitational collapse also occurs on larger scales - at certain stages of the evolution of galactic nuclei. Astronomical observations using orbiting space telescopes in the optical, IR and X-ray ranges convincingly indicate the presence of massive black holes weighing from several million to several billion solar masses at the centers of some galaxies. In the center of our Galaxy there is a “point” invisible object - a black hole with a mass of 3 million solar masses, determined from the orbits of neighboring stars revolving around it. Such black holes initially arise due to gravitational collapse and then gradually increase their mass, absorbing surrounding matter.
Gravitational collapse is associated with the loss of stability of an object in relation to compression under the influence of gravitational forces. After losing stability over time, the object deviates more and more from the initial state of hydrostatic equilibrium, and the forces of gravity begin to prevail over the forces of pressure, which causes further acceleration of compression. The gravitational collapse during the birth of stars and the formation of neutron stars and black holes are based on completely different physical processes. However, the hydrodynamic picture of the development of gravitational collapse is basically the same in both cases.
The birth of stars is associated with gravitational instability of the interstellar medium. During the formation of neutron stars and black holes, the impetus for the onset of gravitational collapse is the loss of star stability due to the dissociation of atomic nuclei into their constituent nucleons and/or neutronization of the star’s matter (massive capture of electrons by atomic nuclei), accompanied by intense energy losses through the emission of electron neutrinos.
The gravitational collapse that has begun is developing at an increasingly accelerated pace, mainly for two reasons. Firstly, energy expenditure on the splitting of particles of matter (dissociation of molecules and ionization of atoms during the compression of protostellar clouds, dissociation of atomic nuclei during the formation of neutron stars) leads to a decrease in the rate of increase in pressure that prevents the compression of matter. Secondly, intense energy losses due to radiation during gravitational collapse further slow down the pressure increase.
A detailed description of gravitational collapse can only be obtained using high-speed computers, taking into account specific mechanisms of energy loss (IR radiation or neutrinos) and other physical properties of the collapsing substance. The greater the density of matter inside the collapsing volume, the faster the gravitational collapse develops. Therefore, the region near the center of the star (central core) collapses first. After the gravitational collapse of the core stops, the substance of the shell collides with it at supersonic speed, forming a strong shock wave (SW). In the central region of the object, excess pressure arises, under the influence of which the shock wave moves in the outer direction. The shock not only stops the fall of the shell, but can also give the outer layers a speed directed away from the center. This effect, discovered in detailed calculations of gravitational collapse, is called hydrodynamic reflection (rebound). Its existence is important for diagnosing gravitational collapse in observations, in particular for the theory of supernova explosions.
After the fall of the main mass of the shell onto the core and the attenuation caused by the hydrodynamic reflection of the core pulsations, the gravitational collapse actually ends. However, a significant portion of the energy released during the gravitational collapse does not have time to be emitted and ends up stored in the form of heat in the resulting dense hydrostatically equilibrium object (in a protostar or in a hot neutron star). As energy is emitted, the protostar continues to slowly contract. In accordance with the virial theorem, the temperature at the center of the protostar increases and, ultimately, reaches a value sufficient for thermonuclear reactions to occur - the protostar turns into an ordinary star.
At the final stages of the evolution of massive stars, conditions can be created that are favorable for the formation of stellar cores that are unstable to gravitational collapse with a mass exceeding the limiting mass of a neutron star (2-3 solar masses). Under such circumstances, the gravitational collapse can no longer stop at the intermediate state of an equilibrium neutron star and continues indefinitely with the formation of a black hole. The main role here is played by the effects of the general theory of relativity, therefore such a gravitational collapse is called relativistic.
Gravitational collapse can be significantly affected by the rotation of the collapsing object and its magnetic field. While maintaining angular momentum and magnetic flux, the rotation speed and magnetic field increase during the compression process, which can change the picture of gravitational collapse not only quantitatively, but also qualitatively. For example, in the absence of spherical symmetry, energy loss through the emission of gravitational waves becomes possible. A sufficiently strong initial rotation can lead to stopping the gravitational collapse at an intermediate stage, when further compression will be possible only in the presence of any mechanisms for loss of angular momentum or when the object is fragmented into smaller clumps. The quantitative theory of gravitational collapse taking into account rotation and/or magnetic field is just beginning to develop and is based on the achievements of modern computational mathematics. The results obtained for gravitational collapse without taking into account rotation and magnetic field, nevertheless have important applied significance and in a number of cases are, apparently, a good approximation to reality.
The study of gravitational collapse has gained particular interest in connection with the achievements of infrared astronomy, which makes it possible to observe the birth of stars, as well as the construction of underground neutrino observatories capable of detecting a burst of neutrino radiation in the event of the formation of neutron stars and black holes in our Galaxy.
Lit.: Zeldovich Ya. B., Novikov I. D. Theory of gravity and the evolution of stars. M., 1971; Shklovsky I. S. Stars: their birth, life and death. 3rd ed. M., 1984; Physics of space: A small encyclopedia. 2nd ed. M., 1986: Physical encyclopedia. M., 1988. T. 1.
The main component of an eclipsing binary has an absolute visual magnitude; bolometric correction corresponding to its spectrum is about , so that: the Sun emits more energy than the Sun, 2.5121484 = 860,000 times, but its mass is 19 times greater than that of the Sun and therefore per 1 g of matter it emits 45,000 times more than the Sun. The Sun produces radiation per 1 g of mass. In the same way, we find that component B of the visual double star Kruger 60 emits 80 times less matter per 1 g than the Sun, i.e. for it. The specific radiation of Sirius B, a white dwarf, is even lower: . Meanwhile, the average temperature T of a star changes incomparably less for the same stars (except, perhaps, for a white dwarf) (see p. 196). It is difficult to assume in advance that in all three cases the energy generation mechanism is the same, but if it is the same, then, obviously, it is very sensitive to changes in the physical conditions inside the star, in particular, temperature. Of the various possible types of energy generation in stars, the following two are significant:
a) gravitational compression,
b) thermonuclear processes.
GRAVITATIONAL COMPRESSION
If a rarefied ball is compressed, then its potential energy decreases [see. (15.8)]; this decrease goes to an increase in the kinetic energy of the particles of the ball, i.e., to an increase in temperature when the ball is gas (see (15.9)).
The internal thermal energy of an ideal gas that has reached temperature is equal to 1 g. For the entire star this will be
The integral is equal to . Substituting here instead the expression from (15.9), in which , and adding the expression for potential energy from (15.8), we can easily obtain
Total Energy
For a monatomic gas and, therefore, neglecting the radiation pressure of the star (for which ), we will have
that is, the total energy is equal to half the potential energy and its change is only half the change in potential energy.
The polytropic model, which is quite broad in applicability, has the potential energy
Here n is the polytropy class (at which the energy becomes positive, i.e. the ball has infinitely large dimensions) and for the convective model
and for the standard model
The rate of energy change should obviously be identified with the luminosity of the star in the compression stage:
As can be seen from equality (17.4). the changes in total energy, which we equate in (17.8) with luminosity, account for only half the change in the potential energy of the star. The other half goes to warming it up.
If we substitute into the right side of (17.9) instead of L the ray emission of the Sun, and instead of R the mass and radius of the Sun, then we will have
(17.10)
Taking a formal approach to the last calculation, we can say that if we assume the Sun is contracting, then with the current characteristics of the Sun, the radius of the Sun is “enough” for only years to compensate for the loss of heat by radiation. Essentially, we must say that under gravitational compression the Sun changes significantly over 25 million years. But the geological history of the Earth teaches us that the Sun more or less invariably irradiates the Earth for about 3 billion years and, therefore, the indicated time scale of about 20 million years, the so-called Kelvin-Helmholtz contraction time scale, is not suitable for explaining the modern evolution of the Sun. It is quite suitable for the evolution of condensing stars when they are heated during compression, until the heating becomes so strong that thermonuclear reactions come into operation.
GRAVITATIONAL COLLAPSE
rapid compression and disintegration of an interstellar cloud or star under the influence of its own gravity. Gravitational collapse is a very important astrophysical phenomenon; it is involved both in the formation of stars, star clusters and galaxies, and in the death of some of them. In interstellar space there are many clouds consisting mainly of hydrogen with a density of approx. 1000 at/cm3, sizes from 10 to 100 St. years. Their structure and, in particular, density continuously change under the influence of mutual collisions, heating by stellar radiation, pressure of magnetic fields, etc. When the density of a cloud or part of it becomes so great that gravity exceeds gas pressure, the cloud begins to shrink uncontrollably - it collapses. Small initial density inhomogeneities become stronger during the collapse process; As a result, the cloud fragments, i.e. breaks up into parts, each of which continues to shrink. Generally speaking, when a gas is compressed, its temperature and pressure increase, which can prevent further compression. But while the cloud is transparent to infrared radiation, it cools easily, and the compression does not stop. However, as the density of individual fragments increases, their cooling becomes more difficult and the increasing pressure stops the collapse - this is how a star is formed, and the entire set of cloud fragments that have turned into stars forms a star cluster. The collapse of a cloud into a star or star cluster lasts about a million years - relatively quickly on a cosmic scale. After this, thermonuclear reactions occurring in the bowels of the star maintain temperature and pressure, which prevents compression. During these reactions, light chemical elements are converted into heavier ones, releasing enormous energy (similar to what happens when a hydrogen bomb explodes). The released energy leaves the star in the form of radiation. Massive stars emit very intense radiation and burn their “fuel” in just a few tens of millions of years. Low-mass stars have enough fuel to last many billions of years of slow burning. Sooner or later, any star runs out of fuel, thermonuclear reactions in the core stop and, deprived of a heat source, it remains at the mercy of its own gravity, inexorably leading the star to death.
Collapse of low-mass stars. If, after losing the envelope, the remnant of the star has a mass of less than 1.2 solar, then its gravitational collapse does not go too far: even a shrinking star deprived of heat sources gains a new ability to resist gravity. At a high density of matter, electrons begin to intensively repel each other; this is not due to their electrical charge, but to their quantum mechanical properties. The resulting pressure depends only on the density of the substance and does not depend on its temperature. Physicists call this property of electrons degeneracy. In low-mass stars, the pressure of degenerate matter can resist gravity. The contraction of a star stops when it becomes approximately the size of Earth. Such stars are called white dwarfs because they shine weakly, but immediately after compression they have a rather hot (white) surface. However, the temperature of the white dwarf gradually decreases, and after several billion years such a star is already difficult to notice: it becomes a cold, invisible body.
Collapse of massive stars. If the mass of the star is more than 1.2 solar, then the pressure of degenerate electrons is not able to resist gravity, and the star cannot become a white dwarf. Its uncontrollable collapse continues until the substance reaches a density comparable to the density of atomic nuclei (approximately 3 * 10 14 g/cm3). In this case, most of the matter turns into neutrons, which, like electrons in a white dwarf, become degenerate. The pressure of degenerate neutron matter can stop the contraction of a star if its mass does not exceed approximately 2 solar masses. The resulting neutron star has a diameter of only ca. 20 km. When the rapid contraction of a neutron star suddenly stops, all the kinetic energy turns into heat and the temperature rises to hundreds of billions of kelvins. As a result, a giant flare of the star occurs, its outer layers are thrown out at high speed, and the luminosity increases several billion times. Astronomers call this a "supernova explosion." After about a year, the brightness of the explosion products decreases, the ejected gas gradually cools, mixes with interstellar gas, and in subsequent epochs becomes part of stars of new generations. The neutron star that emerged during the collapse rotates rapidly in the first millions of years and is observed as a variable emitter - a pulsar. If the mass of the collapsing star significantly exceeds 2 solar, then the compression does not stop at the neutron star stage, but continues until its radius decreases to several kilometers. Then the gravitational force on the surface increases so much that even a ray of light cannot leave the star. A star that has collapsed to such an extent is called a black hole. Such an astronomical object can only be studied theoretically, using Einstein's general theory of relativity. Calculations show that the compression of the invisible black hole continues until the matter reaches an infinitely high density.
see also PULSAR; BLACK HOLE .
LITERATURE
Shklovsky I.S., Stars: their birth, life and death. M., 1984
Collier's Encyclopedia. - Open Society. 2000 .
See what "GRAVITATIONAL COLLAPSE" is in other dictionaries:
The process is hydrodynamic. compression of the body under the influence of its own. forces of gravity. This process in nature is possible only in fairly massive bodies, in particular stars. A necessary condition for G.K. a decrease in elasticity in VA inside a star, to a swarm leads to ... ... Physical encyclopedia
Catastrophically fast compression of massive bodies under the influence of gravitational forces. Gravitational collapse can end the evolution of stars with a mass exceeding two solar masses. After the exhaustion of nuclear fuel in such stars, they lose their... ... encyclopedic Dictionary
Model of the mechanism of gravitational collapse Gravitational collapse is a catastrophically rapid compression of massive bodies under the influence of gravitational forces. Gravitational to... Wikipedia
Catastrophically fast compression of massive bodies under the influence of gravitational forces. The evolution of stars with a mass exceeding two solar masses can end with gravitational collapse. After the exhaustion of nuclear fuel in such stars, they lose their... ... Astronomical Dictionary
Gravitational collapse- (from gravity and lat. collapsus fallen) (in astrophysics, astronomy) catastrophically fast compression of a star in the last stages of evolution under the influence of its own gravitational forces, exceeding the weakening pressure forces of heated gas (matter) ... ... The beginnings of modern natural science
See Gravitational collapse... Great Soviet Encyclopedia
Catastrophically fast compression of massive bodies under the influence of gravity. strength GK may end the evolution of stars with a mass of St. two solar masses. After the exhaustion of nuclear fuel in such stars, they lose their mechanical properties. sustainability and... Natural science. encyclopedic Dictionary
See Gravitational Collapse... Big Encyclopedic Dictionary
See gravitational collapse. * * * COLLAPSE GRAVITATIONAL COLLAPSE GRAVITATIONAL, see gravitational collapse (see GRAVITATIONAL COLLAPSE) ... encyclopedic Dictionary
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- Einstein's vision. , Wheeler J.A. , The book by the outstanding American physicist D. A. Wheeler is devoted to an elementary presentation of geometrodynamics - the embodiment of Einstein’s dream “to reduce all physics to geometry.” The author begins with... Category: Mathematics and science Series: Publisher:
In interstellar space there are many clouds consisting mainly of hydrogen with a density of approx. 1000 at/cm 3, sizes from 10 to 100 sv. years. Their structure and, in particular, density continuously change under the influence of mutual collisions, heating by stellar radiation, pressure of magnetic fields, etc. When the density of a cloud or part of it becomes so great that gravity exceeds gas pressure, the cloud begins to shrink uncontrollably - it collapses. Small initial density inhomogeneities become stronger during the collapse process; As a result, the cloud fragments, i.e. breaks up into parts, each of which continues to shrink.
Generally speaking, when a gas is compressed, its temperature and pressure increase, which can prevent further compression. But while the cloud is transparent to infrared radiation, it cools easily, and the compression does not stop. However, as the density of individual fragments increases, their cooling becomes more difficult and the increasing pressure stops the collapse - this is how a star is formed, and the entire set of cloud fragments that have turned into stars forms a star cluster.
The collapse of a cloud into a star or star cluster lasts about a million years—relatively fast on a cosmic scale. After this, thermonuclear reactions occurring in the bowels of the star maintain temperature and pressure, which prevents compression. During these reactions, light chemical elements are converted into heavier ones, releasing enormous energy (similar to what happens when a hydrogen bomb explodes). The released energy leaves the star in the form of radiation. Massive stars emit very intense radiation and burn their “fuel” in just a few tens of millions of years. Low-mass stars have enough fuel to last many billions of years of slow burning. Sooner or later, any star runs out of fuel, thermonuclear reactions in the core stop and, deprived of a heat source, it remains at the mercy of its own gravity, inexorably leading the star to death.
Collapse of low-mass stars.
If, after losing the envelope, the remnant of the star has a mass of less than 1.2 solar, then its gravitational collapse does not go too far: even a shrinking star deprived of heat sources gains a new ability to resist gravity. At a high density of matter, electrons begin to intensively repel each other; this is not due to their electrical charge, but to their quantum mechanical properties. The resulting pressure depends only on the density of the substance and does not depend on its temperature. Physicists call this property of electrons degeneracy. In low-mass stars, the pressure of degenerate matter can resist gravity. The contraction of a star stops when it becomes approximately the size of Earth. Such stars are called white dwarfs because they shine weakly, but immediately after compression they have a rather hot (white) surface. However, the temperature of the white dwarf gradually decreases, and after several billion years such a star is already difficult to notice: it becomes a cold, invisible body.
Collapse of massive stars.
If the mass of the star is more than 1.2 solar, then the pressure of degenerate electrons is not able to resist gravity, and the star cannot become a white dwarf. Its uncontrollable collapse continues until the substance reaches a density comparable to the density of atomic nuclei (approximately 3H 10 14 g/cm 3). In this case, most of the matter turns into neutrons, which, like electrons in a white dwarf, become degenerate. The pressure of degenerate neutron matter can stop the contraction of a star if its mass does not exceed approximately 2 solar masses. The resulting neutron star has a diameter of only ca. 20 km. When the rapid contraction of a neutron star suddenly stops, all the kinetic energy turns into heat and the temperature rises to hundreds of billions of kelvins. As a result, a giant flare of the star occurs, its outer layers are thrown out at high speed, and the luminosity increases several billion times. Astronomers call this a “supernova explosion.” After about a year, the brightness of the explosion products decreases, the ejected gas gradually cools, mixes with interstellar gas, and in subsequent epochs becomes part of stars of new generations. The neutron star that emerged during the collapse rotates rapidly in the first millions of years and is observed as a variable emitter - a pulsar.
If the mass of the collapsing star significantly exceeds 2 solar, then the compression does not stop at the neutron star stage, but continues until its radius decreases to several kilometers. Then the gravitational force on the surface increases so much that even a ray of light cannot leave the star. A star that has collapsed to such an extent is called a black hole. Such an astronomical object can only be studied theoretically, using Einstein's general theory of relativity. Calculations show that the compression of the invisible black hole continues until the matter reaches an infinitely high density.
