This post is a summary for the fifth lesson of the astrophysics course program for high school. It contains a description of supernova explosions, the processes of formation of neutron stars (pulsars) and black holes of stellar masses, both single and in stellar pairs. And a few words about brown dwarfs.
First, I will repeat the picture showing the classification of star types and their evolution depending on their masses:
1. Flashes of new and supernovae.
Helium burnout in the interiors of stars ends with the formation of red giants and their outbursts as new with education white dwarfs or the formation of red supergiants and their outbursts as supernovae with education neutron stars or black holes as well as nebulae from their shells thrown off by these stars. Often the masses of the ejected shells exceed the masses of the "mummies" of these stars - neutron stars and black holes. To understand the scale of this phenomenon, I will give a video of the outburst of the supernova 2015F at a distance of 50 million light years from us. years of the galaxy NGC 2442:
Another example is the supernova of 1054 in our Galaxy, as a result of which the Crab Nebula and a neutron star were formed at a distance of 6.5 thousand light years from us. years. In this case, the mass of the formed neutron star is ~ 2 solar masses, and the mass of the ejected shell is ~ 5 solar masses. Contemporaries estimated the brightness of this supernova as about 4-5 times greater than that of Venus. If such a supernova flared up a thousand times closer (6.5 light years), then it would sparkle in our sky 4000 times brighter than the Moon, but a hundred times weaker than the Sun.
2. Neutron stars.
Stars of high masses (classes Oh, V, A) after hydrogen burnout into helium and during helium burnup predominantly into carbon, oxygen and nitrogen enter a rather short stage red supergiant and at the end of the helium-carbon cycle, they also drop the shell and flare up as "Supernovae". Their bowels also shrink under the influence of gravity. But the pressure of the degenerate electron gas can no longer, as in white dwarfs, stop this gravitational self-compression. Therefore, the temperature in the depths of these stars rises and thermonuclear reactions begin to take place in them, as a result of which the following elements of the periodic table are formed. Up to gland.
Why precisely to iron? Because the formation of nuclei with a large atomic number does not come with the release of energy, but with its absorption. And taking it from other nuclei is not so easy. Of course, elements with a large atomic number are formed in the depths of these stars. But in much smaller quantities than iron.
But further evolution splits. Not too massive stars (of classes BUT and partially AT) turn into neutron stars. In which electrons are literally imprinted into protons and most of the star's body turns into a huge neutron nucleus. Consisting of contacting and even pressed into each other ordinary neutrons. The density of a substance in which is about several billion tons per cubic centimeter. A typical neutron star diameter- about 10-20 kilometers. The neutron star is the second stable type of "mummy" of a dead star. Their masses, as a rule, lie in the range from about 1.3 to 2.1 solar masses (according to observations).
Single neutron stars are almost impossible to see in optics due to their extremely low luminosity. But some of them find themselves as pulsars. What it is? Almost all stars rotate around their axis and have a fairly strong magnetic field. For example, our Sun rotates around its axis in about a month.
Now imagine that its diameter will decrease a hundred thousand times. It is clear that due to the law of conservation of angular momentum, it will rotate much faster. And the magnetic field of such a star near its surface will be many orders of magnitude stronger than the solar one. Most neutron stars have a period of rotation around their axis in tenths - hundredths of a second. It is known from observations that the fastest rotating pulsar makes just over 700 revolutions around its axis per second, and the slowest rotating one makes one revolution in more than 23 seconds.
Now imagine that the magnetic axis of such a star, like that of the Earth, does not coincide with the axis of rotation. Hard radiation from such a star will be concentrated in narrow cones along the magnetic axis. And if this cone "touches" the Earth with the rotation period of the star, then we will see this star as a pulsating source of radiation. Like a flashlight rotated by our hand.
Such a pulsar (neutron star) was formed after a supernova explosion in 1054, which happened just during the visit of Cardinal Humbert to Constantinople. As a result of which there was a final break between the Catholic and Orthodox churches. This pulsar itself makes 30 revolutions per second. And the shell thrown by him with a mass of ~ 5 solar masses looks like crab nebula:
3. Black holes (stellar masses).
Finally, sufficiently massive stars (of classes O and partially AT) finish their life path the third type of "mummy" - black hole. Such an object arises when the mass of the remnant of the star is so large that the pressure of adjoining neutrons (the pressure of the degenerate neutron gas) in the interior of this remnant cannot resist its gravitational self-compression. Observations show that the mass boundary between neutron stars and black holes lies in the vicinity of ~2.1 solar masses.
It is impossible to observe a single black hole directly. For no particle can escape from its surface (if it exists). Even a particle of light is a photon.
4. Neutron stars and black holes in binary star systems.
Single neutron stars and stellar-mass black holes are practically unobservable. But in cases where they are one of two or more stars in close star systems, such observations become possible. Since their gravitation can "suck" the outer shells of their neighbors that are still normal stars.
With such "suction" around a neutron star or a black hole is formed accretion disk, the matter of which partially "slides" towards the neutron star or black hole and is partially thrown away from it in two jets. This process can be fixed. An example is the binary star system in SS433, one of whose components is either a neutron star or a black hole. And the second is still an ordinary star:
5. Brown dwarfs.
Stars with masses noticeably smaller than the solar mass and up to ~ 0.08 solar masses are class M red dwarfs. They will operate on the hydrogen-helium cycle for a time greater than the age of the universe. In objects with masses below this limit, for a number of reasons, a stationary, long-running thermonuclear fusion is not possible. Such stars are called brown dwarfs. Their surface temperature is so low that they are almost invisible in optics. But they shine in the IR range. For these reasons, they are often referred to as understars.
The mass range of brown dwarfs is from 0.012 to 0.08 solar masses. Objects with a mass less than 0.012 solar masses (~12 Jupiter masses) can only be planets. gas giants. Radiating due to the slow gravitational self-compression noticeably more energy than they receive from the parent stars. So, Jupiter, in the sum of all ranges, radiates about twice as much energy as it receives from the Sun.
What black hole ? Why is it called black? What is happening in the stars? How are neutron stars and black holes related? Is the Large Hadron Collider capable of creating black holes, and what does this mean for us?
What star??? In case you don't already know, our Sun is also a star. This is an object large sizes capable of emitting electromagnetic waves using thermonuclear fusion (this is not the most accurate definition). If it is not clear, you can say this: a star is a large spherical object, inside which, with the help of nuclear reactions, very, very, very a large number of energy, some of which is used to emit visible light. In addition to ordinary light, heat (infrared radiation), radio waves, and ultraviolet radiation, etc. are also emitted.
In any star, nuclear reactions take place in the same way as in nuclear power plants, with only two main differences.
1. In the stars, nuclear fusion reactions occur, that is, the combination of nuclei, and in nuclear power plants – nuclear decay. In the first case, 3 times more energy is released, thousands of times less cost, since only hydrogen is needed, and it is relatively inexpensive. Also, in the first case, there is no harmful waste: only harmless helium is released. Now, of course, you are interested in why nuclear power plants do not use such reactions? Because it is UNCONTROLLED and easily leads to nuclear explosion Moreover, this reaction requires a temperature of several million degrees. For humans, nuclear fusion is the most important and most difficult task (no one has yet figured out a way to control fusion), given that our energy sources are running out.
2. In stars, more matter is involved in reactions than in nuclear power plants, and, naturally, more energy is obtained there.
Now about the evolution of stars. Each star is born, grows, grows old and dies (extinguishes). Stars according to the style of evolution are divided into three categories depending on their mass.
First category – stars with a mass less than 1.4 times the mass of the sun. In such stars, all the “fuel” slowly turns into metal, because due to the fusion (combination) of nuclei, more and more “multinuclear” (heavy) elements appear, and these are metals. Truth, last stage the evolution of such stars has not been recorded (it is difficult to fix metal balls), this is just a theory.
Second category –
stars in mass exceeding the mass of stars of the first category, but less than three masses of the Sun. As a result of evolution, such stars lose the balance of internal forces of attraction and repulsion. As a result, their outer shell is ejected into space, and the inner one (from the law of conservation of momentum) begins to “furiously” shrink. A neutron star is formed. It consists almost entirely of neutrons, that is, of particles that do not have an electric charge. The most remarkable thing about a neutron star –
this is its density, because in order to become a neutron star, you need to shrink to a ball with a diameter of only about 300 km, and this is very small. So its density is very high - about tens of trillions of kg in one cubic meter, which is billions of times greater than the density of the densest substances on Earth. Where did this density come from? The fact is that all substances on Earth are made up of atoms, which in turn are made up of nuclei. Each atom can be thought of as a large empty ball (absolutely empty), in the center of which is a small nucleus. The entire mass of the atom is contained in the nucleus (except for the nucleus, there are only electrons in the atom, but their mass is very small). The nucleus is 1000 times smaller than an atom in diameter. This means that the volume of the nucleus is 1000 * 1000 * 1000 = 1 billion times smaller than the atom. And hence the density of the nucleus is billions of times greater than the density of the atom. What happens in a neutron star? Atoms cease to exist as a form of matter, they are replaced by nuclei. That is why the density of such stars is billions of times greater than the density of terrestrial substances.
We all know that heavy objects (planets, stars) strongly attract everything around them. Neutron stars are found that way. They strongly bend the orbits of others visible stars nearby.
The third category of stars – stars with a mass greater than three times the mass of the Sun. Such stars, becoming neutron ones, shrink further and turn into black holes. Their density is tens of thousands of times greater than the density of neutron stars. Having such a huge density, a black hole acquires the ability of very strong gravity (the ability to attract surrounding bodies). With such gravity, the star does not allow even electromagnetic waves, and therefore light, to leave its limits. That is, a black hole does not emit light. The absence of any light – it's dark, that's why a black hole is called a black hole. It is always black, it cannot be seen in any telescope. Everyone knows that due to their gravity, black holes are able to suck in all the surrounding bodies in large volume. That is why people are wary of launching the Large Hadron Collider, in the work of which, according to scientists, the appearance of black microholes is not ruled out. However, these microholes are very different from ordinary ones: they are unstable, because their lifetime is very short, and have not been proven in practice. Moreover, scientists claim that these microholes are of a completely different nature, unlike ordinary black holes, and are not capable of absorbing matter.
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For stars with a mass below a certain critical gravitational contraction stops at the stage of the so-called "white dwarf".
The density of the white dwarf is greater than 10 7 g/cm 3 , the surface temperature is ~ 10 4 K. At such high temperature the atoms must be completely ionized and inside the star the nuclei must be immersed in a sea of electrons forming a degenerate electron gas. The pressure of this gas prevents further gravitational collapse of the star.
The pressure of a degenerate electron gas has a quantum nature. It arises as a consequence of the Pauli principle, to which electrons obey.
The Pauli principle establishes a limit on the minimum amount of space that can be occupied by each electron. External pressure is not able to reduce this volume. In a white dwarf, all electrons have reached their minimum volume and the gravitational contraction is balanced internal pressure electron gas.
The limit on the mass of a white dwarf is about 1.5M s . This limiting mass is called the Chandrasekhar limit (M s is the mass of the Sun, equal to ~ 1.99 10 30 kg).
It is usually believed that the maximum mass of a white dwarf is 1.4M s . Thus, the electron degeneracy pressure cannot hold masses greater than 1.4M s . If 0.5Ms< M < 1.4M s , ядро белого карлика состоит из углерода и кислорода. Если M < 0.5M s , ядро белого карлика состоит из гелия.
The density of a white dwarf with a mass close to Chandrasekhar's is 6x10 6 g/cm 3 , radius is 5x10 3 km.
The luminosity of white dwarfs is 10 -2 -10 -4 of the luminosity of the Sun. Their radiation is provided by the thermal energy stored in them.
neutron star
Calculations show that the explosion of a supernova with M ~ 25M s leaves a dense neutron core (neutron star) with a mass of ~ 1.6M s .
In stars with a residual mass M > 1.4M s that have not reached the supernova stage, the pressure of the degenerate electron gas is also unable to balance the gravitational forces, and the star shrinks to the state of nuclear density. The mechanism of this gravitational collapse is the same as in a supernova explosion.
The pressure and temperature inside the star reach such values at which electrons and protons seem to be "pressed" into each other and as a result of the reaction
p + e -> n + v e
after the ejection of neutrinos, neutrons are formed, occupying a much smaller phase volume than electrons.
A so-called neutron star appears, the density of which reaches 10 14 - 10 15 g/cm 3 . The characteristic size of a neutron star is 10-15 km.
In a sense, a neutron star is a giant atomic nucleus.
Further gravitational contraction is prevented by the pressure of nuclear matter, which arises due to the interaction of neutrons. This is the same degeneracy pressure as earlier in the case of a white dwarf, but it is the degeneracy pressure of a much denser neutron gas. This pressure is able to hold masses up to 3.2M s .
The neutrinos produced at the moment of collapse cool the neutron star rather quickly. According to theoretical estimates, its temperature drops from 10 11 to 10 9 K in ~ 100 s. Further, the rate of cooling decreases somewhat. However, it is quite high in astronomical terms. The decrease in temperature from 10 9 to 10 8 K occurs in 100 years and to 10 6 K in a million years.
Detecting neutron stars with optical methods is quite difficult due to their small size and low temperature.
In 1967, at the University of Cambridge, Hewish and Bell discovered cosmic sources of periodic electromagnetic radiation - pulsars. The pulse repetition periods of most pulsars lie in the range from 3.3·10 -2 to 4.3 s.
According to modern concepts, pulsars are rotating neutron stars with a mass of 1-3M s and a diameter of 10-20 km.
Only compact objects with the properties of neutron stars can maintain their shape without collapsing at such rotational speeds.
The conservation of angular momentum and magnetic field during the formation of a neutron star leads to the birth of rapidly rotating pulsars with a strong magnetic field B ~ 10 12 G.
B is the magnetic induction vector, the main power characteristic of the magnetic field. It is measured in gauss (Gs) in the CGS system (centimeter-gram-second) and in teslas (T) in the International System of Units (SI). 1 T = 10 4 Gs.
It is believed that a neutron star has a magnetic field whose axis does not coincide with the axis of rotation of the star. In this case, the radiation of the star (radio waves and visible light) glides across the Earth like the rays of a beacon. When the beam crosses the Earth, an impulse is registered.
The very radiation of a neutron star arises due to the fact that charged particles from the surface of the star move outward along the magnetic field lines, emitting electromagnetic waves. This pulsar radio emission mechanism, first proposed by Gold, is shown in the figure below.
If the radiation beam hits an earthly observer, then the radio telescope detects short pulses of radio emission with a period equal to the rotation period of the neutron star.
The shape of the pulse can be very complex, which is due to the geometry of the magnetosphere of a neutron star and is characteristic of each pulsar.
The rotation periods of pulsars are strictly constant and the measurement accuracy of these periods reaches 14-digit figures.
Pulsars that are part of binary systems have now been discovered. If the pulsar orbits around the second component, then variations in the period of the pulsar due to the Doppler effect should be observed.
When the pulsar approaches the observer, the recorded period of radio pulses decreases due to the Doppler effect, and when the pulsar moves away from us, its period increases. Based on this phenomenon, pulsars that are part of binary stars were discovered.
For the first discovered pulsar PSR 1913 + 16, which is part of a binary system, the orbital period of revolution was 7 hours 45 minutes. The proper period of revolution of the pulsar PSR 1913 + 16 is 59 ms.
The radiation of the pulsar should lead to a decrease in the speed of rotation of the neutron star. This effect has also been found. A neutron star, which is part of a binary system, can also be a source of intense X-rays.
The formation of neutron stars is not always the result of a supernova explosion. Another mechanism for the formation of neutron stars during the evolution of white dwarfs in close binary star systems is also possible.
The flow of matter from the companion star to the white dwarf gradually increases the mass of the white dwarf, and upon reaching the critical mass (the Chandrasekhar limit), the white dwarf turns into a neutron star.
In the case when the flow of matter continues after the formation of a neutron star, its mass can increase significantly and, as a result of gravitational collapse, it can turn into a black hole. This corresponds to the so-called "silent" collapse.
There is a limit to the mass of a star that can be held in equilibrium by densely packed neutrons. This limit cannot be calculated exactly, since the behavior of matter at densities significantly exceeding the density of nuclear matter has not been sufficiently studied.
Estimates of the mass of a star that can no longer be stabilized by degenerate neutrons give a value of ~ 3M s .
Thus, if a residual mass M > 3M s is preserved during a supernova explosion, then it cannot exist in the form of a stable neutron star.
The nuclear repulsive forces at small distances are not able to resist further gravitational contraction of the star. An unusual object appears - a black hole.
The main property of a black hole is that no signals emitted by it can go beyond its limits and reach an external observer.
A star of mass M, collapsing into a black hole, reaches a sphere of radius r g (Schwarzschild sphere):
r g \u003d 2GM / c 2,
(formally, this relation can be arrived at by putting in the well-known formula for the second space velocity v k2 = (2GM/R) 1/2 the limiting value of this speed, equal to the speed of light).
When an object reaches the size of a Schwarzschild sphere, its gravitational field becomes so strong that even electromagnetic radiation cannot leave this object. The Schwarzschild radius of the Sun is 3 km, the Earth's is 1 cm.
The Schwarzschild black hole belongs to non-rotating objects and is the remnant of a massive non-rotating star. A rotating massive star collapses into a rotating black hole (Kerr black hole).
A black hole can only be detected by indirect signs, in particular, if it is part of a binary star system with a visible star. In this case, the black hole will suck in the star's gas. This gas will heat up, becoming a source of intense x-rays that can be detected.
There is currently no direct experimental evidence for the existence of black holes. There are several space objects whose behavior can be explained by the presence of black holes.
So there is the Cygnus XI object, which is a binary system with a rotation period of 5.6 days. The system includes a blue giant with a mass of 22M s and an invisible source of pulsating X-ray radiation with a mass of 8M s , which is possibly a black hole (an object of such a large mass cannot be a neutron star).
Along with black holes formed during the collapse of stars, there may be black holes in the Universe that arose long before the appearance of the first stars due to the inhomogeneity of the Big Bang.
The resulting clumps of matter could shrink to the state of black holes, while the rest of the matter expanded. Black holes, formed at the earliest stage of the Universe, are called relict. It is assumed that the size of some of them can be significantly smaller size proton.
In 1974, Hawking showed that black holes must emit particles. The source of these particles is the process of formation of virtual particle-antiparticle pairs in vacuum. In ordinary fields, these pairs annihilate so quickly that they cannot be observed. However, in very strong fields virtual particle and the antiparticle can separate and become real.
Powerful tidal forces act at the edge of a black hole. Under the action of these forces, some of the particles (antiparticles) that were part of the virtual pairs can fly out of the black hole. Since many of them annihilate, the black hole must become a source of radiation.
The energy radiated into space by a black hole comes from its depths. Therefore, in the process of such emission of particles, the mass and size of the black hole must decrease. This is the mechanism of "evaporation" of a black hole.
The temperature of a black hole is inversely proportional to its mass, so the more massive ones evaporate more slowly, because their lifetime is proportional to the cube of the mass (in four-dimensional space-time). For example, the lifetime of a black hole with a mass M of the solar order exceeds the age of the Universe, while a microhole with M = 1 teraelectronvolt (10 12 eV, approximately 2x10 -30 kg) lives for about 10 -27 seconds (Science and Life, BLACK HOLES).
For large black holes, the rate of "evaporation" is very slow and practically negligible. A black hole with a mass of 10 solar masses will evaporate in 10 69 years. The evaporation time of supermassive (billion solar masses) black holes, which may be at the center of large galaxies, can be 10 96 years.
The processes of transformation of stars into white dwarfs, neutron stars or black holes, as a rule, are accompanied by emissions of colossal energy. More about this kind of energy emissions and other space explosions is described in the next video.
Video: The most brutal and largest explosions in space. Explosions of galaxies, stars, planets.
Gravity is the main subject of many of these questions. It is the defining force in space. It holds the planets in their orbits, links the stars and galaxies, determines the fate of our universe. Created by Isaac Newton in the 17th century, the theoretical description of gravity remains accurate enough to calculate the trajectories of spacecraft when flying to Mars, Jupiter and beyond. But after 1905, when Albert Einstein showed in special theory relativity that instantaneous transmission of information is impossible, physicists realized that Newton's laws would no longer be adequate when the speed of gravity-induced motion approached the speed of light. However, Einstein's general theory of relativity (published in 1916) describes quite consistently even those situations where gravity is extremely strong. General relativity is seen as one of the two pillars of 20th-century physics; the second is quantum theory, a revolution in ideas that anticipated our modern understanding of atoms and their nuclei. Einstein's intellectual feat was especially impressive, because, unlike the pioneers of quantum theory, he did not have an incentive in the form of an experimental problem. Only 50 years later, astronomers discovered objects with a sufficiently strong gravitational field in which the most characteristic and striking features of the theory could appear Einstein. In the early 60s, objects with a very high luminosity - quasars - were discovered. They seemed to require an even more efficient source of energy than the nuclear fusion that makes the stars shine; gravitational collapse seemed the most attractive explanation. The American theorist Thomas Gold expressed the excitement that then seized the theorists. In his afternoon talk at the first big conference on the new object of relativistic astrophysics, which took place in Dallas in 1963, he said: "Relativists with their sophisticated work are not only a brilliant decoration of culture, but they can be useful to science! Everyone is happy: relativists, who feel that their work is recognized, that they have suddenly become experts in a field they did not even know existed, astrophysicists who have expanded their field of activity ... All this is very pleasant, let's hope that this is correct. "Observations using new methods of radio and x-ray astronomy supported Gold's optimism. In the 1950s, the best optical telescopes in the world were concentrated in the United States, especially in California. This movement from Europe was due to both climatic and financial reasons. However, radio waves from space can pass through clouds, so in Europe and Australia the new science of radio astronomy could develop unaffected by the weather. Some of the strongest sources of space radio noise have been identified. One was the Crab Nebula, the expanding remnants of a supernova explosion seen by Eastern astronomers in 1054 B.C. Other sources were distant extragalactic objects, in which, as we now understand, energy generation took place near giant black holes. These discoveries were unexpected. The physical processes responsible for the emission of radio waves, which are now quite well understood, were not predicted. The most remarkable unexpected achievement of radio astronomy was the discovery of neutron stars in 1967 by Anthony Hewish and Jocelyn Bell. These stars are dense remnants left in the center after some supernova explosions. They were discovered like pulsars: they rotate (sometimes several times per second) and emit a powerful beam of radio waves that passes through our line of sight once per revolution. The importance of neutron stars lies in their extreme physical conditions: colossal densities, strong magnetic and gravitational fields. In 1969, a very fast (30 Hz) pulsar was discovered in the center of the Crab Nebula. Careful observations showed that the frequency of the pulses gradually decreased. It was natural if the energy of the star's rotation is gradually converted into a wind of particles that keep the nebula glowing in blue light. Interestingly, the pulse rate of the pulsar - 30 per second - is so high that the eye sees it as permanent source. If it were as bright but rotated more slowly - say, 10 times per second - the remarkable properties of this small star could have been discovered as early as 70 years ago. How would the development of physics in the 20th century change if superdense matter were discovered in the 1920s, before neutrons were discovered on Earth? Although no one knows this, there is no doubt that the importance of astronomy for fundamental physics would have been realized much earlier. Neutron stars were discovered by accident. No one expected that they would emit such strong and clear radio pulses. If theorists had been asked in the early 1960s how best to detect neutron stars, most would have suggested looking for X-rays. Indeed, if neutron stars radiate as much energy as ordinary stars from a much smaller area, they must be hot enough to emit X-rays. Thus, X-ray astronomers seemed to be in the best position to discover neutron stars. X-rays from space objects, however, are absorbed in the Earth's atmosphere and can only be observed from space. X-ray astronomy, like radio astronomy, was given impetus to development as a result of the use of military technology and experience. In this area, US scientists have taken a leading position, especially the late Herbert Friedman and his colleagues at the Naval research laboratory USA. Their first rocket-mounted X-ray detectors only worked for a few minutes before falling to the ground. X-ray astronomy made great progress in the 1970s, when NASA launched the first X-ray satellite, which collected information over several years. This project, and many that have followed, have shown that X-ray astronomy has opened an important new window on the universe. X-rays are emitted by unusually hot gas and particularly powerful sources. Therefore, the hottest and most powerful objects in space stand out on the x-ray map of the sky. Among them are neutron stars, in which a mass, at least not less than the mass of the Sun, is concentrated in a volume with a diameter of slightly more than 10 kilometers. The gravitational force on them is so strong that relativistic corrections reach up to 30%. It is currently assumed that some remnants of stars, when collapsing, can exceed the density of neutron stars and turn into black holes, which distort time and space even more than neutron stars. An astronaut who dares to go inside the horizon of a black hole will not be able to transmit light signals into the surrounding world - as if space itself is being sucked in faster than light is moving through it. An outside observer will never know the astronaut's final fate. It will seem to him that any clock falling inward will go slower and slower. So the astronaut will be, as it were, nailed to the horizon, having stopped in time. Russian theorists Yakov Zeldovich and Igor Novikov, who studied how time is distorted near collapsed objects, proposed the term "frozen stars" in the early 1960s. The term "black hole" was coined in 1968 when John Wheeler described how "light and particles falling from outside ... fall on a black hole, only increasing its mass and gravitational pull." Black holes, which are the final evolutionary state of stars, have radii from 10 to 50 kilometers. But now there is compelling evidence that black holes with masses of millions or even billions of solar masses exist at the centers of most galaxies. Some of them manifest themselves as quasars - bundles of energy that shine brighter than all the stars of the galaxies in which they are located, or as powerful sources of cosmic radio emission. Others, including the black hole at the center of our Galaxy, are not so active, but influence the orbits of stars that come close to them. Black holes, when viewed from the outside, are standardized objects: there are no signs by which one could determine how a certain black hole was formed or what objects are swallowed up by it. In 1963, New Zealander Roy Kerr discovered a solution to Einstein's equations, which described a collapsing rotating object. "Kerr's solution" has acquired a very importance, when theorists realized that it describes the space-time around any black hole. A collapsing object quickly comes to a standardized state characterized by just two numbers measuring its mass and spin. Roger Penrose, the mathematical physicist who arguably did the most to revive the theory of relativity in the 1960s, remarked: “There is some irony in the fact that for the strangest and least familiar astrophysical object - the black hole - our theoretical the picture is most complete." The discovery of black holes paved the way for testing the most remarkable consequences of Einstein's theory. Radiation from such objects is mainly due to hot gas falling in a spiral into the "gravitational well". It shows a strong Doppler effect and also has an additional redshift due to the strong gravitational field. A spectroscopic study of this radiation, especially X-rays, will make it possible to probe the flow very close to the black hole and determine whether the shape of space is consistent with the predictions of the theory.
White dwarfs, neutron stars and black holes are various forms final stage of stellar evolution. Young stars draw their energy from thermonuclear reactions taking place in the stellar interior; These reactions convert hydrogen into helium. After a certain amount of hydrogen has been used up, the resulting helium core begins to shrink. The further evolution of a star depends on its mass, or rather on how it correlates with a certain critical value called the Chandrasekhar limit. If the mass of the star is less than this value, then the pressure of the degenerate electron gas stops the compression (collapse) of the helium core before its temperature reaches such high value when thermonuclear reactions begin, during which helium is converted into carbon. Meanwhile, the outer layers of the evolving star are shed relatively quickly. (It is assumed that this is the way planetary nebulae are formed.) A white dwarf is a helium core surrounded by a more or less extended hydrogen shell.
In more massive stars, the helium core continues to shrink until the helium "burns up". The energy released in the process of converting helium into carbon prevents the core from further contraction - but not for long. After the helium is completely used up, the compression of the core continues. The temperature rises again, other nuclear reactions begin, which proceed until the energy stored in atomic nuclei is exhausted. By this time, the core of the star already consists of pure iron, which plays the role of nuclear "ash". Now nothing can prevent the further collapse of the star - it continues until the density of its matter reaches the density of atomic nuclei. The sharp compression of matter in the central regions of the star generates an explosion of tremendous force, as a result of which the outer layers of the star fly apart at tremendous speeds. It is these explosions that astronomers associate with the phenomenon of supernovae.
The fate of the collapsing remnant of a star depends on its mass. If the mass is less than about 2.5 M 0 (the mass of the Sun), then the pressure due to the "zero" motion of neutrons and protons is large enough to prevent further gravitational contraction of the star. Objects whose matter density is equal to (or even exceeds) the density of atomic nuclei are called neutron stars. Their properties were first studied in the 30s by R. Oppenheimer and G. Volkov.
According to Newton's theory, the radius of a collapsing star decreases to zero in a finite time, while the gravitational potential increases indefinitely. Einstein's theory paints a different scenario. The photon's speed decreases as it approaches the center of the black hole, becoming equal to zero. This means that from the point of view of an external observer, a photon falling into a black hole will never reach its center. Since particles of matter cannot move faster than a photon, the radius of a black hole will reach its limit value in infinite time. Moreover, photons emitted from the surface of the black hole experience an ever-increasing redshift during the collapse. From the point of view of an external observer, the object from which the black hole is formed is initially compressed at an ever-increasing rate; then its radius begins to decrease more and more slowly.
Not having internal sources energy, neutron stars and black holes are rapidly cooling down. And since their surface area is very small - only a few tens of square kilometers - it should be expected that the brightness of these objects is extremely low. Indeed, thermal radiation from the surface of neutron stars or black holes has not yet been observed. However, some neutron stars are powerful sources of nonthermal radiation. We are talking about the so-called pulsars, discovered in 1967 by Jocelyn Bell - a graduate student University of Cambridge. Bell studied radio signals recorded using equipment developed by Anthony Hewish to study the radiation of oscillating radio sources. Among the many records of chaotically flickering sources, she noticed one where bursts were repeated with a clear periodicity, although they varied in intensity. More detailed observations confirmed the exactly periodic nature of the pulse repetition, and when studying other records, two more sources with the same properties were discovered. Observations and theoretical analysis show that pulsars are rapidly rotating neutron stars with an unusually strong magnetic field. The pulsating nature of the radiation is due to a beam of rays emerging from "hot spots" on (or near) the surface of a rotating neutron star. The detailed mechanism of this radiation is still a mystery to scientists.
Several neutron stars that are part of close binary systems have been discovered. It is these (and no other) neutron stars that are powerful sources of X-rays. Let us imagine a close binary, one component of which is a giant or supergiant, and the other is a compact star. Under the action of the gravitational field of a compact star, gas can flow out of the rarefied atmosphere of a giant: such gas flows in close binary systems, discovered long ago by spectral analysis methods, have received a corresponding theoretical interpretation. If the compact star in a binary system is a neutron star or a black hole, then the molecules of gas escaping from another component of the system can be accelerated to very high energies. Due to collisions between molecules, the kinetic energy of the gas falling on a compact star is eventually converted into heat and into radiation. Estimates show that the energy released in this case fully explains the observed X-ray intensity in binary systems of this type.
AT general theory In Einstein's theory of relativity, black holes occupy the same place as ultrarelativistic particles in his special theory of relativity. But if the world of ultrarelativistic particles - high energy physics - is full of amazing phenomena that play an important role in experimental physics and observational astronomy, then the phenomena associated with black holes are still surprising. In time, black hole physics will produce results that are important for cosmology, but right now this branch of science is basically " playground» for theorists. Doesn't it follow from this that Einstein's theory of gravity gives us less information about the Universe than Newton's theory, although theoretically it is much superior to it? Not at all! Unlike Newton's theory, Einstein's theory forms the foundation of a self-consistent model of the real Universe as a whole, that this theory has many amazing and testable predictions, and, finally, it provides a causal relationship between freely falling, non-rotating reference frames and distribution, as well as the motion of mass in space space.
