The birth and evolution of stars: the giant factory of the universe. Life cycle of a star - description, diagram and interesting facts

It occupies a point in the upper right corner: it has a high luminosity and a low temperature. The main radiation occurs in the infrared range. Radiation from the cold dust shell reaches us. In the process of evolution, the position of the star on the diagram will change. The only source of energy at this stage is gravitational contraction. Therefore, the star moves quite quickly parallel to the y-axis.

The surface temperature does not change, but the radius and luminosity decrease. The temperature in the center of the star rises, reaching a value at which reactions begin with light elements: lithium, beryllium, boron, which quickly burn out, but manage to slow down the compression. The track turns parallel to the y-axis, the temperature on the surface of the star rises, and the luminosity remains almost constant. Finally, in the center of the star, the reactions of the formation of helium from hydrogen (hydrogen combustion) begin. The star enters the main sequence.

The duration of the initial stage is determined by the mass of the star. For stars like the Sun, it is about 1 million years, for a star with a mass of 10 M☉ about 1000 times smaller, and for a star with a mass of 0.1 M☉ thousands of times more.

Young low mass stars

At the beginning of its evolution, a low-mass star has a radiant core and a convective envelope (Fig. 82, I).

At the main sequence stage, the star shines due to the release of energy in the nuclear reactions of the conversion of hydrogen into helium. The supply of hydrogen ensures the luminosity of a star of mass 1 M☉ Approximately within 10 10 years. Stars of greater mass consume hydrogen faster: for example, a star with a mass of 10 M☉ will use up hydrogen in less than 10 7 years (luminosity is proportional to the fourth power of mass).

low mass stars

As the hydrogen burns out, the central regions of the star are strongly compressed.

Stars of high mass

After entering the main sequence, the evolution of a large-mass star (>1.5 M☉) is determined by the conditions of combustion of nuclear fuel in the interior of the star. At the main sequence stage, this is the combustion of hydrogen, but unlike low-mass stars, reactions of the carbon-nitrogen cycle dominate in the core. In this cycle, the C and N atoms play the role of catalysts. The rate of energy release in the reactions of such a cycle is proportional to T 17 . Therefore, a convective core is formed in the core, surrounded by a zone in which energy transfer is carried out by radiation.

The luminosity of large mass stars is much higher than the luminosity of the Sun, and hydrogen is consumed much faster. This is due to the fact that the temperature in the center of such stars is also much higher.

As the proportion of hydrogen in the substance of the convective core decreases, the rate of energy release decreases. But since the rate of release is determined by the luminosity, the core begins to shrink, and the rate of energy release remains constant. At the same time, the star expands and passes into the region of red giants.

low mass stars

By the time the hydrogen is completely burned out, a small helium core is formed in the center of a low-mass star. In the core, the matter density and temperature reach 10 9 kg/m and 10 8 K, respectively. Hydrogen combustion occurs on the surface of the nucleus. As the temperature in the core rises, the rate of hydrogen burning increases, and the luminosity increases. The radiant zone gradually disappears. And because of the increase in the speed of convective flows, the outer layers of the star swell. Its size and luminosity increase - the star turns into a red giant (Fig. 82, II).

Stars of high mass

When the hydrogen of a large mass star is completely exhausted, a triple helium reaction begins in the core and at the same time the reaction of oxygen formation (3He => C and C + He => 0). At the same time, hydrogen begins to burn on the surface of the helium core. The first layer source appears.

The supply of helium is exhausted very quickly, since in the described reactions in each elementary act, relatively little energy is released. The picture repeats itself, and two layer sources appear in the star, and the C + C => Mg reaction begins in the core.

The evolutionary track in this case turns out to be very complex (Fig. 84). In the Hertzsprung-Russell diagram, the star moves along the sequence of giants or (with a very large mass in the supergiant region) periodically becomes a cephei.

Old low-mass stars

In a star of low mass, in the end, the speed of the convective flow at some level reaches the second space velocity, the shell comes off, and the star turns into a white dwarf, surrounded by a planetary nebula.

The evolutionary track of a low-mass star on the Hertzsprung-Russell diagram is shown in Figure 83.

Death of high mass stars

At the end of evolution, a large mass star has a very complex structure. Each layer has its own chemical composition, in several layered sources flow nuclear reactions, and an iron core is formed in the center (Fig. 85).

Nuclear reactions with iron do not proceed, since they require the expenditure (and not release) of energy. Therefore, the iron core is rapidly compressed, the temperature and density in it increase, reaching fantastic values ​​- a temperature of 10 9 K and a pressure of 10 9 kg / m 3. material from the site

At this moment, two most important processes begin, going on in the nucleus simultaneously and very quickly (apparently, in minutes). The first is that during the collision of nuclei, iron atoms decay into 14 helium atoms, the second is that electrons are “pressed” into protons, forming neutrons. Both processes are associated with the absorption of energy, and the temperature in the core (also pressure) drops instantly. The outer layers of the star begin to fall towards the center.

The fall of the outer layers leads to a sharp increase in temperature in them. Hydrogen, helium, carbon begin to burn. This is accompanied by a powerful stream of neutrons that comes from the central core. As a result, a powerful nuclear explosion occurs, throwing off the outer layers of the star, which already contain all the heavy elements, up to californium. According to modern views, all atoms of heavy chemical elements (i.e., heavier than helium) were formed in the Universe precisely in flares

The evolution of stars is a change in physical. characteristics, internal buildings and chem. composition of stars over time. The most important tasks theory of E.z. - explanation of the formation of stars, changes in their observed characteristics, study of the genetic relationship of various groups of stars, analysis of their final states.

Since in the part of the Universe known to us approx. 98-99% of the mass of the observed matter is contained in stars or has passed the stage of stars, the explanation of E.z. yavl. one of the most important problems in astrophysics.

A star in a stationary state is a gas ball, which is in a hydrostatic state. and thermal equilibrium (i.e., the action of gravitational forces is balanced by internal pressure, and energy losses due to radiation are compensated by the energy released in the interior of the star, see). The "birth" of a star is the formation of a hydrostatically equilibrium object, the radiation of which is supported by its own. energy sources. The "death" of a star is an irreversible imbalance leading to the destruction of the star or to its catastrophic failure. compression.

Separation of gravity. energy can play a decisive role only when the temperature of the interior of the star is insufficient for the nuclear energy release to compensate for energy losses, and the star as a whole or part of it must contract to maintain equilibrium. The illumination of thermal energy becomes important only after the depletion of nuclear energy reserves. Thus, E.z. can be represented as a successive change of energy sources of stars.

The characteristic time of E.z. too large to be able to follow the whole evolution directly. Therefore, the main research method E.z. yavl. construction of sequences of models of stars that describe changes in the internal. buildings and chem. composition of stars over time. Evolution. the sequences are then compared with the results of observations, for example, with (G.-R.d.), which summarizes the observations of a large number of stars at different stages of evolution. Of particular importance is the comparison with G.-R.d. for star clusters, since all cluster stars have the same initial chem. composition and formed almost simultaneously. According to G.-R.d. clusters of different ages, it was possible to establish the direction of the E.z. Evolutionary detail. sequences are calculated by numerically solving a system of differential equations describing the distribution of mass, density, temperature and luminosity in a star, to which are added, the laws of energy release and opacity of stellar matter and equations describing the change in chemical. star composition over time.

The evolution of a star depends mainly on its mass and initial chem. composition. A certain, but not fundamental role can be played by the rotation of the star and its magn. field, but the role of these factors in E.z. not yet sufficiently explored. Chem. The composition of a star depends on the time it was formed and on its position in the galaxy at the time of formation. The stars of the first generation were formed from matter, the composition of which was determined by the cosmological. conditions. Apparently, it contained approximately 70% by mass of hydrogen, 30% of helium, and a negligible admixture of deuterium and lithium. In the course of the evolution of stars of the first generation, heavy elements (following helium) were formed, which were ejected into interstellar space as a result of the outflow of matter from stars or during star explosions. The stars of subsequent generations were already formed from matter containing up to 3-4% (by mass) of heavy elements.

The most direct indication that star formation is taking place in the Galaxy at the present time is yavl. existence of massive bright stars spectrum. classes O and B, the lifetime of which cannot exceed ~ 10 7 years. The rate of star formation in modern epoch is estimated at 5 per year.

2. Star formation, stage of gravitational contraction

According to the most common view, stars are formed as a result of gravity. condensation of matter in the interstellar medium. The necessary separation of the interstellar medium into two phases - dense cold clouds and a rarefied medium with a higher temperature - can occur under the influence of the Rayleigh-Taylor thermal instability in the interstellar magnetic field. field. Gas-dust complexes with mass , characteristic size (10-100) pc and particle concentration n~10 2 cm -3 . actually observed due to their emission of radio waves. Compression (collapse) of such clouds requires certain conditions: gravitational. particles of the cloud must exceed the sum of the energy of the thermal motion of particles, the energy of rotation of the cloud as a whole and the magnetic. cloud energy (Jeans criterion). If only energy is taken into account thermal motion, then up to a factor of the order of one, the Jeans criterion is written as follows: align="absmiddle" width="205" height="20">, where is the mass of the cloud, T- gas temperature in K, n- number of particles in 1 cm 3 . With typical modern interstellar clouds temp-pax K can only collapse clouds with a mass not less than . The Jeans criterion indicates that for the formation of stars with a really observed mass spectrum, the concentration of particles in collapsing clouds should reach (10 3 -10 6) cm -3 , i.e. 10-1000 times higher than observed in typical clouds. However, such concentrations of particles can be achieved in the depths of clouds that have already begun to collapse. It follows from this that what is happening is by means of a successive process carried out in several stages, fragmentation of massive clouds. This picture naturally explains the birth of stars in groups - clusters. At the same time, issues related to the heat balance in the cloud, the velocity field in it, and the mechanism that determines the mass spectrum of fragments still remain unclear.

Collapsing objects of stellar mass called. protostars. The collapse of a spherically symmetric non-rotating protostar without magnetic. fields include several. stages. At the initial moment of time, the cloud is uniform and isothermal. It is transparent to the public. radiation, so the collapse occurs with volumetric energy losses, Ch. arr. due to thermal radiation of dust, a swarm transmit their kinetic. the energy of a gas particle. In a homogeneous cloud, there is no pressure gradient and the compression begins in the free fall regime with the characteristic time , where G- , - cloud density. With the onset of compression, a rarefaction wave arises, moving towards the center at the speed of sound, and since the collapse occurs faster where the density is higher, the protostar is divided into a compact core and an extended shell, in which the matter is distributed according to the law . When the concentration of particles in the core reaches ~ 10 11 cm -3, it becomes opaque for the IR radiation of dust particles. The energy released in the core slowly seeps to the surface due to radiant heat conduction. The temperature begins to rise almost adiabatically, this leads to an increase in pressure, and the core enters a hydrostatic state. balance. The shell continues to fall on the nucleus, and appears on its periphery. The parameters of the core at this time weakly depend on the total mass of the protostar: K. As the mass of the core increases due to accretion, its temperature changes almost adiabatically until it reaches 2000 K, when the dissociation of H 2 molecules begins. As a result of energy consumption for dissociation, and not an increase in kinetic. particle energy, the value of the adiabatic index becomes less than 4/3, pressure changes are not able to compensate for the gravitational forces, and the core collapses again (see ). A new core is formed with parameters , surrounded by a shock front, onto which the remnants of the first core are accreted. A similar rearrangement of the nucleus occurs with hydrogen.

Further growth of the core due to the material of the shell continues until all the matter falls on the star or is scattered under the action of or , if the core is sufficiently massive (see ). For protostars with the characteristic time of the shell matter t a >t kn, so their luminosity is determined by the energy release of contracting nuclei.

Evolution. tracks of nuclei of protostars with constant mass at the hydrostatic stage. compression are shown in fig. 1. In low-mass stars, at the moment when hydrostatic is established. equilibrium, the conditions in the nuclei are such that energy is transferred in them. Calculations show that the surface temperature of a fully convective star is almost constant. The radius of the star is continuously decreasing, because. she keeps shrinking. With a constant surface temperature and a decreasing radius, the luminosity of the star should also fall on the G.-R.d. this stage of evolution corresponds to the vertical segments of the tracks.

As the compression continues, the temperature in the interior of the star rises, the matter becomes more transparent, and stars with align="absmiddle" width="90" height="17"> have radiant cores, but the shells remain convective. Less massive stars remain fully convective. Their luminosity is regulated by a thin radiant layer in the photosphere. The more massive the star and the higher its effective temperature, the larger its radiant core (in stars with align="absmiddle" width="74" height="17">, the radiant core appears immediately). In the end, almost the entire star (with the exception of the surface convective zone in stars with mass ) passes into a state of radiative equilibrium, at which all the energy released in the core is transferred by radiation.

3. Evolution based on nuclear reactions

When the hydrogen content in the core decreases to 1%, the expansion of the shells of stars with align="absmiddle" width="66" height="17"> is replaced by the general contraction of the star, which is necessary to maintain energy release. Compression of the shell causes heating of hydrogen in the layer adjacent to the helium core to the temperature of its thermonuclear combustion, and a layer source of energy release appears. For stars with mass , for which it depends to a lesser extent on temperature and the region of energy release is not so strongly concentrated towards the center, there is no stage of general compression.

E.z. after hydrogen burnout depends on their mass. The most important factor, affecting the course of evolution of stars with mass , yavl. degeneracy of the electron gas at high densities. Due to the high density, the number of quantum states with low energy is limited due to the Pauli principle, and electrons fill quantum levels with high energy, much higher than the energy of their thermal motion. The most important feature of a degenerate gas is that its pressure p depends only on density: for non-relativistic degeneracy and for relativistic degeneracy. The electron gas pressure is much greater than the ion pressure. This implies the fundamental for E.z. conclusion: since the gravitational force acting on a unit volume of a relativistically degenerate gas, , depends on the density in the same way as the pressure gradient , there must be a limiting mass (see ), such that for align="absmiddle" width="66" height ="15"> The pressure of the electrons cannot counteract gravity and compression begins. Mass limit align="absmiddle" width="139" height="17">. The boundary of the region in which the electron gas is degenerate is shown in fig. 3 . In low-mass stars, degeneracy plays an appreciable role already in the process of formation of helium nuclei.

The second factor determining E.z. in the later stages, these are neutrino energy losses. In the depths of the stars T~10 8 To main. the role in the birth is played by: photoneutrino process, decay of quanta of plasma oscillations (plasmons) into neutrino-antineutrino pairs (), annihilation of electron-positron pairs () and (see). The most important feature of neutrinos is that the matter of the star is practically transparent for them, and neutrinos freely carry away energy from the star.

The helium core, in which the conditions for helium combustion have not yet arisen, is compressed. The temperature in the layered source adjacent to the core increases, and the rate of hydrogen burning increases. The need to transfer the increased energy flow leads to the expansion of the shell, for which part of the energy is spent. Since the luminosity of the star does not change, the temperature of its surface drops, and on G.-R.d. the star moves into the region occupied by red giants. The restructuring time of the star is two orders of magnitude shorter than the hydrogen burnout time in the core; therefore, there are few stars between the MS band and the region of red supergiants. With a decrease in the temperature of the shell, its transparency increases, as a result of which an external. convective zone and the luminosity of the star increases.

The removal of energy from the core through the thermal conduction of degenerate electrons and neutrino losses in stars delays the moment of helium ignition. The temperature begins to grow noticeably only when the core becomes almost isothermal. Combustion 4 He determines the E.z. from the moment when the energy release exceeds the energy losses due to heat conduction and neutrino radiation. The same condition applies to the combustion of all subsequent types of nuclear fuel.

A remarkable feature of neutrino-cooled stellar nuclei from degenerate gas is "convergence" - the convergence of tracks, which characterize the ratio of density and temperature T c in the center of the star (Fig. 3). The rate of energy release during compression of the nucleus is determined by the rate of attachment of matter to it through a layer source, which depends only on the mass of the nucleus for a given type of fuel. A balance of inflow and outflow of energy must be maintained in the core, so the same distribution of temperature and density is established in the cores of stars. By the time of ignition of 4 He, the mass of the nucleus depends on the content of heavy elements. In degenerate gas nuclei, the ignition of 4 He has the character of a thermal explosion, since the energy released during combustion goes to increase the energy of the thermal motion of electrons, but the pressure almost does not change with increasing temperature until the thermal energy of the electrons is equal to the energy of the degenerate gas of electrons. Then the degeneracy is removed and the core rapidly expands - a helium flash occurs. Helium flashes are probably accompanied by the loss of stellar matter. At , where massive stars have long since completed their evolution and red giants have masses , stars at the helium burning stage are on the horizontal branch of the G.-R.d.

In helium cores of stars with align="absmiddle" width="90" height="17"> the gas is not degenerate, 4 He ignites quietly, but the cores also expand due to increasing T c. In the most massive stars, the 4 He ignition occurs even when they are yavl. blue supergiants. The expansion of the core leads to a decrease T in the region of the hydrogen layer source, and the luminosity of the star decreases after the helium flash. To maintain thermal equilibrium, the shell contracts, and the star leaves the red supergiant region. When 4 He in the core is depleted, the compression of the core and the expansion of the shell begin again, the star again becomes a red supergiant. A layered combustion source 4 He is formed, which dominates in the energy release. Outside appears again. convective zone. As helium and hydrogen burn out, the thickness of the layered sources decreases. A thin layer of helium combustion turns out to be thermally unstable, because with a very strong sensitivity of energy release to temperature (), the thermal conductivity of the substance is insufficient to extinguish thermal perturbations in the combustion layer. During thermal flashes, convection occurs in the layer. If it penetrates into layers rich in hydrogen, then as a result of a slow process ( s-process, see) elements are synthesized with atomic masses from 22 Ne to 209 B.

The radiation pressure on the dust and molecules formed in the cold extended shells of red supergiants leads to a continuous loss of matter at a rate of up to per year. Continuous mass loss can be supplemented by losses due to the instability of stratified combustion or pulsations, which can lead to the release of one or more. shells. When the amount of matter above the carbon-oxygen core becomes less than a certain limit, the shell, in order to maintain the temperature in the combustion layers, is forced to contract until the compression is able to sustain combustion; star on G.-R.d. shifts almost horizontally to the left. At this stage, the instability of the combustion layers can also lead to expansion of the shell and loss of matter. As long as the star is hot enough, it is observed as a core with one or more. shells. When layer sources are displaced to the surface of the star so that the temperature in them becomes lower than necessary for nuclear combustion, the star cools down, turning into a white dwarf with radiating due to the consumption of thermal energy of the ionic component of its substance. The characteristic cooling time for white dwarfs is ~109 years. The lower limit on the masses of single stars turning into white dwarfs is unclear, it is estimated at 3-6 . In stars with electron gas degenerates at the stage of growth of carbon-oxygen (C,O-) stellar cores. As in the helium cores of stars, due to neutrino energy losses there is a "convergence" of conditions in the center and by the time carbon is ignited in the C,O core. The ignition of 12 C under such conditions most likely has the character of an explosion and leads to the complete destruction of the star. Complete destruction may not occur if . Such a density is achievable when the core growth rate is determined by the accretion of the satellite's matter in a close binary system.

Contemplating the clear night sky away from city lights, it is easy to see that the universe is full of stars. How did nature manage to create a myriad of these objects? After all, according to estimates only Milky Way about 100 billion stars. In addition, stars are still being born today, 10-20 billion years after the formation of the Universe. How are stars formed? What changes does a star undergo before it reaches a steady state, like our Sun?

From the point of view of physics, a star is a ball of gas

From the point of view of physics, it is a gas ball. The heat and pressure generated in nuclear reactions - mainly in the reactions of fusion of helium from hydrogen - prevent the star from collapsing under its own gravity. The life of this relatively simple object follows a well-defined scenario. First, a star is born from a diffuse cloud of interstellar gas, then goes a long doomsday. But eventually, when all the nuclear fuel is exhausted, it will turn into a faintly luminous white dwarf, neutron star or black hole.

This description may give the impression that a detailed analysis of the formation and early stages of stellar evolution should not cause significant difficulties. But the interplay of gravity and thermal pressure causes stars to behave in unpredictable ways.
Consider, for example, the evolution of luminosity, that is, the change in the amount of energy emitted by the stellar surface per unit time. The internal temperature of a young star is too low for the fusion of hydrogen atoms, so its luminosity must be relatively low. It can increase when nuclear reactions begin, and only then can it gradually fall. In fact, a very young star is extremely bright. Its luminosity decreases with age, reaching a temporary minimum during the burning of hydrogen.

In the early stages of evolution, various physical processes take place in stars.

In the early stages of evolution, a variety of physical processes take place in stars, some of which are still poorly understood. Only in the last two decades have astronomers begun to build a detailed picture of the evolution of stars on the basis of advances in theory and observation.
Stars are born from large, unobservable visible light clouds located in disks spiral galaxies. Astronomers call these objects giant molecular complexes. The term "molecular" reflects the fact that the gas in the complexes is primarily composed of hydrogen in molecular form. Such clouds are the largest formations in the Galaxy, sometimes reaching more than 300 sv. years across.

In a more thorough analysis of the evolution of the star

A closer analysis reveals that stars form from individual condensations—compact zones—in a giant molecular cloud. Astronomers have studied the properties of compact zones with large radio telescopes, the only instruments capable of detecting faint millimoclouds. It follows from observations of this radiation that a typical compact zone has a diameter of several light months, a density of 30,000 hydrogen molecules per cm^, and a temperature of 10 Kelvin.
Based on these values, it was concluded that the pressure of the gas in compact zones is such that it can withstand compression under the action of self-gravitational forces.

Therefore, in order for a star to form, the compact zone must contract from an unstable state, such that the gravitational forces exceed the internal gas pressure.
It is not yet clear how compact zones condense from the initial molecular cloud and acquire such an unstable state. Nevertheless, even before the discovery of compact zones, astrophysicists had the opportunity to simulate the process of star formation. As early as the 1960s, theorists used computer simulations to determine how clouds compress in an unstable state.
Although a wide range of initial conditions was used for theoretical calculations, the results obtained coincided: for a cloud that is too unstable, the inner part is compressed first, that is, free fall the substance in the center is exposed first, while the peripheral regions remain stable. Gradually, the compression region expands outward, covering the entire cloud.

Deep in the bowels of a shrinking region begins the evolution of stars

Deep in the bowels of the shrinking region, star formation begins. The diameter of a star is only one light second, that is, one millionth of the diameter of the compact zone. For such relatively small sizes overall picture cloud compression is not significant, and the main role here is played by the speed of matter falling onto the star

The rate of fall of matter can be different, but it directly depends on the temperature of the cloud. The higher the temperature, the faster the speed. Calculations show that a mass equal to the mass of the Sun can accumulate in the center of a collapsing compact zone over a period of 100 thousand to 1 million years. A body formed in the center of a collapsing cloud is called a protostar. Using computer simulations, astronomers have developed a model that describes the structure of a protostar.
It turned out that the falling gas hits the surface of the protostar with a very high speed. Therefore, a powerful shock front is formed (a sharp transition to a very high pressure). Within the shock front, the gas heats up to almost 1 million Kelvin, then, during radiation near the surface, it rapidly cools to about 10,000 K, forming a protostar layer by layer.

The presence of a shock front explains the high brightness of young stars

The presence of a shock front explains the high brightness of young stars. If the mass of a protosis-star is equal to one solar mass, then its luminosity can exceed the solar one by ten times. But it is caused not by thermonuclear fusion reactions, as in ordinary stars, but by the kinetic energy of matter acquired in the gravitational field.
Protostars can be observed, but not with conventional optical telescopes.
All interstellar gas, including the one from which stars are formed, contains "dust" - a mixture of solid submicron particles. The radiation of the shock front encounters on its way a large number of these particles, which, together with the gas, fall on the surface of the protostar.
Cold dust particles absorb photons emitted by the shock front and re-emit them with longer wavelengths. This long-wavelength radiation is in turn absorbed and then re-emitted by even more distant dust. Therefore, while a photon makes its way through clouds of dust and gas, its wavelength is in the infrared range of the electromagnetic spectrum. But already at a distance of several light-hours from the protostar, the wavelength of the photon becomes too large, so that the dust cannot absorb it, and it can finally rush unhindered to Earth-sensitive telescopes that are sensitive to infrared radiation.
Despite the wide capabilities of modern detectors, astronomers cannot claim that telescopes actually register the radiation of protostars. Apparently, they are deeply hidden in the bowels of the compact zones registered in the radio range. The uncertainty in registration is due to the fact that detectors cannot distinguish a protostar from older stars interspersed in gas and dust.
For reliable identification, an infrared or radio telescope must detect a Doppler shift in the spectral emission lines of a protostar. The Doppler shift would show the true movement of the gas falling on its surface.
As soon as, as a result of the fall of matter, the mass of the protostar reaches several tenths of the mass of the Sun, the temperature in the center becomes sufficient for the start of thermonuclear fusion reactions. However, thermonuclear reactions in protostars are fundamentally different from reactions in middle-aged stars. The energy source of such stars is the reactions of thermonuclear fusion of helium from hydrogen.

Hydrogen is the most common chemical element in the universe

Hydrogen is the most abundant chemical element in the universe. At the birth of the Universe (Big Bang), this element was formed in its usual form with a nucleus consisting of one proton. But two out of every 100,000 nuclei are deuterium nuclei, made up of a proton and a neutron. This isotope of hydrogen is present in the modern era in the interstellar gas from which it enters the stars.
It is noteworthy that this meager admixture plays a dominant role in the life of protostars. The temperature in their depths is insufficient for the reactions of ordinary hydrogen, which occur at 10 million Kelvin. But as a result of gravitational compression, the temperature in the center of the protostar can easily reach 1 million Kelvin, when the fusion of deuterium nuclei begins, at which colossal energy is also released.

The opacity of protostellar matter is too great

The opacity of protostellar matter is too great for this energy to be transmitted by radiative transfer. Therefore, the star becomes convectively unstable: gas bubbles heated by "nuclear fire" float to the surface. These ascending flows are balanced by cold gas flows descending towards the center. Similar convective movements, but on a much smaller scale, take place in a steam-heated room. In a protostar, convective vortices carry deuterium from the surface to its interior. Thus, the fuel needed for thermonuclear reactions reaches the core of the star.
Despite the very low concentration of deuterium nuclei, the heat released during their merger has a strong effect on the protostar. The main consequence of deuterium combustion reactions is the "swelling" of the protostar. Due to the efficient transfer of heat by convection as a result of the "burning" of deuterium, the protostar increases in size, which depends on its mass. A protostar of one solar mass has a radius equal to five solar masses. With a mass equal to three solar, the protostar swells up to a radius equal to 10 solar.
The mass of a typical compact zone is greater than the mass of its generated star. Therefore, there must be some mechanism that removes excess mass and stops the fall of matter. Most astronomers are convinced that a strong stellar wind is responsible for this, escaping from the surface of the protostar. The stellar wind blows the incident gas backwards and eventually disperses the compact zone.

stellar wind idea

The "idea of ​​a stellar wind" does not follow from theoretical calculations. And astonished theorists were given evidence of this phenomenon: observations of molecular gas flows moving from infrared radiation sources. These flows are associated with the protostellar wind. Its origin is one of the deepest mysteries of young stars.
When the compact zone dissipates, an object that can be observed in the optical range is exposed - a young star. Like a protostar, it has a high luminosity that is more determined by gravity than by fusion. The pressure in the interior of the star prevents a catastrophic gravitational collapse. However, the heat responsible for this pressure is radiated from the stellar surface, so the star shines very brightly and contracts slowly.
As it contracts, its internal temperature gradually rises and eventually reaches 10 million Kelvin. Then the fusion reactions of hydrogen nuclei begin with the formation of helium. The heat released creates pressure that prevents compression, and the star will shine for a long time until nuclear fuel runs out in its depths.
Our Sun, a typical star, took about 30 million years to shrink from protostellar to modern size. Thanks to the heat released during thermonuclear reactions, it has retained these dimensions for about 5 billion years.
This is how stars are born. But despite such obvious successes of scientists who allowed us to learn one of the many secrets of the universe, many more known properties young stars are not yet fully understood. This refers to their irregular variability, colossal stellar wind, unexpected bright flashes. There are no definite answers to these questions yet. But these unresolved problems should be seen as breaks in a chain, the main links of which have already been soldered. And we will be able to close this chain and complete the biography of young stars if we find the key created by nature itself. And this key flickers in the clear sky above us.

Birth of a star video:

Thermonuclear fusion in the interior of stars

At this time, for stars with a mass greater than 0.8 solar masses, the core becomes transparent to radiation, and radiative energy transfer in the core will prevail, while the shell at the top remains convective. No one knows for sure what kind of stars of smaller mass arrive on the main sequence, since the time these stars spend in the category of young ones exceeds the age of the Universe. All our ideas about the evolution of these stars are based on numerical calculations.

As the star contracts, the pressure of the degenerate electron gas begins to increase, and at some radius of the star, this pressure stops growth. central temperature and then starts to decrease it. And for stars less than 0.08, this turns out to be fatal: the energy released during nuclear reactions will never be enough to cover the cost of radiation. Such under-stars are called brown dwarfs, and their fate is a constant contraction until the pressure of the degenerate gas stops it, and then a gradual cooling with a stop to all nuclear reactions.

Young stars of intermediate mass

Young stars of intermediate mass (from 2 to 8 solar masses) qualitatively evolve in exactly the same way as their smaller sisters, with the exception that they do not have convective zones until the main sequence.

Objects of this type are associated with the so-called. Ae\Be Herbit stars are irregular variables of spectral type B-F5. They also have bipolar jet disks. The exhaust velocity, luminosity, and effective temperature are substantially greater than for τ Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.

Young stars with a mass greater than 8 solar masses

In fact, these are already normal stars. While the mass of the hydrostatic core was accumulating, the star managed to skip all the intermediate stages and heat up the nuclear reactions to such an extent that they compensate for the losses due to radiation. For these stars, the outflow of mass and luminosity is so high that it not only stops the collapse of the remaining outer regions, but pushes them back. Thus, the mass of the formed star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars more than 100-200 solar masses.

mid-life cycle of a star

Among the formed stars there is a huge variety of colors and sizes. They range in spectral type from hot blues to cool reds, and in mass from 0.08 to more than 200 solar masses. The luminosity and color of a star depends on the temperature of its surface, which, in turn, is determined by its mass. All new stars "take their place" on the main sequence according to their chemical composition and mass. We are not talking about the physical movement of the star - only about its position on the indicated diagram, which depends on the parameters of the star. That is, we are talking, in fact, only about changing the parameters of the star.

What happens next depends again on the mass of the star.

Later years and the death of stars

Old stars with low mass

To date, it is not known for certain what happens to light stars after the depletion of the hydrogen supply. Since the universe is 13.7 billion years old, which is not enough to deplete the supply of hydrogen fuel, current theories are based on computer simulations of the processes occurring in such stars.

Some stars can only fuse helium in certain active regions, which causes instability and strong solar winds. In this case, the formation of a planetary nebula does not occur, and the star only evaporates, becoming even smaller than a brown dwarf.

But a star with a mass of less than 0.5 solar mass will never be able to synthesize helium even after reactions involving hydrogen cease in the core. Their stellar shell is not massive enough to overcome the pressure produced by the core. Such stars include red dwarfs (such as Proxima Centauri), whose main sequence lifetimes are hundreds of billions of years. After the termination of thermonuclear reactions in their core, they, gradually cooling down, will continue to weakly radiate in the infrared and microwave ranges of the electromagnetic spectrum.

medium sized stars

When a star reaches an average size (from 0.4 to 3.4 solar masses) of the red giant phase, its outer layers continue to expand, the core contracts, and reactions of carbon synthesis from helium begin. The fusion releases a lot of energy, giving the star a temporary reprieve. For a star similar in size to the Sun, this process can take about a billion years.

Changes in the amount of energy emitted cause the star to go through periods of instability, including changes in size, surface temperature, and energy release. The release of energy is shifted towards low-frequency radiation. All this is accompanied by an increasing mass loss due to strong solar winds and intense pulsations. The stars in this phase are called late-type stars, OH-IR stars or Mira-like stars, depending on their exact characteristics. The ejected gas is relatively rich in heavy elements produced in the interior of the star, such as oxygen and carbon. The gas forms an expanding shell and cools as it moves away from the star, allowing the formation of dust particles and molecules. With strong infrared radiation from the central star, ideal conditions are formed in such shells for the activation of masers.

Helium combustion reactions are very sensitive to temperature. Sometimes this leads to great instability. Violent pulsations occur, which eventually impart enough kinetic energy to the outer layers to be ejected and become a planetary nebula. In the center of the nebula, the core of the star remains, which, cooling down, turns into a helium white dwarf, as a rule, having a mass of up to 0.5-0.6 solar and a diameter of the order of the diameter of the Earth.

white dwarfs

The vast majority of stars, including the Sun, end their evolution by shrinking until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a factor of a hundred and the density becomes a million times that of water, the star is called a white dwarf. It is deprived of sources of energy and, gradually cooling down, becomes dark and invisible.

In stars more massive than the Sun, the pressure of degenerate electrons cannot contain the compression of the core, and it continues until most of the particles turn into neutrons, packed so densely that the size of the star is measured in kilometers, and the density is 100 million times greater than the density water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.

supermassive stars

After the outer layers of the star, with a mass greater than five solar masses, have scattered to form a red supergiant, the core begins to shrink due to gravitational forces. As the compression increases, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, heavy elements are synthesized, which temporarily restrains the collapse of the nucleus.

Ultimately, as more and more heavy elements of the periodic system are formed, iron -56 is synthesized from silicon. Up to this point, the synthesis of elements released a large amount of energy, but it is the iron-56 nucleus that has the maximum mass defect and the formation of heavier nuclei is unfavorable. Therefore, when the iron core of a star reaches a certain value, the pressure in it is no longer able to withstand the colossal force of gravity, and an immediate collapse of the core occurs with the neutronization of its matter.

What happens next is not entirely clear. But whatever it is, in a matter of seconds, it leads to the explosion of a supernova of incredible force.

The accompanying burst of neutrinos provokes a shock wave. Strong neutrino jets and a rotating magnetic field push out most of the material accumulated by the star - the so-called seating elements, including iron and lighter elements. The expanding matter is bombarded by neutrons escaping from the nucleus, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and possibly even California). Thus, supernova explosions explain the presence of elements heavier than iron in the interstellar matter.

The blast wave and jets of neutrinos carry material away from the dying star and into interstellar space. Subsequently, moving through space, this supernova material can collide with other space debris, and possibly participate in the formation of new stars, planets or satellites.

The processes that take place during the formation of a supernova are still being studied, and so far this issue is not clear. It is also questionable what actually remains of the original star. However, two options are being considered:

neutron stars

In some supernovae, the strong gravity in the supergiant's interior is known to cause electrons to fall into the atomic nucleus, where they fuse with protons to form neutrons. The electromagnetic forces separating nearby nuclei disappear. The core of a star is now a dense ball of atomic nuclei and individual neutrons.

Such stars, known as neutron stars, are extremely small - no larger than a major city - and have unimaginably high densities. Their orbital period becomes extremely short as the size of the star decreases (due to conservation of angular momentum). Some make 600 revolutions per second. When the axis connecting the north and south magnetic poles of this rapidly rotating star points to the Earth, it is possible to detect a pulse of radiation repeating at intervals equal to the period of rotation of the star. Such neutron stars were called "pulsars", and became the first discovered neutron stars.

Black holes

Not all supernovae become neutron stars. If the star has a sufficiently large mass, then the collapse of the star will continue and the neutrons themselves will begin to fall inward until its radius becomes less than the Schwarzschild radius. The star then becomes a black hole.

The existence of black holes was predicted by the general theory of relativity. According to general relativity, matter and information cannot leave black hole no way. However, quantum mechanics makes exceptions to this rule possible.

A number of open questions remain. Chief among them: "Are there any black holes at all?" Indeed, in order to say for sure that a given object is a black hole, it is necessary to observe its event horizon. All attempts to do so ended in failure. But there is still hope, since some objects cannot be explained without involving accretion, moreover, accretion onto an object without a solid surface, but the very existence of black holes does not prove this.

Questions are also open: is it possible for a star to collapse directly into a black hole, bypassing a supernova? Are there supernovae that will eventually become black holes? What is the exact effect of the initial mass of a star on the formation of objects at the end of its life cycle?

Star evolution is change over time physical characteristics, internal structure and chemical composition of stars. Modern theory The evolution of stars is capable of explaining the general course of the development of stars in satisfactory agreement with the data of astronomical observations. The evolution of a star depends on its mass and initial chemical composition. The stars of the first generation were formed from matter whose composition was determined by cosmological conditions (about 70% hydrogen, 30% helium, negligible admixture of deuterium and lithium). During the evolution of the first generation of stars, heavy elements were formed that were ejected into interstellar space as a result of the outflow of matter from stars or during star explosions. The stars of subsequent generations were formed from matter containing 3–4% of heavy elements.

The birth of a star is the formation of an object whose radiation is maintained by own sources energy. The process of star formation continues uninterruptedly, it is happening at the present time.

To explain the structure of the mega world, the most important is the gravitational interaction. In gas and dust nebulae, under the influence of gravitational forces, unstable inhomogeneities are formed, due to which diffuse matter breaks up into a number of clumps. If such clumps persist long enough, they turn into stars over time. It is important to note that the process of the birth of not a single star, but of stellar associations takes place. The resulting gaseous bodies are attracted to each other, but do not necessarily combine into one huge body. They usually begin to rotate relative to each other, and the centrifugal forces of this movement counteract the forces of attraction, leading to further concentration.

Young stars are those that are still in the stage of initial gravitational contraction. The temperature at the center of such stars is still insufficient for thermonuclear reactions to take place. The glow of stars occurs only due to the conversion of gravitational energy into heat. Gravitational contraction is the first stage in the evolution of stars. It leads to the heating of the central zone of the star to the temperature of the beginning of a thermonuclear reaction (10 - 15 million K) - the conversion of hydrogen into helium.

The huge energy radiated by stars is formed as a result of nuclear processes occurring inside stars. The energy generated inside a star allows it to radiate light and heat for millions and billions of years. For the first time, the assumption that the source of stellar energy is thermonuclear reactions of helium synthesis from hydrogen was put forward in 1920 by the English astrophysicist A.S. Eddington. In the interiors of stars, two types of thermonuclear reactions involving hydrogen are possible, called hydrogen (proton-proton) and carbon (carbon-nitrogen) cycles. In the first case, only hydrogen is required for the reaction to proceed, in the second, the presence of carbon, which serves as a catalyst, is also necessary. The starting material is protons, from which helium nuclei are formed as a result of nuclear fusion.

Since two neutrinos are born during the transformation of four protons into a helium nucleus, 1.8∙10 38 neutrinos are generated every second in the depths of the Sun. The neutrino weakly interacts with matter and has a high penetrating power. Having passed through the huge thickness of the solar matter, neutrinos retain all the information that they received in thermonuclear reactions in the bowels of the Sun. The flux density of solar neutrinos incident on the Earth's surface is 6.6∙10 10 neutrinos per 1 cm 2 in 1 s. Measuring the flux of neutrinos incident on the Earth makes it possible to judge the processes occurring inside the Sun.

Thus, the source of energy for most stars is hydrogen thermonuclear reactions in the central zone of the star. As a result of a thermonuclear reaction, an outward flow of energy arises in the form of radiation in a wide range of frequencies (wavelengths). The interaction between radiation and matter leads to a steady state of equilibrium: the pressure of the outward radiation is balanced by the pressure of gravity. Further contraction of the star stops as long as enough energy is produced in the center. This state is fairly stable and the size of the star remains constant. Hydrogen is the main component cosmic matter and the most important type of nuclear fuel. A star has enough hydrogen reserves for billions of years. This explains why stars are so stable long time. Until all the hydrogen in the central zone burns out, the properties of the star change little.

The field of hydrogen burnout in the central zone of the star forms a helium core. Hydrogen reactions continue to take place, but only in a thin layer near the surface of the nucleus. Nuclear reactions move to the periphery of the star. The structure of the star at this stage is described by models with a layered energy source. The burnt-out core begins to shrink, and the outer shell expands. The shell swells to colossal proportions, the external temperature becomes low. The star becomes a red giant. From this moment on, the life of a star begins to decline. Red giants are characterized by low temperatures and huge sizes (from 10 to 1000 R s). The average density of matter in them does not even reach 0.001 g/cm 3 . Their luminosity is hundreds of times higher than the luminosity of the Sun, but the temperature is much lower (about 3000 - 4000 K).

It is believed that our Sun, during the transition to the stage of a red giant, can increase so much that it fills the orbit of Mercury. True, the Sun will become a red giant in 8 billion years.

A red giant is characterized by a low external temperature, but a very high internal temperature. With its increase, ever heavier nuclei are included in thermonuclear reactions. At a temperature of 150 million K, helium reactions begin, which are not only a source of energy, but during them the synthesis of heavier chemical elements is carried out. After the formation of carbon in the helium core of a star, the following reactions are possible:

It should be noted that the synthesis of the next heavier nucleus requires more and more high energy. By the time magnesium is formed, all the helium in the core of the star is depleted, and in order for further nuclear reactions to become possible, a new compression of the star and an increase in its temperature are necessary. However, this is not possible for all stars, only for sufficiently large ones, the mass of which exceeds the mass of the Sun by more than 1.4 times (the so-called Chandrasekhar limit). In stars of smaller mass, the reactions end at the stage of magnesium formation. In stars whose mass exceeds the Chandrasekhar limit, due to gravitational contraction, the temperature rises to 2 billion degrees, the reactions continue, forming heavier elements - up to iron. Elements heavier than iron are formed when stars explode.

As a result of increasing pressure, pulsations, and other processes, the red giant continuously loses matter, which is ejected into interstellar space in the form of a stellar wind. When the internal fusion power sources are completely depleted, further fate star depends on its mass.

With a mass less than 1.4 solar masses, the star passes into a stationary state with a very high density (hundreds of tons per 1 cm 3). Such stars are called white dwarfs. In the process of turning a red giant into a white dwarf, the race can shed its outer layers like a light shell, exposing the core. The gaseous envelope glows brightly under the influence of powerful radiation from the star. This is how planetary nebulae are formed. At high densities of matter inside a white dwarf, the electron shells of atoms are destroyed, and the matter of the star is an electron-nuclear plasma, and its electronic component is a degenerate electron gas. White dwarfs are in equilibrium due to the equality of forces between gravity (compression factor) and the pressure of degenerate gas in the interior of the star (expansion factor). White dwarfs can exist for billions of years.

The thermal reserves of the star are gradually depleted, the star is slowly cooling, which is accompanied by ejections of the stellar envelope into interstellar space. The star gradually changes its color from white to yellow, then to red, and finally it ceases to radiate, becomes a small lifeless object, a dead cold star, the size of which smaller sizes Earth, and the mass is comparable to the mass of the Sun. The density of such a star is billions of times greater than the density of water. Such stars are called black dwarfs. This is how most stars end their lives.

When the mass of the star is more than 1.4 solar masses, the stationary state of the star without internal energy sources becomes impossible, because The pressure inside the star cannot balance the force of gravity. Gravitational collapse begins - compression of matter towards the center of the star under the influence of gravitational forces.

If particle repulsion and other causes stop the collapse, then powerful explosion─ a supernova explosion with the ejection of a significant part of the matter into the surrounding space and the formation of gaseous nebulae. The name was proposed by F. Zwicky in 1934. A supernova explosion is one of the intermediate stages in the evolution of stars before they turn into white dwarfs, neutron stars or black holes. An explosion releases energy of 10 43 ─ 10 44 J at a radiation power of 10 34 W. In this case, the brightness of the star increases by tens of magnitudes in a few days. The luminosity of a supernova can exceed the luminosity of the entire galaxy in which it burst.

The gaseous nebula formed during a supernova explosion consists partly of the upper layers of the star ejected by the explosion, and partly of interstellar matter, compacted and heated by the expanding products of the explosion. The most famous gaseous nebula is the Crab Nebula in the constellation Taurus - the remnant of the supernova of 1054. Young supernova remnants are expanding at speeds of 10-20 thousand km / s. The collision of the expanding shell with the stationary interstellar gas generates a shock wave in which the gas heats up to millions of Kelvin and becomes a source of X-rays. The propagation of a shock wave in a gas leads to the appearance of fast charged particles (cosmic rays), which, moving in an interstellar magnetic field compressed and enhanced by the same wave, radiate in the radio range.

Astronomers recorded supernova explosions in 1054, 1572, 1604. In 1885, a supernova was observed in the Andromeda Nebula. Its brightness exceeded the brightness of the entire Galaxy and turned out to be 4 billion times more intense than the brightness of the Sun.

Already by 1980, more than 500 supernova explosions had been discovered, but not a single one was observed in our Galaxy. Astrophysicists have calculated that supernovae in our Galaxy flare with a period of 10 million years in the immediate vicinity of the Sun. On average, a supernova explosion occurs in the Metagalaxy every 30 years.

In this case, doses of cosmic radiation on Earth can exceed the normal level by 7000 times. This will lead to the most serious mutations in living organisms on our planet. Some scientists explain the sudden death of dinosaurs in this way.

Part of the mass of an exploded supernova may remain in the form of a superdense body - a neutron star or a black hole. The mass of neutron stars is (1.4 - 3) M s, the diameter is about 10 km. The density of a neutron star is very high, higher than the density of atomic nuclei ─ 10 15 g/cm 3 . With an increase in compression and pressure, the reaction of absorption of electrons by protons becomes possible As a result, all the matter of the star will consist of neutrons. The neutronization of a star is accompanied by a powerful burst of neutrino radiation. During the burst of supernova SN1987A, the duration of the neutrino burst was 10 s, and the energy carried away by all neutrinos reached 3∙10 46 J. The temperature of a neutron star reaches 1 billion K. Neutron stars cool very quickly, their luminosity weakens. But they intensely radiate radio waves in a narrow cone in the direction of the magnetic axis. Stars whose magnetic axis does not coincide with the axis of rotation are characterized by radio emission in the form of repetitive pulses. Therefore, neutron stars are called pulsars. The first pulsars were discovered in 1967. The frequency of radiation pulsations, determined by the speed of rotation of the pulsar, is from 2 to 200 Hz, which indicates their small size. For example, the pulsar in the Crab Nebula has a pulse period of 0.03 s. There are currently hundreds of neutron stars known. A neutron star may appear as a result of the so-called "silent collapse". If a white dwarf enters a binary system of closely spaced stars, then the phenomenon of accretion occurs when matter from a neighboring star flows onto a white dwarf. The mass of the white dwarf grows and at some point exceeds the Chandrasekhar limit. A white dwarf turns into a neutron star.

If the final mass of the white dwarf exceeds 3 solar masses, then the degenerate neutron state is unstable, and gravitational contraction continues until an object called a black hole is formed. The term "black hole" was introduced by J. Wheeler in 1968. However, the concept of such objects arose several centuries earlier, after the discovery by I. Newton in 1687 of the law gravity. In 1783, J. Mitchell suggested that dark stars must exist in nature, the gravitational field of which is so strong that light cannot escape from them. In 1798 the same idea was expressed by P. Laplace. In 1916, the physicist Schwarzschild, solving Einstein's equations, came to the conclusion about the possibility of the existence of objects with unusual properties, later called black holes. A black hole is a region of space in which the gravitational field is so strong that the second cosmic velocity for bodies located in this region must exceed the speed of light, i.e. nothing can escape from a black hole, neither particles nor radiation. In accordance with general theory In relativity, the characteristic size of a black hole is determined by the gravitational radius: R g =2GM/c 2 , where M is the mass of the object, c is the speed of light in vacuum, and G is the gravitational constant. The gravitational radius of the Earth is 9 mm, the Sun is 3 km. The boundary of the region beyond which no light escapes is called the event horizon of a black hole. Rotating black holes have an event horizon radius smaller than the gravitational radius. Of particular interest is the possibility of capture by a black hole of bodies arriving from infinity.

The theory allows the existence of black holes with a mass of 3–50 solar masses, which are formed in the late stages of the evolution of massive stars with a mass of more than 3 solar masses, supermassive black holes in the nuclei of galaxies with a mass of millions and billions of solar masses, primordial (relic) black holes formed in the early stages of the evolution of the universe. Relic black holes weighing more than 10 15 g (mass middle mountain on Earth) due to the mechanism of quantum evaporation of black holes proposed by S.W.Hawking.

Astronomers detect black holes by powerful x-rays. An example of this type of star is the powerful X-ray source Cygnus X-1, whose mass exceeds 10 M s. Often black holes are found in X-ray binary star systems. Dozens of stellar-mass black holes have already been discovered in such systems (m black hole = 4-15 M s). Based on the effects of gravitational lensing, several single stellar-mass black holes (m black holes = 6-8 M s) have been discovered. In case of close double star the phenomenon of accretion is observed - the flow of plasma from the surface ordinary star under the influence of gravitational forces on a black hole. Matter flowing into a black hole has an angular momentum. Therefore, the plasma forms a rotating disk around the black hole. The temperature of the gas in this rotating disk can reach 10 million degrees. At this temperature, the gas emits in the X-ray range. From this radiation it is possible to determine the presence in this place black hole.

Of particular interest are supermassive black holes in the cores of galaxies. Based on the study of the X-ray image of the center of our Galaxy, obtained with the help of the CHANDRA satellite, the presence of a supermassive black hole, the mass of which is 4 million times greater than the mass of the Sun, has been established. As a result of recent research, American astronomers have discovered a unique superheavy black hole located in the center of a very distant galaxy, the mass of which is 10 billion times the mass of the Sun. In order to reach such unimaginably huge sizes and density, a black hole had to form over many billions of years, continuously attracting and absorbing matter. Scientists estimate its age at 12.7 billion years, i.e. it began to form about one billion years after the Big Bang. To date, more than 250 supermassive black holes have been discovered in the nuclei of galaxies (m black holes = (10 6 – 10 9) M s).

The question of the origin of chemical elements is closely related to the evolution of stars. If hydrogen and helium are elements that are left over from the early stages of the evolution of the expanding universe, then heavier chemical elements could only be formed in the interiors of stars during thermonuclear reactions. Inside stars during thermonuclear reactions, up to 30 chemical elements (including iron) can be formed.

In my own way physical condition stars can be divided into normal and degenerate. The former consist mainly of low-density matter; thermonuclear fusion reactions take place in their depths. Degenerate stars include white dwarfs and neutron stars, they represent the final stage of stellar evolution. The fusion reactions in them have ended, and the equilibrium is maintained by the quantum-mechanical effects of degenerate fermions: electrons in white dwarfs and neutrons in neutron stars. White dwarfs, neutron stars, and black holes are collectively referred to as "compact remnants".

At the end of evolution, depending on the mass, the star either explodes or releases more calmly matter, already enriched with heavy chemical elements. This creates the rest of the elements. periodic system. From the interstellar medium enriched with heavy elements, the stars of the next generations are formed. For example, the Sun is a second-generation star formed from matter that has already been in the interiors of stars and enriched with heavy elements. Therefore, the age of stars can be judged from their chemical composition determined by spectral analysis.