Studying stellar evolution is impossible by observing just one star - many changes in stars occur too slowly to be noticed even after many centuries. Therefore, scientists study many stars, each of which is at a certain stage of its life cycle. Over the past few decades, modeling of the structure of stars using computer technology has become widespread in astrophysics.
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Thermonuclear fusion in the interior of stars
Young stars
The process of star formation can be described in a unified way, but according to next stages The evolution of a star depends almost entirely on its mass, and only at the very end of the star’s evolution can its chemical composition play a role.
Young low mass stars
Young low-mass stars (up to three solar masses) [ ], which are approaching the main sequence, are completely convective - the convection process covers the entire body of the star. These are essentially protostars, in the centers of which they are just beginning nuclear reactions, and all radiation occurs mainly due to gravitational compression. Until hydrostatic equilibrium is established, the star's luminosity decreases at a constant effective temperature. On the Hertzsprung-Russell diagram, such stars form an almost vertical track called the Hayashi track. As the compression slows, the young star approaches the main sequence. Objects of this type are associated with T Tauri 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 becomes predominant, since convection is increasingly hampered by the increasing compaction of stellar matter. In the outer layers of the star’s body, convective energy transfer prevails.
It is not known for certain what characteristics do stars of lower mass have at the moment they enter the main sequence, since the time these stars spent in the young category exceeds the age of the Universe [ ] . All ideas about the evolution of these stars are based only on numerical calculations and mathematical modeling.
As the star contracts, the pressure of the degenerate electron gas begins to increase and when a certain radius of the star is reached, the compression stops, which leads to a stop in the further increase in temperature in the core of the star caused by the compression, and then to its decrease. For stars smaller than 0.0767 solar masses, this does not happen: the energy released during nuclear reactions is never enough to balance the internal pressure and gravitational compression. Such “understars” emit more energy than is produced during thermonuclear reactions, and are classified as so-called brown dwarfs. Their fate is constant compression until the pressure of the degenerate gas stops it, and then gradual cooling with the cessation of all thermonuclear reactions that have begun.
Young intermediate mass stars
Young stars of intermediate mass (from 2 to 8 solar masses) [ ] evolve qualitatively in exactly the same way as their smaller sisters and brothers, with the exception that they do not have convective zones up to the main sequence.
Objects of this type are associated with the so-called. Ae\Be Herbig stars with irregular variables of spectral class B-F0. They also exhibit disks and bipolar jets. The rate of outflow of matter from the surface, luminosity and effective temperature are significantly higher than for T Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.
Young stars with a mass greater than 8 solar masses
Stars with such masses already have the characteristics of normal stars, since they went through all the intermediate stages and were able to achieve such a rate of nuclear reactions that compensated for the energy lost to radiation while mass accumulated to achieve hydrostatic equilibrium of the core. For these stars, the outflow of mass and luminosity are so great that they not only stop the gravitational collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, disperse them away. Thus, the mass of the resulting star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars with a mass greater than about 300 solar masses.
Mid-life cycle of a star
Stars come in a wide variety of colors and sizes. By spectral class they range from hot blue to cold red, by mass - from 0.0767 to about 300 solar masses. latest estimates. The luminosity and color of a star depend on its surface temperature, 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. Naturally, we are not talking about the physical movement of the star - only about its position on the indicated diagram, depending on the parameters of the star. In fact, the movement of a star along the diagram corresponds only to a change in the parameters of the star.
The thermonuclear “burning” of matter, resumed at a new level, causes a monstrous expansion of the star. The star "swells", becoming very "loose", and its size increases approximately 100 times. So the star becomes a red giant, and the helium burning phase lasts about several million years. Almost all red giants are variable stars.
Final stages of stellar evolution
Old stars with low mass
At present, it is not known for certain what happens to light stars after the supply of hydrogen in their cores is depleted. Since the age of the Universe is 13.7 billion years, which is not enough for the hydrogen fuel supply in such stars to be depleted, modern theories are based on computer modeling processes occurring in such stars.
Some stars can only synthesize helium in certain active zones, causing instability and strong stellar 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 [ ] .
A star with a mass less than 0.5 solar is not able to convert helium even after reactions involving hydrogen stop in its core - the mass of such a star is too small to provide a new phase of gravitational compression to a degree sufficient to “ignite” helium Such stars include red dwarfs, such as Proxima Centauri, whose residence time on the main sequence ranges from tens of billions to tens of trillions of years. After the cessation of thermonuclear reactions in their cores, they, gradually cooling, will continue to weakly emit in the infrared and microwave ranges of the electromagnetic spectrum.
Medium sized stars
Upon reaching star average size(from 0.4 to 3.4 solar masses) [ ] of the red giant phase, hydrogen runs out in its core, and reactions of synthesis of carbon from helium begin. This process occurs at more high temperatures and therefore the flow of energy from the core increases and, as a result, the outer layers of the star begin to expand. The beginning of carbon synthesis marks a new stage in the life of a star and continues for some time. 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. Energy output shifts towards low frequency radiation. All this is accompanied by increasing mass loss due to strong stellar winds and intense pulsations. Stars in this phase are called “late-type stars” (also “retired 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, making possible education dust particles and molecules. With strong infrared radiation from the source star, ideal conditions for the activation of cosmic masers are formed in such shells.
Thermonuclear combustion reactions of helium are very sensitive to temperature. Sometimes this leads to great instability. Strong pulsations arise, which as a result impart sufficient acceleration to the outer layers to be thrown off and turn into a planetary nebula. In the center of such a nebula there remains a bare core of the star, in which the thermonuclear reactions, and as it cools, it turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar masses and a diameter on the order of the diameter of the Earth.
The vast majority of stars, including the Sun, complete their evolution by contracting until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a hundred times, and the density becomes a million times higher than the density of water, the star is called a white dwarf. It is deprived of energy sources and, gradually cooling, becomes an invisible black dwarf.
In stars more massive than the Sun, the pressure of degenerate electrons cannot stop further compression nuclei, and electrons begin to “press” into atomic nuclei, which turns protons into neutrons, between which there are no electrostatic repulsion forces. This neutronization of matter leads to the fact that the size of the star, which is now, in fact, one huge atomic nucleus, is measured in several kilometers, and its density is 100 million times higher than the density of water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.
Supermassive stars
After a star with a mass greater than five solar masses enters the red supergiant stage, its core begins to shrink under the influence of gravity. As compression increases, temperature and density increase, and new sequence thermonuclear reactions. In such reactions, increasingly heavier elements are synthesized: helium, carbon, oxygen, silicon and iron, which temporarily restrains the collapse of the core.
As a result, as increasingly heavier elements of the Periodic Table are formed, iron-56 is synthesized from silicon. At this stage, further exothermic thermonuclear fusion becomes impossible, since the iron-56 nucleus has a maximum mass defect and the formation of heavier nuclei with the release of energy is impossible. Therefore, when the iron core of a star reaches a certain size, the pressure in it is no longer able to withstand the weight of the overlying layers of the star, and immediate collapse of the core occurs with neutronization of its matter.
What happens next is not yet completely clear, but, in any case, the processes taking place in a matter of seconds lead to a supernova explosion of incredible power.
Strong neutrino jets and a rotating magnetic field push out much of the star's accumulated material. [ ] - so-called seating elements, including iron and lighter elements. The exploding matter is bombarded by neutrons escaping from the stellar core, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and perhaps even californium). Thus, supernova explosions explain the presence in interstellar matter elements heavier than iron, but this is not the only possible way their formation, which, for example, is demonstrated by technetium stars.
blast wave And jets of neutrinos carry matter away from the dying star [ ] into interstellar space. Subsequently, as it cools and moves through space, this supernova material can collide with other cosmic “salvage” and, possibly, participate in the formation of new stars, planets or satellites.
The processes occurring during the formation of a supernova are still being studied, and so far there is no clarity on this issue. Also questionable is what actually remains of the original star. However, two options are being considered: neutron stars and black holes.
Neutron stars
It is known that in some supernovae, strong gravity in the depths of the supergiant forces electrons to be absorbed by the atomic nucleus, where they merge with protons to form neutrons. This process is called neutronization. Electromagnetic forces, separating nearby nuclei, disappear. The star's core is now a dense ball of atomic nuclei and individual neutrons.
Such stars, known as neutron stars, are extremely small - no more than the size of a large city - and have an unimaginably high density. Their orbital period becomes extremely short as the size of the star decreases (due to the conservation of angular momentum). Some neutron stars rotate 600 times per second. For some of them, the angle between the radiation vector and the axis of rotation may be such that the Earth falls into the cone formed by this radiation; in this case, it is possible to detect a radiation pulse repeating at intervals equal to the star’s orbital period. Such neutron stars were called “pulsars”, and became the first neutron stars to be discovered.
Black holes
Not all stars, after going through the supernova explosion phase, become neutron stars. If the star has a sufficiently large mass, then the collapse of such a star will continue, and the neutrons themselves will begin to fall inward until its radius becomes less than the Schwarzschild radius. After this, the star becomes a black hole.
The existence of black holes was predicted by the general theory of relativity. According to this theory,
The evolution of stars is a change in physicality. characteristics, internal structures and chemistry composition of stars over time. The most important tasks theories of E.z. - explanation of the formation of stars, changes in their observable characteristics, research genetic connection various groups 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, explanation by 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 bowels 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 its catastrophe. compression.
Isolation of gravitational energy can play a decisive role only when the temperature of the star’s interior is insufficient for nuclear energy release to compensate for energy losses, and the star as a whole or part of it must contract to maintain equilibrium. The release of thermal energy becomes important only after nuclear energy reserves have been exhausted. T.o., E.z. can be represented as a consistent change in the energy sources of stars.
Characteristic time E.z. too large for all evolution to be traced directly. Therefore the main E.Z. research method yavl. construction of sequences of star models describing changes in internal structures and chemistry composition of stars over time. Evolution. the sequences are then compared with the results of observations, for example, with (G.-R.d.), summing up the observations large number stars at different stages of evolution. Especially important role plays a comparison with G.-R.d. for star clusters, since all stars in a cluster have the same initial chemical. 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. Evolution in detail. sequences are calculated by numerically solving a system of differential equations describing the distribution of mass, density, temperature and luminosity over a star, to which are added the laws of energy release and opacity of stellar matter and equations describing changes in chemical properties. star composition over time.
The course of a star's evolution depends mainly on its mass and initial chemistry. composition. The rotation of the star and its magnetic field can play a certain, but not fundamental, role. field, however, the role of these factors in E.Z. has not yet been sufficiently researched. Chem. The composition of a star depends on the time at which it was formed and on its position in the Galaxy at the time of formation. Stars of the first generation were formed from matter, the composition of which was determined by cosmology. conditions. Apparently, it contained approximately 70% by mass hydrogen, 30% helium and an insignificant admixture of deuterium and lithium. During the evolution of first-generation stars, heavy elements (following helium) were formed, which were ejected into interstellar space as a result of the outflow of matter from stars or during stellar explosions. Stars of subsequent generations were formed from matter containing up to 3-4% (by mass) of heavy elements.
The most direct indication that star formation in the Galaxy is still ongoing is the phenomenon. 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 times. era is estimated at 5 per year.
2. Star formation, stage of gravitational compression
According to the most common point of view, stars are formed as a result of gravitational forces. condensation of matter in the interstellar medium. The necessary division of the interstellar medium into two phases - dense cold clouds and a rarefied medium with a higher temperature - can occur under the influence of 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 . are actually observed due to their emission of radio waves. Compression (collapse) of such clouds requires certain conditions: gravity. particles of the cloud must exceed the sum of the energy of thermal motion of the particles, the rotational energy of the cloud as a whole and the magnetic field. cloud energy (Jeans criterion). If only the energy of thermal motion is taken into account, then, accurate to a factor of the order of unity, the Jeans criterion is written in the form: align="absmiddle" width="205" height="20">, where is the mass of the cloud, T- gas temperature in K, n- number of particles per 1 cm3. With typical modern interstellar clouds temperature K can only collapse clouds with a mass not less than . The Jeans criterion indicates that for the formation of stars of the actually observed mass spectrum, the concentration of particles in collapsing clouds must 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 it happens through a sequential process, carried out in several steps. stages, fragmentation of massive clouds. This picture naturally explains the birth of stars in groups - clusters. At the same time, questions related to the thermal balance in the cloud, the velocity field in it, and the mechanism determining the mass spectrum of fragments still remain unclear.
Collapsed stellar mass objects are called protostars. Collapse of a spherically symmetric non-rotating protostar without a magnetic field. fields includes several. stages. At the initial moment of time, the cloud is homogeneous and isothermal. It is transparent to its own. radiation, so the collapse comes with volumetric energy losses, Ch. arr. due to the thermal radiation of the dust, the cut transmits its kinetic. energy of a gas particle. In a homogeneous cloud there is no pressure gradient and compression begins in free fall with characteristic time, Where G- , - cloud density. With the beginning of compression, a rarefaction wave appears, moving towards the center at the speed of sound, and since collapse occurs faster where the density is higher, the protostar is divided into a compact core and an extended shell, into 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 to the IR radiation of dust grains. The energy released in the core slowly seeps to the surface due to radiative thermal conduction. The temperature begins to increase almost adiabatically, this leads to an increase in pressure, and the core becomes hydrostatic. balance. The shell continues to fall onto the core, and it appears at its periphery. The kernel parameters at this time weakly depend on total mass protostars: 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 adiabatic index value becomes less than 4/3, pressure changes are not able to compensate for gravitational forces and the core collapses again (see). A new core with parameters is formed, surrounded shock front, onto which the remains of the first core are accreted. A similar rearrangement of the nucleus occurs with hydrogen.
Further growth of the core at the expense of the shell matter continues until all the matter falls onto the star or is scattered under the influence of or, if the core is sufficiently massive (see). Protostars with a characteristic time of shell matter t a >t kn, therefore their luminosity is determined by the energy release of the collapsing nuclei.
A star, consisting of a core and an envelope, is observed as an IR source due to the processing of radiation in the envelope (the dust of the envelope, absorbing photons of UV radiation from the core, emits in the IR range). When the shell becomes optically thin, the protostar begins to be observed as an ordinary object of stellar nature. The most massive stars retain their shells until thermonuclear burning of hydrogen begins at the center of the star. Radiation pressure limits the mass of stars to probably . Even if more massive stars are formed, they turn out to be pulsationally unstable and may lose their power. part of the mass at the stage of hydrogen combustion in the core. The duration of the stage of collapse and scattering of the protostellar shell is of the same order as the free fall time for the parent cloud, i.e. 10 5 -10 6 years. Illuminated by the core, clumps of dark matter from the remnants of the shell, accelerated by the stellar wind, are identified with Herbig-Haro objects (stellar clumps with an emission spectrum). Low-mass stars, when they become visible, are in the G.-R.D. region occupied by T Tauri stars (dwarf), more massive ones are in the region where Herbig emission stars are located (irregular early spectral classes with emission lines in spectra).Evolution. tracks of protostar cores with constant mass at the hydrostatic stage. compressions are shown in Fig. 1. For stars of low mass, at the moment when hydrostatic is established. equilibrium, the conditions in the nuclei are such that energy is transferred to 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 continues to shrink. 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 vertical sections of tracks.
As the compression continues, the temperature in the interior of the star increases, 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 completely convective. Their luminosity is controlled by a thin radiant layer in the photosphere. The more massive the star and the higher its effective temperature, the larger its radiative core (in stars with align="absmiddle" width="74" height="17"> the radiative core appears immediately). In the end, almost the entire star (with the exception of the surface convective zone for stars with a mass) goes into a state of radiative equilibrium, in which all the energy released in the core is transferred by radiation.
3. Evolution based on nuclear reactions
At a temperature in the nuclei of ~ 10 6 K, the first nuclear reactions begin - deuterium, lithium, boron burn out. The primary quantity of these elements is so small that their burnout practically does not withstand compression. The compression stops when the temperature at the center of the star reaches ~ 10 6 K and hydrogen ignites, because The energy released during thermonuclear combustion of hydrogen is sufficient to compensate for radiation losses (see). Homogeneous stars, in the cores of which hydrogen burns, form on the G.-R.D. initial main sequence (IMS). Massive stars reach the NGP faster than low-mass stars, because their rate of energy loss per unit mass, and therefore the rate of evolution, is higher than that of low-mass stars. Since entering the NGP E.z. occurs on the basis of nuclear combustion, the main stages of which are summarized in table. Nuclear combustion can occur before the formation of iron group elements, which have the highest binding energy among all nuclei. Evolution. tracks of stars on G.-R.D. are shown in Fig. 2. Evolution central values the temperature and density of stars is shown in Fig. 3. At K main. source of energy yavl. reaction of the hydrogen cycle, at large T- reactions of the carbon-nitrogen (CNO) cycle (see). Side effect CNO cycle phenomenon establishing equilibrium concentrations of nuclides 14 N, 12 C, 13 C - 95%, 4% and 1% by weight, respectively. The predominance of nitrogen in the layers where hydrogen combustion occurred is confirmed by the results of observations, in which these layers appear on the surface as a result of the loss of external. layers. In stars in the center of which the CNO cycle is realized ( align="absmiddle" width="74" height="17">), a convective core appears. The reason for this is very strong addiction energy release depending on temperature: . The flow of radiant energy ~ T 4(see), therefore, it cannot transfer all the energy released, and convection must occur, which is more efficient than radiative transfer. In the most massive stars, more than 50% of the stellar mass is covered by convection. The importance of the convective core for evolution is determined by the fact that nuclear fuel is uniformly depleted in a region much larger than the region of effective combustion, while in stars without a convective core it initially burns out only in a small vicinity of the center, where the temperature is quite high. The hydrogen burnout time ranges from ~ 10 10 years for to years for . The time of all subsequent stages of nuclear combustion does not exceed 10% of the time of hydrogen combustion, therefore stars at the stage of hydrogen combustion form on the G.-R.D. densely populated region - (GP). In stars with a temperature in the center that never reaches the values necessary for the combustion of hydrogen, they shrink indefinitely, turning into “black” dwarfs. Burnout of hydrogen leads to an increase in avg. molecular weight of the core substance, and therefore to maintain hydrostatic. equilibrium, the pressure in the center must increase, which entails an increase in the temperature in the center and the temperature gradient across the star, and consequently, the luminosity. An increase in luminosity also results from a decrease in the opacity of matter with increasing temperature. The core contracts to maintain nuclear energy release conditions with a decrease in hydrogen content, and the shell expands due to the need to transfer the increased energy flow from the core. On G.-R.d. the star moves to the right of the NGP. A decrease in opacity leads to the death of convective cores in all but the most massive stars. The rate of evolution of massive stars is the highest, and they are the first to leave the MS. The lifetime on the MS is for stars with ca. 10 million years, from ca. 70 million years, and from ca. 10 billion years.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 a general contraction of the star 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 arises. In stars with mass , in which it depends less 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 burns out depends on their mass. The most important factor, influencing the course of evolution of stars with mass , yavl. degeneration of electron gas at high densities. In due 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, significantly exceeding the energy of their thermal motion. Key Feature degenerate gas is that its pressure p depends only on the density: for non-relativistic degeneracy and for relativistic degeneracy. The gas pressure of electrons is much greater than the pressure of ions. This follows what is fundamental for E.Z. conclusion: since the gravitational force acting on a unit volume of a relativistically degenerate gas depends on density in the same way as the pressure gradient, there must be a limiting mass (see), such that at align="absmiddle" width="66" height ="15"> electron pressure cannot counteract gravity and compression begins. Limit weight 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 a noticeable role already in the process of formation of helium nuclei.
The second factor determining E.z. at later stages, these are neutrino energy losses. In the depths of the stars T~10 8 K main. a role in the birth is played by: photoneutrino process, decay of plasma oscillation quanta (plasmons) into neutrino-antineutrino pairs (), annihilation of electron-positron pairs () and (see). The most important feature of neutrinos is that the star’s matter is almost transparent to them and neutrinos freely carry energy away from the star.
The helium core, in which 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 combustion increases. The need to transfer an increased energy flow leads to expansion of the shell, for which part of the energy is wasted. Since the luminosity of the star does not change, the temperature of its surface drops, and on the G.-R.D. the star moves to the region occupied by red giants. The star's restructuring time is two orders of magnitude less than the time it takes for hydrogen to burn out in the core, so there are few stars between the MS strip 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 appearance appears. convective zone and the luminosity of the star increases.
The removal of energy from the core through the thermal conductivity of degenerate electrons and neutrino losses in stars delays the moment of helium combustion. The temperature begins to increase noticeably only when the core becomes almost isothermal. Combustion of 4 He determines the E.Z. from the moment when the energy release exceeds the energy loss through thermal conductivity and neutrino radiation. The same condition applies to the combustion of all subsequent types of nuclear fuel.
A remarkable feature of stellar cores made of degenerate gas, cooled by neutrinos, is “convergence” - the convergence of tracks, which characterize the relationship between density and temperature Tc in the center of the star (Fig. 3). The rate of energy release during compression of the core is determined by the rate of addition of matter to it through a layer source, and depends only on the mass of the core for a given type of fuel. A balance of inflow and outflow of energy must be maintained in the core, therefore the same distribution of temperature and density is established in the cores of stars. By the time 4 He ignites, the mass of the nucleus depends on the content of heavy elements. In nuclei of degenerate gas, the combustion of 4 He has the character of a thermal explosion, because the energy released during combustion goes to increase the energy of the thermal motion of electrons, but the pressure remains almost unchanged 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 flares are likely accompanied by the loss of stellar matter. In , where massive stars have long finished evolution and red giants have masses, stars at the helium burning stage are on the horizontal branch of the G.-R.D.
In the 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 Tc. In the most massive stars, the combustion of 4 He occurs even when they are active. blue supergiants. Expansion of the core leads to a decrease T in the region of the hydrogen layer source, and the luminosity of the star after the helium burst decreases. To maintain thermal equilibrium, the shell contracts, and the star leaves the region of red supergiants. When the 4 He in the core is depleted, compression of the core and expansion of the shell begin again, the star again becomes a red supergiant. A layered combustion source of 4 He is formed, which dominates the energy release. External appears again. convective zone. As helium and hydrogen burn out, the thickness of the layer 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 disturbances in the combustion layer. During thermal outbreaks, 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.
Radiation pressure on dust and molecules formed in the cold, extended shells of red supergiants leads to continuous loss of matter at a rate of up to a year. Continuous mass loss can be supplemented by losses caused by instability of layer combustion or pulsations, which can lead to the release of one or more. shells. When the amount of substance 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 compress until the compression is capable of maintaining combustion; star on G.-R.D. moves 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. While the star is hot enough, it is observed as a core with one or more. shells. When layer sources shift toward the surface of the star so much that the temperature in them becomes lower than that required for nuclear combustion, the star cools, turning into a white dwarf with , radiating due to the consumption of thermal energy of the ionic component of its matter. The characteristic cooling time of white dwarfs is ~ 10 9 years. The lower limit on the masses of single stars turning into white dwarfs is unclear, it is estimated at 3-6. In c stars, the 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, a “convergence” of conditions occurs in the center and at the moment of combustion of carbon in the C,O core. The combustion of 12 C under such conditions most likely has the nature 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 satellite matter in a close binary system.
Life cycle of stars
A typical star releases energy by fusing hydrogen into helium in a nuclear furnace at its core. After the star uses up hydrogen in the center, it begins to burn out in the shell of the star, which increases in size and swells. The size of the star increases, its temperature decreases. This process gives rise to red giants and supergiants. The lifespan of each star is determined by its mass. Massive stars end their life cycle with an explosion. Stars like the Sun shrink, becoming dense white dwarfs. During the process of transforming from a red giant to a white dwarf, a star can shed its outer layers like a light gas shell, exposing the core.
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Evolution of Stars of Different Masses
Astronomers cannot observe the life of a single star from beginning to end, because even the shortest-lived stars exist for millions of years - longer life of all humanity. Change over time physical characteristics And chemical composition stars, i.e. Astronomers study stellar evolution by comparing the characteristics of many stars at different stages of evolution.
Physical patterns connecting the observed characteristics of stars are reflected in the color-luminosity diagram - the Hertzsprung - Russell diagram, on which the stars form separate groups - sequences: the main sequence of stars, sequences of supergiants, bright and faint giants, subgiants, subdwarfs and white dwarfs.
For most of its life, any star is on the so-called main sequence of the color-luminosity diagram. All other stages of the star's evolution before the formation of a compact remnant take no more than 10% of this time. This is why most of the stars observed in our Galaxy are modest red dwarfs with the mass of the Sun or less. The main sequence contains about 90% of all observed stars.
The lifespan of a star and what it turns into at the end life path, is completely determined by its mass. Stars with masses greater than the Sun live much less than the Sun, and the lifetime of the most massive stars is only millions of years. For the vast majority of stars, the lifetime is about 15 billion years. After a star exhausts its energy sources, it begins to cool and contract. The end product of stellar evolution is compact, massive objects whose density is many times greater than that of ordinary stars.
Stars of different masses end up in one of three states: white dwarfs, neutron stars or black holes. If the mass of the star is small, then the gravitational forces are relatively weak and the compression of the star (gravitational collapse) stops. It transitions to a stable white dwarf state. If the mass exceeds a critical value, compression continues. At very high density electrons combine with protons to form neutrons. Soon, almost the entire star consists of only neutrons and has such an enormous density that the huge stellar mass is concentrated in a very small ball with a radius of several kilometers and the compression stops - a neutron star is formed. If the mass of the star is so great that even the formation of a neutron star will not stop the gravitational collapse, then the final stage of the star’s evolution will be a black hole.
Each of us has looked at the starry sky at least once in our lives. Someone looked at this beauty, experiencing romantic feelings, another tried to understand where all this beauty comes from. Life in space, unlike life on our planet, flows at a different speed. Time in outer space lives in its own categories, the distances and sizes in the Universe are colossal. We rarely think about the fact that the evolution of galaxies and stars is constantly happening before our eyes. Every object in endless space is a consequence of a certain physical processes. Galaxies, stars and even planets have main phases of development.
Our planet and we all depend on our star. How long will the Sun delight us with its warmth, breathing life into the Solar System? What awaits us in the future after millions and billions of years? In this regard, it is interesting to learn more about what are the stages of evolution of astronomical objects, where stars come from and how the life of these wonderful luminaries in the night sky ends.
Origin, birth and evolution of stars
The evolution of the stars and planets inhabiting our Milky Way galaxy and the entire Universe, for the most part well studied. In space, the laws of physics are unshakable, which help to understand the origin space objects. Rely on in this case adopted by the Big Bang theory, which is now the dominant doctrine about the process of the origin of the Universe. The event that shook the universe and led to the formation of the universe is, by cosmic standards, lightning fast. For the cosmos, moments pass from the birth of a star to its death. Vast distances create the illusion of the constancy of the Universe. A star that flares up in the distance shines on us for billions of years, at which time it may no longer exist.
The theory of evolution of the galaxy and stars is a development of the Big Bang theory. The doctrine of the birth of stars and the emergence star systems differs in the scale of what is happening and the time frame, which, unlike the Universe as a whole, can be observed modern means Sciences.
When studying the life cycle of stars, you can use the example of the closest star to us. The Sun is one of hundreds of trillions of stars in our field of vision. In addition, the distance from the Earth to the Sun (150 million km) provides a unique opportunity to study the object without leaving solar system. The information obtained will make it possible to understand in detail how other stars are structured, how quickly these gigantic heat sources are depleted, what are the stages of development of a star, and what will be the ending of this brilliant life - quiet and dim or sparkling, explosive.
After big bang tiny particles formed interstellar clouds, which became a “maternity hospital” for trillions of stars. It is characteristic that all stars were born at the same time as a result of compression and expansion. Compression in the clouds of cosmic gas occurred under the influence of its own gravity and similar processes in new stars in the neighborhood. The expansion arose as a result of the internal pressure of interstellar gas and under the influence of magnetic fields within the gas cloud. At the same time, the cloud rotated freely around its center of mass.
The gas clouds formed after the explosion consist of 98% atomic and molecular hydrogen and helium. Only 2% of this massif consists of dust and solid microscopic particles. Previously it was believed that at the center of any star lies a core of iron, heated to a temperature of a million degrees. It was this aspect that explained the gigantic mass of the star.
In confrontation physical strength compression forces prevailed, since the light resulting from the release of energy does not penetrate into the gas cloud. The light, along with part of the released energy, spreads outward, creating a subzero temperature and a low-pressure zone inside the dense accumulation of gas. Being in this state, the cosmic gas rapidly contracts, the influence of gravitational attraction forces leads to the fact that particles begin to form stellar matter. When a gas accumulation is dense, intense compression leads to the formation of star cluster. When the size of the gas cloud is small, compression leads to the formation of a single star.
A brief description of what is happening is that the future star goes through two stages - fast and slow compression to the state of a protostar. To put it simply and in clear language, fast compression is the fall of stellar matter towards the center of the protostar. Slow compression occurs against the background of the formed center of the protostar. Over the next hundreds of thousands of years, the new formation shrinks in size, and its density increases millions of times. Gradually, the protostar becomes opaque due to the high density of stellar matter, and the ongoing compression triggers the mechanism of internal reactions. An increase in internal pressure and temperature leads to the formation of the future star’s own center of gravity.
The protostar remains in this state for millions of years, slowly giving off heat and gradually shrinking, decreasing in size. As a result, the contours of the new star emerge, and the density of its matter becomes comparable to the density of water.
On average, the density of our star is 1.4 kg/cm3 - almost the same as the density of water in the salty Dead Sea. At the center, the Sun has a density of 100 kg/cm3. Stellar matter is not in liquid state, but exists in the form of plasma.
Under the influence of enormous pressure and temperature of approximately 100 million K, thermonuclear reactions of the hydrogen cycle begin. The compression stops, the mass of the object increases when the gravitational energy transforms into thermonuclear combustion of hydrogen. From this moment on, the new star, emitting energy, begins to lose mass.
The above-described version of star formation is just a primitive diagram that describes the initial stage of the evolution and birth of a star. Today, such processes in our galaxy and throughout the Universe are practically invisible due to the intense depletion of stellar material. In the entire conscious history of observations of our Galaxy, only isolated appearances of new stars have been noted. On the scale of the Universe, this figure can be increased hundreds and thousands of times.
For most of their lives, protostars are hidden from the human eye by a dusty shell. The radiation from the core can only be observed in the infrared, which is the only way to see the birth of a star. For example, in the Orion Nebula in 1967, astrophysicists discovered in the infrared range new star, the radiation temperature of which was 700 degrees Kelvin. Subsequently, it turned out that the birthplace of protostars are compact sources that exist not only in our galaxy, but also in other distant corners of the Universe. Besides infrared radiation The birthplaces of new stars are marked by intense radio signals.
The process of studying and the evolution of stars
The entire process of knowing the stars can be divided into several stages. At the very beginning, you should determine the distance to the star. Information about how far the star is from us and how long the light has been coming from it gives an idea of what happened to the star throughout this time. After man learned to measure the distance to distant stars, it became clear that stars are the same as suns, only different sizes and with different fates. Knowing the distance to the star, the level of light and the amount of energy emitted can be used to trace the process of thermonuclear fusion of the star.
After determining the distance to the star, you can use spectral analysis to calculate the chemical composition of the star and find out its structure and age. Thanks to the advent of the spectrograph, scientists have the opportunity to study the nature of starlight. This device can determine and measure gas composition stellar matter that a star possesses different stages of its existence.
By studying the spectral analysis of the energy of the Sun and other stars, scientists came to the conclusion that the evolution of stars and planets has common roots. All cosmic bodies have the same type, similar chemical composition and originated from the same matter that arose as a result of the Big Bang.
Stellar matter consists of the same chemical elements (even iron) as our planet. The only difference is in the amount of certain elements and in the processes occurring on the Sun and inside the earth's solid surface. This is what distinguishes stars from other objects in the Universe. The origin of stars should also be considered in the context of another physical discipline — quantum mechanics. According to this theory, the matter that determines stellar matter consists of constantly dividing atoms and elementary particles that create their own microcosm. In this light, the structure, composition, structure and evolution of stars is of interest. As it turned out, the bulk of the mass of our star and many other stars consists of only two elements - hydrogen and helium. Theoretical model, which describes the structure of a star, will allow us to understand their structure and the main difference from other space objects.
The main feature is that many objects in the Universe have a certain size and shape, while a star can change size as it develops. A hot gas is a combination of atoms loosely bound to each other. Millions of years after the formation of a star, the surface layer of stellar matter begins to cool. The star gives off most of its energy into outer space, decreasing or increasing in size. Heat and energy are transferred from the interior of the star to the surface, affecting the intensity of radiation. In other words, the same star in different periods its existence looks different. Thermonuclear processes based on reactions of the hydrogen cycle contribute to the transformation of light hydrogen atoms into heavier elements - helium and carbon. According to astrophysicists and nuclear scientists, such a thermonuclear reaction is the most efficient in terms of the amount of heat generated.
Why doesn’t thermonuclear fusion of the nucleus end with the explosion of such a reactor? The thing is that the forces of the gravitational field in it can hold stellar matter within a stabilized volume. From this we can draw an unambiguous conclusion: any star is a massive body that maintains its size due to the balance between the forces of gravity and the energy of thermonuclear reactions. The result of such an ideal natural model is a heat source capable of operating long time. It is assumed that the first forms of life on Earth appeared 3 billion years ago. The sun in those distant times warmed our planet just as it does now. Consequently, our star has changed little, despite the fact that the scale of the emitted heat and solar energy colossal - more than 3-4 million tons every second.
It is not difficult to calculate how much weight our star has lost over the years of its existence. This will be a huge figure, but due to its enormous mass and high density, such losses on the scale of the Universe look insignificant.
Stages of star evolution
The fate of the star depends on the initial mass of the star and its chemical composition. While the main reserves of hydrogen are concentrated in the core, the star remains in the so-called main sequence. As soon as there is a tendency for the size of the star to increase, it means that the main source for thermonuclear fusion has dried up. The long final path of transformation of the celestial body has begun.
The luminaries formed in the Universe are initially divided into three most common types:
- normal stars (yellow dwarfs);
- dwarf stars;
- giant stars.
Low-mass stars (dwarfs) slowly burn up their hydrogen reserves and live their lives quite calmly.
Such stars are the majority in the Universe, and our star, a yellow dwarf, is one of them. With the onset of old age, a yellow dwarf becomes a red giant or supergiant.
Based on the theory of the origin of stars, the process of star formation in the Universe has not ended. The brightest stars in our galaxy are not only the largest, compared to the Sun, but also the youngest. Astrophysicists and astronomers call such stars blue supergiants. In the end, they will suffer the same fate as trillions of other stars. First a rapid birth, brilliant and ardent life, after which a period of slow decay begins. Stars the size of the Sun have a long life cycle, being in the main sequence (in its middle part).
Using data on the mass of the star, we can assume it evolutionary path development. A clear illustration of this theory is the evolution of our star. Nothing lasts forever. As a result of thermonuclear fusion, hydrogen is converted into helium, therefore, its original reserves are consumed and reduced. Someday, not very soon, these reserves will run out. Judging by the fact that our Sun continues to shine for more than 5 billion years, without changing in its size, the mature age of the star can still last about the same period.
The depletion of hydrogen reserves will lead to the fact that, under the influence of gravity, the core of the sun will begin to rapidly shrink. The density of the core will become very high, as a result of which thermonuclear processes will move to the layers adjacent to the core. This state is called collapse, which can be caused by the passage of thermonuclear reactions in upper layers stars. As a result high pressure thermonuclear reactions involving helium are triggered.
The reserves of hydrogen and helium in this part of the star will last for millions of years. It will not be long before the depletion of hydrogen reserves will lead to an increase in the intensity of radiation, to an increase in the size of the shell and the size of the star itself. As a result, our Sun will become very large. If you imagine this picture tens of billions of years from now, then instead of a dazzling bright disk, a hot red disk of gigantic proportions will hang in the sky. Red giants are a natural phase of the evolution of a star, its transition state into the category of variable stars.
As a result of this transformation, the distance from the Earth to the Sun will decrease, so that the Earth will fall into the zone of influence solar corona and will begin to “fry” in it. The temperature on the surface of the planet will increase tenfold, which will lead to the disappearance of the atmosphere and the evaporation of water. As a result, the planet will turn into a lifeless rocky desert.
The final stages of stellar evolution
Having reached the red giant phase, normal star influenced gravitational processes becomes a white dwarf. If the mass of a star is approximately equal to the mass of our Sun, all the main processes in it will occur calmly, without impulses or explosive reactions. The white dwarf will die for a long time, burning out to the ground.
In cases where the star initially had a mass greater than 1.4 times the Sun, the white dwarf will not be the final stage. With a large mass inside the star, processes of compaction of stellar matter on the atomic, molecular level. Protons turn into neutrons, the density of the star increases, and its size rapidly decreases.
Neutron stars known to science have a diameter of 10-15 km. With such a small size, a neutron star has a colossal mass. One cubic centimeter stellar matter can weigh billions of tons.
In the event that we initially dealt with a star large mass, the final stage of evolution takes other forms. The fate of a massive star is a black hole - an object with an unexplored nature and unpredictable behavior. The huge mass of the star contributes to an increase in gravitational forces, driving compression forces. It is not possible to pause this process. The density of matter increases until it becomes infinite, forming a singular space (Einstein's theory of relativity). The radius of such a star will eventually become equal to zero, becoming a black hole in outer space. There would be significantly more black holes if massive and supermassive stars occupied most of the space.
It should be noted that when a red giant transforms into neutron star or into a black hole, the universe can survive unique phenomenon— the birth of a new space object.
The birth of a supernova is the most spectacular final stage in the evolution of stars. Valid here natural law nature: the cessation of the existence of one body gives rise to a new life. The period of such a cycle as the birth of a supernova mainly concerns massive stars. The exhausted reserves of hydrogen lead to the inclusion of helium and carbon in the process of thermonuclear fusion. As a result of this reaction, the pressure increases again, and an iron core is formed in the center of the star. Under the influence of strong gravitational forces, the center of mass shifts to the central part of the star. The core becomes so heavy that it is unable to resist its own gravity. As a result, rapid expansion of the core begins, leading to an instant explosion. The birth of a supernova is an explosion, a shock wave of monstrous force, a bright flash in the vast expanses of the Universe.
It should be noted that our Sun is not massive star, therefore, a similar fate does not threaten it, and our planet should not be afraid of such an ending. In most cases, supernova explosions occur in distant galaxies, which is why they are rarely detected.
Finally
The evolution of stars is a process that extends over tens of billions of years. Our idea of the processes taking place is just a mathematical and physical model, a theory. Earthly time is only a moment in the huge time cycle in which our Universe lives. We can only observe what happened billions of years ago and imagine what subsequent generations of earthlings may face.
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