The internal life of a star is regulated by the influence of two forces: the force of gravity, which counteracts the star and holds it, and the force released during nuclear reactions occurring in the core. On the contrary, it tends to “push” the star into distant space. During the formation stages, a dense and compressed star is strongly influenced by gravity. As a result, strong heating occurs, the temperature reaches 10-20 million degrees. This is enough to start nuclear reactions, as a result of which hydrogen is converted into helium.
Then, over a long period, the two forces balance each other, the star is in a stable state. When the nuclear fuel in the core gradually runs out, the star enters an instability phase, two forces opposing each other. A critical moment comes for a star; a variety of factors come into play - temperature, density, chemical composition. The mass of the star comes first; the future of this celestial body depends on it - either the star will explode like a supernova, or turn into a white dwarf, a neutron star or a black hole.
How hydrogen runs out
Only the very largest among celestial bodies (about 80 times the mass of Jupiter) become stars, the smaller ones (about 17 times smaller than Jupiter) become planets. There are also bodies of medium mass, they are too large to belong to the class of planets, and too small and cold for events to occur in their depths. nuclear reactions, characteristic of stars.
These dark-colored celestial bodies have low luminosity and are quite difficult to distinguish in the sky. They are called “brown dwarfs”.
So, a star is formed from clouds consisting of interstellar gas. As already noted, quite long time the star is in a balanced state. Then comes a period of instability. The further fate of the star depends on various factors. Consider a hypothetical small star whose mass ranges from 0.1 to 4 solar masses. A characteristic feature of stars with low mass is the absence of convection in the inner layers, i.e. The substances that make up the star do not mix, as happens in stars with a large mass.
This means that when the hydrogen in the core runs out, there are no new reserves of this element in the outer layers. Hydrogen burns and turns into helium. Little by little the core heats up, the surface layers destabilize their own structure, and the star, as can be seen from the H-R diagram, slowly leaves the Main Sequence phase. In the new phase, the density of matter inside the star increases, the composition of the core “degenerates,” and as a result, a special consistency appears. It is different from normal matter.
Modification of matter
When matter changes, pressure depends only on the density of the gases, not on temperature.
In the Hertzsprung–Russell diagram, the star moves to the right and then upward, approaching the red giant region. Its dimensions increase significantly, and because of this, the temperature of the outer layers drops. The diameter of a red giant can reach hundreds of millions of kilometers. When ours enters this phase, it will “swallow” or Venus, and if it cannot capture the Earth, it will heat it up to such an extent that life on our planet will cease to exist.
During the evolution of a star, the temperature of its core increases. First nuclear reactions occur, then upon reaching optimal temperature Helium begins to melt. When this happens, a sudden increase in core temperature causes a flare and the star quickly moves to the left G-R diagrams. This is the so-called “helium flash”. At this time, the core containing helium burns together with hydrogen, which is part of the shell surrounding the core. On the H-R diagram, this stage is recorded by moving to the right along a horizontal line.
Last phases of evolution
When helium is transformed into carbon, the nucleus is modified. Its temperature rises until (if the star is large) until the carbon begins to burn. A new outbreak occurs. In any case, during the last phases of the star's evolution, a significant loss of its mass is noted. This can happen gradually or suddenly, during an outburst, when the outer layers of the star burst like a large bubble. In the latter case, a planetary nebula is formed - a spherical shell, spreading in outer space at a speed of several tens or even hundreds of km/sec.
The final fate of a star depends on the mass remaining after everything that happens in it. If it ejected a lot of matter during all transformations and flares and its mass does not exceed 1.44 solar masses, the star turns into a white dwarf. This figure is called the “Chandra-sekhar limit” in honor of the Pakistani astrophysicist Subrahmanyan Chandrasekhar. This is the maximum mass of a star at which a catastrophic end may not occur due to the pressure of electrons in the core.
After the explosion of the outer layers, the core of the star remains, and its surface temperature is very high - about 100,000 °K. The star moves to the left edge of the H-R diagram and goes down. Its luminosity decreases as its size decreases.
The star is slowly reaching the white dwarf zone. These are stars of small diameter (like ours), but characterized by a very high density, one and a half million times the density of water. A cubic centimeter of the material that makes up a white dwarf would weigh about one ton on Earth!
A white dwarf represents the final stage of star evolution, without outbursts. She is gradually cooling down.
Scientists believe that the end of the white dwarf is very slow, at least since the beginning of the Universe, it seems that not a single white dwarf has suffered from “thermal death”.
If the star is large, and its mass bigger than the sun, it will explode like a supernova. During a flare, a star may collapse completely or partially. In the first case, what will be left behind is a cloud of gas with residual matter from the star. In the second, a celestial body of the highest density remains - a neutron star or a black hole.
The Universe is a constantly changing macrocosm, where every object, substance or matter is in a state of transformation and change. These processes last for billions of years. Compared to duration human life this incomprehensible period of time is enormous. On a cosmic scale, these changes are quite fleeting. The stars that we now see in the night sky were the same thousands of years ago, when they could be seen Egyptian pharaohs, however, in fact, all this time the change in the physical characteristics of the celestial bodies did not stop for a second. Stars are born, live and certainly age - the evolution of stars goes on as usual.
Position of constellation stars Big Dipper in different historical periods in the interval 100,000 years ago - our time and after 100 thousand years
Interpretation of the evolution of stars from the point of view of the average person
For the average person, space appears to be a world of calm and silence. In fact, the Universe is a giant physical laboratory where enormous transformations are taking place, during which the chemical composition, physical characteristics and structure of stars change. The life of a star lasts as long as it shines and gives off heat. However, such a brilliant state does not last forever. The bright birth is followed by a period of star maturity, which inevitably ends with the aging of the celestial body and its death.
Formation of a protostar from a gas and dust cloud 5-7 billion years ago
All our information about stars today fits within the framework of science. Thermodynamics gives us an explanation of the processes of hydrostatic and thermal equilibrium in which stellar matter resides. Nuclear and quantum physics allow us to understand the complex process of nuclear fusion that allows a star to exist, emitting heat and giving light to the surrounding space. At the birth of a star, hydrostatic and thermal equilibrium is formed, maintained by its own energy sources. At the end of a brilliant stellar career, this balance is disrupted. A series of irreversible processes begins, the result of which is the destruction of the star or collapse - a grandiose process of instant and brilliant death of the heavenly body.
A supernova explosion is a bright finale to the life of a star born in the early years of the Universe.
Changes in the physical characteristics of stars are due to their mass. The rate of evolution of objects is influenced by their chemical composition and, to some extent, by existing astrophysical parameters - the speed of rotation and the state of the magnetic field. It is not possible to talk exactly about how everything actually happens due to the enormous duration of the processes described. The rate of evolution and the stages of transformation depend on the time of birth of the star and its location in the Universe at the time of birth.
The evolution of stars from a scientific point of view
Any star is born from a clump of cold interstellar gas, which, under the influence of external and internal gravitational forces, is compressed to the state of a gas ball. The process of compression of the gaseous substance does not stop for a moment, accompanied by a colossal release of thermal energy. The temperature of the new formation increases until thermonuclear fusion starts. From this moment, the compression of stellar matter stops, and a balance is reached between the hydrostatic and thermal states of the object. The Universe has been replenished with a new full-fledged star.
The main stellar fuel is the hydrogen atom as a result of a launched thermonuclear reaction.
In the evolution of stars, their sources of thermal energy are of fundamental importance. The radiant and thermal energy escaping into space from the surface of the star is replenished by cooling the inner layers of the celestial body. Constantly occurring thermonuclear reactions and gravitational compression in the bowels of the star make up for the loss. As long as there is sufficient nuclear fuel in the bowels of the star, the star glows with bright light and emits heat. As soon as the process of thermonuclear fusion slows down or stops completely, the mechanism of internal compression of the star is activated to maintain thermal and thermodynamic equilibrium. At this stage, the object is already emitting thermal energy, which is visible only in the infrared range.
Based on the processes described, we can conclude that the evolution of stars represents a consistent change in sources of stellar energy. In modern astrophysics, the processes of transformation of stars can be arranged in accordance with three scales:
- nuclear timeline;
- thermal period of a star's life;
- dynamic segment (final) of the life of a luminary.
In each individual case, the processes that determine the age of the star, its physical characteristics and the type of death of the object are considered. The nuclear timeline is interesting as long as the object is powered by its own heat sources and emits energy that is a product of nuclear reactions. The duration of this stage is estimated by determining the amount of hydrogen that will be converted into helium during thermonuclear fusion. The greater the mass of the star, the greater the intensity of nuclear reactions and, accordingly, the higher the luminosity of the object.
Sizes and masses of various stars, ranging from a supergiant to a red dwarf
The thermal time scale defines the stage of evolution during which a star expends all its thermal energy. This process begins from the moment when the last reserves of hydrogen are used up and nuclear reactions stop. To maintain the equilibrium of the object, a compression process is started. Stellar matter falls towards the center. In this case, the kinetic energy is converted into thermal energy, which is spent on maintaining the necessary temperature balance inside the star. Some of the energy escapes into outer space.
Considering the fact that the luminosity of stars is determined by their mass, at the moment of compression of an object, its brightness in space does not change.
A star on its way to the main sequence
Star formation occurs according to a dynamic time scale. Stellar gas falls freely inward toward the center, increasing the density and pressure in the bowels of the future object. The higher the density at the center of the gas ball, the higher the temperature inside the object. From this moment on, heat becomes the main energy of the celestial body. The greater the density and the higher the temperature, the greater the pressure in the depths of the future star. The free fall of molecules and atoms stops, and the process of compression of stellar gas stops. This state of an object is usually called a protostar. The object is 90% molecular hydrogen. When the temperature reaches 1800K, hydrogen passes into the atomic state. During the decay process, energy is consumed, and the temperature increase slows down.
The Universe is 75% composed of molecular hydrogen, which during the formation of protostars turns into atomic hydrogen - the nuclear fuel of a star
In this state, the pressure inside the gas ball decreases, thereby giving freedom to the compression force. This sequence is repeated each time all the hydrogen is ionized first, and then the helium is ionized. At a temperature of 10⁵ K, the gas is completely ionized, the compression of the star stops, and hydrostatic equilibrium of the object arises. The further evolution of the star will occur in accordance with the thermal time scale, much slower and more consistent.
The radius of the protostar has been decreasing from 100 AU since the beginning of formation. up to ¼ a.u. The object is in the middle of a gas cloud. As a result of the accretion of particles from the outer regions of the stellar gas cloud, the mass of the star will constantly increase. Consequently, the temperature inside the object will increase, accompanying the process of convection - the transfer of energy from the inner layers of the star to its outer edge. Subsequently, with increasing temperature in the interior of the celestial body, convection is replaced by radiative transfer, moving towards the surface of the star. At this moment, the luminosity of the object rapidly increases, and the temperature of the surface layers of the stellar ball also increases.
Convection processes and radiative transfer in a newly formed star before the onset of thermonuclear fusion reactions
For example, for stars with a mass identical to the mass of our Sun, the compression of the protostellar cloud occurs in just a few hundred years. As for the final stage of the formation of the object, the condensation of stellar matter has been stretching for millions of years. The Sun is moving towards the main sequence quite quickly, and this journey will take hundreds of millions or billions of years. In other words, the greater the mass of the star, the longer the period of time spent on the formation of a full-fledged star. A star with a mass of 15M will move along the path to the main sequence for much longer - about 60 thousand years.
Main sequence phase
Despite the fact that some thermonuclear fusion reactions start at lower temperatures, the main phase of hydrogen combustion starts at a temperature of 4 million degrees. From this moment the main sequence phase begins. A new form of stellar energy reproduction comes into play - nuclear. The kinetic energy released during the compression of an object fades into the background. The achieved equilibrium ensures a long and quiet life for a star that finds itself in the initial phase of the main sequence.
The fission and decay of hydrogen atoms during a thermonuclear reaction occurring in the interior of a star
From this moment on, observation of the life of a star is clearly tied to the phase of the main sequence, which is an important part of the evolution of celestial bodies. It is at this stage that the only source of stellar energy is the result of hydrogen combustion. The object is in a state of equilibrium. As nuclear fuel is consumed, only the chemical composition of the object changes. The Sun's stay in the main sequence phase will last approximately 10 billion years. This is how long it will take for our native star to use up its entire supply of hydrogen. As for massive stars, their evolution occurs faster. By emitting more energy, a massive star remains in the main sequence phase for only 10-20 million years.
Less massive stars burn in the night sky for much longer. Thus, a star with a mass of 0.25 M will remain in the main sequence phase for tens of billions of years.
Hertzsprung–Russell diagram assessing the relationship between the spectrum of stars and their luminosity. The points on the diagram are the locations of known stars. The arrows indicate the displacement of stars from the main sequence into the giant and white dwarf phases.
To imagine the evolution of stars, just look at the diagram characterizing the path of a celestial body in the main sequence. Top part The graphics look less object-saturated since this is where the massive stars are concentrated. This location is explained by their short duration life cycle. Of the stars known today, some have a mass of 70M. Objects whose mass exceeds the upper limit of 100M may not form at all.
Heavenly bodies whose mass is less than 0.08 M do not have the opportunity to overcome the critical mass required for the onset of thermonuclear fusion and remain cold throughout their lives. The smallest protostars collapse and form planet-like dwarfs.
A planet-like brown dwarf compared to a normal star (our Sun) and the planet Jupiter
At the bottom of the sequence are concentrated objects dominated by stars with a mass equal to the mass of our Sun and slightly more. The imaginary boundary between the upper and lower parts of the main sequence are objects whose mass is – 1.5M.
Subsequent stages of stellar evolution
Each of the options for the development of the state of a star is determined by its mass and the length of time during which the transformation of stellar matter occurs. However, the Universe is a multifaceted and complex mechanism, so the evolution of stars can take other paths.
When traveling along the main sequence, a star with a mass approximately equal to the mass of the Sun has three main route options:
- live your life calmly and rest peacefully in the vast expanses of the Universe;
- enter the red giant phase and slowly age;
- become a white dwarf, explode as a supernova, and become a neutron star.
Possible options for the evolution of protostars depending on time, the chemical composition of objects and their mass
After the main sequence, the giant phase begins. By this time, the reserves of hydrogen in the bowels of the star are completely exhausted, the central region of the object is a helium core, and thermonuclear reactions shift to the surface of the object. Under the influence of thermonuclear fusion, the shell expands, but the mass of the helium core increases. An ordinary star turns into a red giant.
Giant phase and its features
In stars with low mass, the core density becomes colossal, turning stellar matter into a degenerate relativistic gas. If the mass of the star is slightly more than 0.26 M, an increase in pressure and temperature leads to the beginning of helium synthesis, covering the entire central region of the object. From this moment on, the temperature of the star increases rapidly. The main feature of the process is that the degenerate gas does not have the ability to expand. Under the influence of high temperature, only the rate of helium fission increases, which is accompanied by an explosive reaction. At such moments we can observe a helium flash. The brightness of the object increases hundreds of times, but the agony of the star continues. The star transitions to a new state, where all thermodynamic processes occur in the helium core and in the discharged outer shell.
Structure of a solar-type main sequence star and a red giant with an isothermal helium core and a layered nucleosynthesis zone
This condition is temporary and not stable. Stellar matter is constantly mixed, and a significant part of it is ejected into the surrounding space, forming a planetary nebula. A hot core remains at the center, called a white dwarf.
For large-mass stars, the processes listed above are not so catastrophic. Helium combustion is replaced by the nuclear fission reaction of carbon and silicon. Eventually the stellar core will turn into star iron. The giant phase is determined by the mass of the star. The greater the mass of an object, the lower the temperature at its center. This is clearly not enough to trigger the nuclear fission reaction of carbon and other elements.
The fate of a white dwarf - a neutron star or a black hole
Once in the white dwarf state, the object is in an extremely unstable state. The stopped nuclear reactions lead to a drop in pressure, the core goes into a state of collapse. The energy released in this case is spent on the decay of iron into helium atoms, which further decays into protons and neutrons. The launched process is developing at a rapid speed. The collapse of a star characterizes the dynamic segment of the scale and takes a fraction of a second in time. The combustion of nuclear fuel residues occurs explosively, releasing a colossal amount of energy in a split second. This is quite enough to blow up the upper layers of the object. The final stage of a white dwarf is a supernova explosion.
The star's core begins to collapse (left). The collapse forms a neutron star and creates a flow of energy into the outer layers of the star (center). Energy released when the outer layers of a star are shed during a supernova explosion (right).
The remaining superdense core will be a cluster of protons and electrons, which collide with each other to form neutrons. The Universe has been replenished with a new object - a neutron star. Due to the high density, the core becomes degenerate, and the process of core collapse stops. If the star's mass were large enough, the collapse could continue until the remaining stellar matter finally fell into the center of the object, forming a black hole.
Explaining the final part of stellar evolution
For normal equilibrium stars, the described evolution processes are unlikely. However, the existence of white dwarfs and neutron stars proves the real existence of processes of compression of stellar matter. The small number of such objects in the Universe indicates the transience of their existence. The final stage of stellar evolution can be represented as a sequential chain of two types:
- normal star - red giant - shedding of outer layers - white dwarf;
- massive star – red supergiant – supernova explosion – neutron star or black hole – nothingness.
Diagram of the evolution of stars. Options for the continuation of the life of stars outside the main sequence.
It is quite difficult to explain the ongoing processes from a scientific point of view. Nuclear scientists agree that in the case of the final stage of stellar evolution, we are dealing with fatigue of matter. As a result of prolonged mechanical and thermodynamic influence, matter changes its physical properties. The fatigue of stellar matter, depleted by long-term nuclear reactions, can explain the appearance of degenerate electron gas, its subsequent neutronization and annihilation. If all of the above processes take place from beginning to end, stellar matter ceases to be a physical substance - the star disappears in space, leaving nothing behind.
Interstellar bubbles and gas and dust clouds, which are the birthplace of stars, cannot be replenished only by disappeared and exploded stars. The Universe and galaxies are in an equilibrium state. There is a constant loss of mass, the density of interstellar space decreases in one part of outer space. Consequently, in another part of the Universe, conditions are created for the formation of new stars. In other words, the scheme works: if a certain amount of matter was lost in one place, in another place in the Universe the same amount of matter appeared in a different form.
Finally
By studying the evolution of stars, we come to the conclusion that the Universe is a gigantic rarefied solution in which part of the matter is transformed into molecules of hydrogen, which is building material for the stars. The other part dissolves in space, disappearing from the sphere of material sensations. A black hole in this sense is the place of transition of all material into antimatter. It is quite difficult to fully comprehend the meaning of what is happening, especially if, when studying the evolution of stars, you rely only on the laws of nuclear, quantum physics and thermodynamics. The theory of relative probability should be included in the study of this issue, which allows for the curvature of space, allowing the transformation of one energy into another, one state into another.
Star mass T☼ and radius R can be characterized by its potential energy E . Potential or gravitational energy star is the work that must be expended to disperse the star's matter to infinity. And vice versa, this energy is released when the star contracts, i.e. as its radius decreases. The value of this energy can be calculated using the formula:
The potential energy of the Sun is equal to: E ☼ = 5.9∙10 41 J.
A theoretical study of the process of gravitational compression of a star has shown that a star emits approximately half of its potential energy, while the other half is spent on increasing the temperature of its mass to approximately ten million kelvins. It is not difficult, however, to be convinced that the Sun would have emitted this energy in 23 million years. So, gravitational compression can be a source of energy for stars only in some, quite brief stages their development.
The theory of thermonuclear fusion was formulated in 1938 by German physicists Karl Weizsäcker and Hans Bethe. The prerequisite for this was, firstly, the determination in 1918 by F. Aston (England) of the mass of the helium atom, which is equal to 3.97 masses of the hydrogen atom , secondly, the identification in 1905 of the connection between body weight T and his energy E in the form of Einstein's formula:
where c is the speed of light, thirdly, the discovery in 1929 that, thanks to the tunnel effect, two equally charged particles (two protons) can approach at a distance where the force of attraction is superior, as well as the discovery in 1932 of the positron e+ and neutron n.
The first and most effective of the thermonuclear fusion reactions is the formation of four protons in the nucleus of a helium atom according to the scheme:
What is happening here is very important mass defect: the mass of a helium nucleus is 4.00389 amu, while the mass of four protons is 4.03252 amu. Using Einstein’s formula, we calculate the energy that is released during the formation of one helium nucleus:
It is not difficult to calculate that if the Sun were on initial stage development consisted of only hydrogen, then its transformation into helium would be sufficient for the existence of the Sun as a star with current energy losses of about 100 billion years. In fact, we are talking about the “burning out” of about 10% of hydrogen from the deepest bowels of the star, where the temperature is sufficient for fusion reactions.
Helium synthesis reactions can occur in two ways. The first one is called pp cycle second - WITH NO-cycle. In both cases, twice in each helium nucleus, a proton turns into a neutron according to the following scheme:
,
Where V- neutrino.
Table 1 shows the average time of each thermonuclear fusion reaction, the period during which the number of initial particles will decrease by e once.
Table 1. Helium synthesis reactions.
The efficiency of fusion reactions is characterized by the power of the source, the amount of energy that is released per unit mass of a substance per unit time. It follows from the theory that
, whereas . Temperature limit T, above which the main role will not play rr-, A CNO cycle, is equal to 15∙10 6 K. In the depths of the Sun, the main role will be played by pp- cycle. Precisely because the first of its reactions has a very long characteristic time (14 billion years), the Sun and stars like it go through their evolutionary path for about ten billion years. For more massive white stars, this time is tens and hundreds of times less, since the characteristic time of the main reactions is much shorter CNO- cycle.If the temperature in the interior of a star, after the hydrogen is exhausted there, reaches hundreds of millions of kelvins, and this is possible for stars with a mass T>1.2m ☼ , then the energy source becomes the reaction of converting helium into carbon according to the scheme:
. Calculations show that the star will use up its helium reserves in approximately 10 million years. If its mass is large enough, the core continues to compress and at temperatures above 500 million degrees, synthesis reactions of more complex ones become possible atomic nuclei according to the scheme: 
At higher temperatures the following reactions occur:
etc. up to the formation of iron nuclei. These are reactions exothermic, As a result of their progress, energy is released.
As we know, the energy that a star emits into the surrounding space is released in its depths and gradually seeps to the surface of the star. This transfer of energy through the thickness of the star’s matter can be carried out by two mechanisms: radiant transfer or convection.
In the first case, we are talking about repeated absorption and re-emission of quanta. In fact, during each such event, quanta are fragmented, so instead of hard γ-quanta that arise during thermonuclear fusion in the bowels of a star, millions of low-energy quanta reach its surface. In this case, the law of conservation of energy is fulfilled.
In the theory of energy transfer, the concept of the free path of a quantum of a certain frequency υ was introduced. It is not difficult to understand that in stellar atmospheres, the free path of a quantum does not exceed several centimeters. And the time it takes for energy quanta to leak from the center of a star to its surface is measured in millions of years. However, in the depths of stars, conditions may arise under which such radiative balance is disrupted. Water behaves similarly in a vessel that is heated from below. For a certain time, the liquid here is in a state of equilibrium, since the molecule, having received excess energy directly from the bottom of the vessel, manages to transfer part of the energy due to collisions to other molecules that are located above. This establishes a certain temperature gradient in the vessel from its bottom to the top edge. However, over time, the rate at which molecules can transfer energy upward through collisions becomes less than the rate at which heat is transferred from below. Boiling occurs - heat transfer by direct movement of the substance.
Contemplating the clear night sky away from the city lights, it is easy to notice that the Universe is full of stars. How did nature manage to create a myriad of these objects? After all, it is estimated that there are about 100 billion stars in the Milky Way alone. 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 a physics point of view, a star is a ball of gas
From a physics point of view, it is a gas ball. The heat and pressure generated in nuclear reactions—mainly the fusion of helium from hydrogen—prevents the star from collapsing under its own gravity. The life of this relatively simple object follows a very specific scenario. First, a star is born from a diffuse cloud of interstellar gas, then there is 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 present significant difficulties. But the interaction 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 young star's internal temperature is too low for hydrogen atoms to fuse together, so its luminosity should be relatively low. It can increase when nuclear reactions begin, and only then can it gradually fall. In fact, the very young star is extremely bright. Its luminosity decreases with age, reaching a temporary minimum during hydrogen combustion.
During the early stages of evolution, a variety of physical processes occur in stars.
During the early stages of evolution, stars undergo a variety of physical processes, some of which are still poorly understood. Only in the last two decades have astronomers begun to build a detailed picture of stellar evolution based on advances in theory and observations.
Stars are born from large clouds, not visible in visible light, located in the disks of spiral galaxies. Astronomers call these objects giant molecular complexes. The term "molecular" reflects the fact that the gas in the complexes consists primarily of hydrogen in molecular form. Such clouds are the largest formations in the Galaxy, sometimes reaching more than 300 light years. years in diameter.
Upon closer analysis of the evolution of the star
A more careful analysis reveals that stars are formed from individual condensations - compact zones - in a giant molecular cloud. Astronomers have studied the properties of compact zones using large radio telescopes, the only instruments capable of detecting faint millimo clouds. From observations of this radiation it follows that a typical compact zone has a diameter of several light months, a density of 30,000 hydrogen molecules per 1 cm^ and a temperature of 10 Kelvin.
Based on these values, it was concluded that the gas pressure in the compact zones is such that it can resist compression under the influence of self-gravitational forces.
Therefore, in order for a star to form, the compact zone must be compressed from an unstable state, and 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. However, even before the discovery of compact zones, astrophysicists had the opportunity to simulate the process of star formation. Already in the 1960s, theorists used computer simulations to determine how unstable clouds collapse.
Although a wide range of initial conditions was used for theoretical calculations, the results obtained were the same: in a cloud that is too unstable, the internal part is compressed first, that is, the substance in the center first undergoes free fall, while the peripheral regions remain stable. Gradually, the compression area spreads outward, covering the entire cloud.
Deep in the depths of the contracting region, the evolution of stars begins
Deep within the contracting region, star formation begins. The diameter of the star is only one light second, i.e. one millionth the diameter of the compact zone. For such relatively small sizes, the overall picture of cloud compression is not significant, and the main role here is played by the speed of matter falling onto the star 
The rate at which matter falls may vary, but it directly depends on the temperature of the cloud. The higher the temperature, the greater 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 the protostar.
It turned out that the falling gas hits the surface of the protostar at a very high speed. Therefore, a powerful shock front is formed (an abrupt transition to very high pressure). Within the shock front, the gas heats up to almost 1 million Kelvin, then during radiation at the surface it quickly 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 the protozoan is equal to one solar, then its luminosity can exceed the solar one ten times. But it is not caused 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 that from which stars are formed, contains “dust” - a mixture of solid particles of submicron size. The radiation from the shock front encounters a large number of these particles along its path, falling along with the gas onto the surface of the protostar.
Cold dust particles absorb photons emitted by the shock front and re-emit them at longer wavelengths. This long-wave 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 ends up in the infrared region of the electromagnetic spectrum. But just a few light hours away from the protostar, the photon's wavelength becomes too long for the dust to absorb it, and it can finally rush unhindered to Earth's infrared-sensitive telescopes.
Despite ample opportunities With modern detectors, astronomers cannot claim that telescopes actually record the radiation of protostars. Apparently they are deeply hidden in the depths of compact zones registered in the radio range. Uncertainty in detection stems from the fact that detectors cannot distinguish a protostar from older stars embedded in gas and dust.
For reliable identification, an infrared or radio telescope must detect the Doppler shift of the spectral emission lines of the protostar. The Doppler shift would reveal the true motion of the gas falling onto 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 onset of thermonuclear fusion reactions. However, thermonuclear reactions in protostars are fundamentally different from reactions in middle-aged stars. The source of energy for such stars is the thermonuclear fusion reactions of helium from hydrogen.
Hydrogen is the most abundant 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, consisting of a proton and a neutron. This isotope of hydrogen is present in modern times in interstellar gas, from which it enters stars.
It is noteworthy that this tiny impurity 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 a protostar can easily reach 1 million Kelvin, when the fusion of deuterium nuclei begins, which also releases colossal energy.
The opacity of protostellar matter is too great
The opacity of protostellar matter is too great for this energy to be transferred by radiative transfer. Therefore, the star becomes convectively unstable: gas bubbles heated by “nuclear fire” float to the surface. These upward flows are balanced by downward flows of cold gas towards the center. Similar convective movements, but on a much smaller scale, take place in a room with steam heating. In a protostar, convective vortices transport deuterium from the surface to its interior. In this way, 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 fusion has a strong effect on the protostar. The main consequence of deuterium combustion reactions is the “swelling” of the protostar. Due to the effective 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 inflates to a radius equal to 10 solar. 
The mass of a typical compact zone is greater than the mass of the star it produces. 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 escaping from the surface of the protostar is responsible. The stellar wind blows the falling gas in the opposite direction and eventually disperses the compact zone.
Stellar wind idea
The “idea of stellar wind” does not follow from theoretical calculations. And the surprised theorists were provided with evidence of this phenomenon: observations of streams of molecular gas moving from infrared radiation sources. These flows are associated with the protostellar wind. Its origin is one of the most deep secrets young stars.
When the compact zone dissipates, an object is exposed that can be observed in the optical range - a young star. Like a protostar, it has a high luminosity, which is determined more by gravity than by thermonuclear fusion. Pressure in the interior of a star prevents catastrophic gravitational collapse. However, the heat responsible for this pressure is radiated from the star's surface, so the star shines very brightly and slowly contracts.
As it contracts, its internal temperature gradually rises and eventually reaches 10 million Kelvin. Then the fusion reactions of hydrogen nuclei begin to form helium. The heat generated creates pressure that prevents compression, and the star will shine for a long time until the nuclear fuel in its depths runs out.
Our Sun, a typical star, took about 30 million years to contract from protostellar to modern sizes. Thanks to the heat released during thermonuclear reactions, it has maintained these dimensions for about 5 billion years.
This is how stars are born. But despite such obvious successes of scientists, which 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, and unexpected bright flares. There are no sure answers to these questions yet. But these unresolved problems should be considered as breaks in a chain, the main links of which have already been welded together. 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.
A star is born video:
Stellar evolution is the change over time in the physical characteristics, internal structure and chemical composition of stars. The modern theory of stellar evolution is capable of explaining the general course of stellar development in satisfactory agreement with the data of astronomical observations. The course of a star's evolution depends on its mass and initial chemical composition. The stars of the first generation were formed from matter, the composition of which was determined by cosmological conditions (about 70% hydrogen, 30% helium, an insignificant admixture of deuterium and lithium). During the evolution of first-generation stars, heavy elements were formed that 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 3–4% heavy elements.
The birth of a star is the formation of an object whose radiation is supported by its own energy sources. The process of star formation continues continuously, and it continues to this day.
To explain the structure of the megaworld, the most important is 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 series of condensations. If such condensations persist long enough, then over time they turn into stars. It is important to note that the birth process is not of an individual star, but of stellar associations. The resulting gas 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 attractive forces leading to further concentration.
Young stars are those that are still in the stage of initial gravitational compression. The temperature at the center of such stars is not yet sufficient for thermonuclear reactions to occur. The glow of stars occurs only due to the conversion of gravitational energy into heat. Gravitational compression is the first stage in the evolution of stars. It leads to heating of the central zone of the star to the temperature at which the thermonuclear reaction begins (10 – 15 million K) – the transformation of hydrogen into helium.
The enormous energy emitted by stars is generated as a result of nuclear processes occurring inside stars. The energy generated inside a star allows it to emit 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 the synthesis of helium from hydrogen was put forward in 1920 by the English astrophysicist A.S. Eddington. In the interior of stars, two types of thermonuclear reactions involving hydrogen are possible, called the hydrogen (proton-proton) and carbon (carbon-nitrogen) cycles. In the first case, only hydrogen is required for the reaction to occur; in the second, the presence of carbon is also necessary, serving as a catalyst. The starting material is protons, from which helium nuclei are formed as a result of nuclear fusion.
Since the transformation of four protons into a helium nucleus produces two neutrinos, 1.8∙10 38 neutrinos are generated every second in the depths of the Sun. Neutrinos interact weakly with matter and have great penetrating power. Having passed through a huge thickness of solar matter, neutrinos retain all the information that they received in thermonuclear reactions in the depths of the Sun. The flux density of solar neutrinos falling on the Earth's surface is 6.6∙10 10 neutrinos per 1 cm 2 per 1 s. Measuring the flux of neutrinos falling 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 occurs in the form of radiation over a wide range of frequencies (wavelengths). The interaction between radiation and matter results in a steady state of equilibrium: the pressure of outward radiation is balanced by the pressure of gravity. Further contraction of the star stops as long as a sufficient amount of energy is produced at the center. This state is quite stable, and the size of the star remains constant. Hydrogen is the main one component cosmic matter and most important species nuclear fuel. The star's hydrogen reserves last for billions of years. This explains why stars are stable for such a long time. Until all the hydrogen in the central zone burns out, the properties of the star change little.
The hydrogen burnout field in the central zone of the star forms a helium core. Hydrogen reactions continue to occur, but only in a thin layer near the surface of the core. 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 begins to expand. The shell swells to colossal sizes, the external temperature becomes low. The star enters the red giant stage. From this moment on, the star's life begins to decline. Red giants are different low temperatures and huge sizes (from 10 to 1000 R c). The average density of the substance in them does not 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, when transitioning to the red giant stage, can increase so much that it fills the orbit of Mercury. True, the Sun will become a red giant in 8 billion years.
The red giant is characterized by low external temperatures, but very high internal temperatures. As it increases, increasingly 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 higher and higher energies. By the time magnesium is formed, all the helium in the star's core is depleted, and in order for further nuclear reactions to become possible, the star must contract again and its temperature increase. However, this is not possible for all stars, only for large ones whose mass exceeds the mass of the Sun by more than 1.4 times (the so-called Chandrasekhar limit). In stars of lower mass, reactions end at the stage of magnesium formation. In stars whose mass exceeds the Chandrasekhar limit, due to gravitational compression, the temperature rises to 2 billion degrees, 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 stellar wind. When the internal thermonuclear energy sources are completely depleted, further fate of a star depends on its mass.
With a mass less than 1.4 solar masses, the star enters 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 transforming a red giant into a white dwarf, a race can shed its outer layers like a light shell, exposing the core. The gas shell 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 electron component is a degenerate electron gas. White dwarfs are in an equilibrium state due to the equality of forces between gravity (compression factor) and the pressure of degenerate gas in the bowels 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, finally it stops emitting, becoming 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 existence.
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 the repulsion of particles and other reasons stop the collapse, then a powerful explosion occurs ─ flash supernova with the release of a significant part of the matter into the surrounding space and the formation of gas 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 their transformation into white dwarfs, neutron stars or black holes. During an explosion, energy is released in the amount of 10 43 ─ 10 44 J with 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 exploded.
The gas nebula formed during a supernova explosion consists partly of elements ejected by the explosion. upper layers stars, and partly from interstellar matter, compacted and heated by the scattering products of the explosion. The most famous gas nebula is the Crab Nebula in the constellation Taurus - a 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 stationary interstellar gas generates a shock wave in which the gas is heated to millions of Kelvin and becomes a source of X-ray radiation. The propagation of a shock wave in a gas leads to the appearance of fast charged particles (cosmic rays), which, moving in a compressed interstellar magnetic field enhanced by the same wave, emit radiation in the radio range.
Astronomers recorded supernova explosions in 1054, 1572, 1604. In 1885, a supernova was observed in the Andromeda nebula. Its brilliance exceeded the brilliance of the entire Galaxy and turned out to be 4 billion times more intense than the brilliance of the Sun.
By 1980, more than 500 supernova explosions had been discovered, but not a single one had been observed in our Galaxy. Astrophysicists have calculated that in our Galaxy, supernovae explode with a period of 10 million years in the immediate vicinity of the Sun. On average, a supernova occurs in the Metagalaxy every 30 years.
Doses of cosmic radiation on Earth can exceed the normal level by 7000 times. This will lead to serious mutations in living organisms on our planet. Some scientists explain the sudden death of dinosaurs this way.
Part of the mass of an exploding supernova may remain in the form of a superdense body - a neutron star or 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 . As compression and pressure increase, it becomes possible reaction absorption of electrons by protons 
As a result, all the matter of the star will consist of neutrons. Neutronization of a star is accompanied by powerful flash neutrino radiation. During the supernova explosion 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 the neutron star reaches 1 billion K. Neutron stars cool very quickly, their luminosity weakens. But they intensely emit 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 repeating pulses. That's why neutron stars are called pulsars. The first pulsars were discovered in 1967. The frequency of radiation pulsations, determined by the rotation speed 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 emission period of 0.03 s. Hundreds of neutron stars are currently 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 located stars, then the phenomenon of accretion occurs when matter from the neighboring star flows onto the white dwarf. The mass of the white dwarf grows and at a certain 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 the formation of an object called a black hole. The term “black hole” was introduced by J. Wheeler in 1968. However, the idea of such objects arose several centuries earlier, after the discovery of the law by I. Newton in 1687 universal gravity. In 1783, J. Mitchell suggested that dark stars should 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, 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 fly out of a black hole - neither particles nor radiation. In accordance with general theory 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, 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 light does not escape 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 a black hole capturing bodies arriving from infinity.
The theory allows the existence of black holes with a mass of 3–50 solar masses, 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 cores of galaxies weighing millions and billions of solar masses, primary (relict) black holes formed in the early stages of the evolution of the Universe. Relic black holes weighing more than 10 15 g (the mass of an average mountain on Earth) should have survived to this day due to the mechanism of quantum evaporation of black holes proposed by S.W. Hawking.
Astronomers detect black holes with powerful x-ray radiation. An example of this type of star is the powerful X-ray source Cygnus X-1, whose mass exceeds 10 M s. Black holes often occur in X-ray binaries star systems. Dozens of stellar-mass black holes have already been discovered in such systems (m black holes = 4-15 M s). Based on the effects of gravitational lensing, several single black holes of stellar mass have been discovered (m black holes = 6-8 M s). In the case of a close binary star, the phenomenon of accretion is observed - the flow of plasma from the surface of an ordinary star under the influence of gravitational forces onto a black hole. Matter flowing into a black hole has 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 X-rays. This radiation can be used to determine the presence of a black hole in a given location.
Of particular interest are supermassive black holes in the nuclei of galaxies. Based on the study of the X-ray image of the center of our Galaxy, obtained using the CHANDRA satellite, the presence of a supermassive black hole, the mass of which is 4 million times 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 enormous size and density, the black hole must have formed 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 approximately 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).
Closely related to the evolution of stars is the question of the origin of chemical elements. If hydrogen and helium are elements that remained from the early stages of the evolution of the expanding Universe, then heavier chemical elements could only be formed in the depths of stars during thermonuclear reactions. Inside stars, thermonuclear reactions can produce up to 30 chemical elements (iron inclusive).
Based on their physical state, 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 called “compact remnants”.
At the end of evolution, depending on the mass, the star either explodes or more quietly dumps matter already enriched with heavy chemical elements. In this case, the remaining elements of the periodic table are formed. Stars of the next generations are formed from the interstellar medium enriched with heavy elements. For example, the Sun is a second-generation star, formed from matter that has already been in the bowels of stars and was enriched with heavy elements. Therefore, the age of stars can be judged by their chemical composition, determined by spectral analysis.