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 the duration of human life, this incomprehensible time period 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 the Egyptian pharaohs could see them, but in fact, all this time the change in physical characteristics did not stop for a second heavenly bodies. Stars are born, live and certainly age - the evolution of stars goes on as usual.
The position of the stars of the constellation Ursa Major 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 provide insight into difficult process nuclear fusion, thanks to which a star exists, 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 due to cooling inner layers heavenly 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 object's balance, 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 in the center of the gas ball, the higher 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
Although some fusion reactions are started at more low temperatures, the main phase of hydrogen combustion starts at a temperature of 4 million degrees. From this moment the main sequence phase begins. Comes into play new form reproduction of stellar energy - 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 main sequence phase, which is important part 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. The upper part of the graph looks less saturated with objects, since this is where the massive stars are concentrated. This location is explained by their short 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 comes the giant phase. 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.
The 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 stars with large masses, the processes listed above are not so catastrophic. Helium combustion is replaced by the nuclear fission reaction of carbon and silicon. Eventually the star 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. Energy released in in this case, is spent on the decay of iron into helium atoms, which further decays into protons and neutrons. The running process is developing at a rapid pace. 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. Because of high density the core becomes degenerate, 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 hydrogen molecules, which are the building material for 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, we rely only on the laws of nuclear power, 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.
Although stars seem eternal on the human time scale, they, like everything in nature, are born, live and die. According to the generally accepted gas-dust cloud hypothesis, a star is born as a result of gravitational compression of an interstellar gas-dust cloud. As such a cloud thickens, it first forms protostar, the temperature at its center steadily increases until it reaches the limit necessary for the speed of thermal motion of particles to exceed the threshold after which protons are able to overcome the macroscopic forces of mutual electrostatic repulsion ( cm. Coulomb's Law) and enter into a thermonuclear fusion reaction ( cm. Nuclear decay and fusion).
As a result of a multi-stage thermonuclear fusion reaction, four protons ultimately form a helium nucleus (2 protons + 2 neutrons) and a whole fountain of various elementary particles is released. In the final state, the total mass of the formed particles is less masses of the four original protons, which means that during the reaction, free energy (cm. Theory of relativity). Because of this, the internal core of the newborn star quickly heats up to ultra-high temperatures, and its excess energy begins to splash towards its less hot surface - and out. At the same time, the pressure in the center of the star begins to increase ( cm. Equation of state of an ideal gas). Thus, by “burning” hydrogen in the process of a thermonuclear reaction, the star does not allow the forces of gravitational attraction to compress itself to a super-dense state, countering the gravitational collapse with continuously renewed internal thermal pressure, resulting in a stable energy equilibrium. Stars actively burning hydrogen are said to be in the "primary phase" of their life cycle or evolution ( cm. Hertzsprung-Russell diagram). The transformation of one chemical element into another inside a star is called nuclear fusion or nucleosynthesis.
In particular, the Sun has been at the active stage of burning hydrogen in the process of active nucleosynthesis for about 5 billion years, and the reserves of hydrogen in the core for its continuation should be enough for our luminary for another 5.5 billion years. The more massive the star, the greater the supply of hydrogen fuel it has, but to counteract the forces of gravitational collapse it must burn hydrogen at an intensity that exceeds the growth rate of hydrogen reserves as the mass of the star increases. Thus, the more massive the star, the shorter its lifetime, determined by the depletion of hydrogen reserves, and the largest stars literally burn out in “some” tens of millions of years. The smallest stars, on the other hand, live comfortably for hundreds of billions of years. So, on this scale, our Sun belongs to the “strong middle class”.
Sooner or later, however, any star will use up all the hydrogen suitable for combustion in its thermonuclear furnace. What's next? It also depends on the mass of the star. The sun (and all stars not exceeding its mass by more than eight times) end my life in a very banal way. As the reserves of hydrogen in the bowels of the star are depleted, the forces of gravitational compression, which have been patiently waiting for this hour since the very moment of the birth of the star, begin to gain the upper hand - and under their influence the star begins to shrink and become denser. This process has a twofold effect: The temperature in the layers immediately around the star's core rises to a level at which the hydrogen contained there finally undergoes thermonuclear fusion to form helium. At the same time, the temperature in the core itself, now consisting almost entirely of helium, rises so much that helium itself is a kind of “ash” of the decaying primary reaction nucleosynthesis - enters into a new thermonuclear fusion reaction: three helium nuclei form one carbon nucleus. This process of secondary thermonuclear fusion reaction, for which the products of the primary reaction serve as fuel, is one of key points life cycle of stars.
During the secondary combustion of helium in the core of the star, so much energy is released that the star literally begins to inflate. In particular, the shell of the Sun at this stage of life will expand beyond the orbit of Venus. In this case, the total energy of the star's radiation remains approximately at the same level as during the main phase of its life, but since this energy is now emitted through a much larger surface area, the outer layer of the star cools down to the red part of the spectrum. The star turns into red giant.
For solar-class stars, after the fuel feeding the secondary nucleosynthesis reaction has been depleted, the stage of gravitational collapse begins again—this time final. The temperature inside the core is no longer able to rise to the level necessary to initiate the next level of thermonuclear reaction. Therefore, the star contracts until the forces of gravitational attraction are balanced by the next force barrier. His role is played by degenerate electron gas pressure(cm. Chandrasekhar limit). Electrons, which until this stage played the role of unemployed extras in the evolution of the star, not participating in nuclear fusion reactions and freely moving between nuclei in the process of fusion, at a certain stage of compression find themselves deprived of “living space” and begin to “resist” further gravitational compression of the star. The state of the star stabilizes, and it turns into a degenerate white dwarf, which will radiate residual heat into space until it cools completely.
Stars more massive than the Sun face a much more spectacular end. After the combustion of helium, their mass during compression turns out to be sufficient to heat the core and shell to the temperatures necessary to launch the next nucleosynthesis reactions - carbon, then silicon, magnesium - and so on, as the nuclear masses grow. Moreover, with the start of each new reaction in the core of the star, the previous one continues in its shell. In fact, everything chemical elements up to the iron that makes up the Universe were formed precisely as a result of nucleosynthesis in the depths of dying stars of this type. But iron is the limit; it cannot serve as fuel for nuclear fusion or decay reactions at any temperature or pressure, since both its decay and the addition of additional nucleons to it require an influx of external energy. As a result, a massive star gradually accumulates an iron core inside itself, which cannot serve as fuel for any further nuclear reactions.
Once the temperature and pressure inside the nucleus reach a certain level, electrons begin to interact with the protons of the iron nuclei, resulting in the formation of neutrons. And in a very short period of time - some theorists believe that this takes a matter of seconds - the electrons free throughout the previous evolution of the star literally dissolve in the protons of the iron nuclei, the entire substance of the star’s core turns into a solid bunch of neutrons and begins to rapidly compress in gravitational collapse , since the counteracting pressure of the degenerate electron gas drops to zero. The outer shell of the star, from under which all support is knocked out, collapses towards the center. The energy of the collision of the collapsed outer shell with the neutron core is so high that it rebounds with enormous speed and scatters in all directions from the core - and the star literally explodes in a blinding flash supernova stars. In a matter of seconds, a supernova explosion can release more energy into space than all the stars in the galaxy put together during the same time.
After a supernova explosion and the expansion of the shell of stars with a mass of about 10-30 solar masses, the ongoing gravitational collapse leads to the formation of a neutron star, the matter of which is compressed until it begins to make itself felt pressure of degenerate neutrons - in other words, now neutrons (just as electrons did earlier) begin to resist further compression, requiring to myself living space. This usually occurs when the star reaches a size of about 15 km in diameter. The result is a rapidly rotating neutron star, emitting electromagnetic pulses at the frequency of its rotation; such stars are called pulsars. Finally, if the star's core mass exceeds 30 solar masses, nothing can stop its further gravitational collapse, and a supernova explosion results in
Part one ASTRONOMICAL ASPECT OF THE PROBLEM
4. Evolution of stars Modern astronomy has big amount arguments in favor of the statement that stars are formed by the condensation of clouds of gas and dust in the interstellar medium. The process of star formation from this environment continues to this day. Clarification of this circumstance is one of the greatest achievements of modern astronomy. Until relatively recently, it was believed that all stars were formed almost simultaneously many billions of years ago. The collapse of these metaphysical ideas was facilitated, first of all, by the progress of observational astronomy and the development of the theory of the structure and evolution of stars. As a result, it became clear that many of the observed stars are relatively young objects, and some of them arose when man was already on Earth. An important argument in favor of the conclusion that stars are formed from the interstellar gas and dust medium is the location of groups of obviously young stars (the so-called “associations”) in the spiral arms of the Galaxy. The fact is that, according to radio astronomical observations, interstellar gas is concentrated mainly in the spiral arms of galaxies. In particular, this occurs in our Galaxy. Moreover, from detailed “radio images” of some galaxies close to us, it follows that the highest density of interstellar gas is observed on the inner (relative to the center of the corresponding galaxy) edges of the spiral, which has a natural explanation, the details of which we cannot dwell on here. But it is precisely in these parts of the spirals that “HII zones,” i.e., clouds of ionized interstellar gas, are observed by optical astronomy methods. In ch. 3 it has already been said that the cause of ionization of such clouds can only be ultraviolet radiation from massive hot stars - obviously young objects (see below). Central to the problem of the evolution of stars is the question of the sources of their energy. Indeed, where, for example, does the enormous amount of energy needed to maintain the Sun's radiation at approximately the observed level for several billion years come from? Every second the Sun emits 4x10 33 ergs, and over 3 billion years it has emitted 4x10 50 ergs. There is no doubt that the age of the Sun is about 5 billion years. This follows at least from modern estimates of the age of the Earth using various radioactive methods. It is unlikely that the Sun is “younger” than the Earth. In the last century and at the beginning of this century, various hypotheses were proposed about the nature of the energy sources of the Sun and stars. Some scientists, for example, believed that the source solar energy is the continuous fall of meteoroids onto its surface; others sought the source in the continuous compression of the Sun. The potential energy released during such a process could, under certain conditions, turn into radiation. As we will see below, this source can be quite effective at an early stage of stellar evolution, but it cannot provide radiation from the Sun for the required time. Advances in nuclear physics made it possible to solve the problem of sources of stellar energy back in the late thirties of our century. Such a source is thermonuclear fusion reactions occurring in the depths of stars at the very high temperature prevailing there (on the order of ten million Kelvin). As a result of these reactions, the speed of which strongly depends on temperature, protons turn into helium nuclei, and the released energy slowly “leaks” through the depths of stars and, in the end, significantly transformed, is emitted into outer space. This is an extremely powerful source. If we assume that initially the Sun consisted only of hydrogen, which as a result of thermonuclear reactions was completely transformed into helium, then the amount of energy released will be approximately 10 52 erg. Thus, to maintain radiation at the observed level for billions of years, it is enough for the Sun to “use up” no more than 10% of its initial supply of hydrogen. Now we can imagine the evolution of a star as follows. For some reasons (several of them can be specified), a cloud of interstellar gas and dust medium began to condense. Quite soon (on an astronomical scale, of course!), under the influence of the forces of universal gravity, a relatively dense opaque gas ball will form from this cloud. Strictly speaking, this ball cannot yet be called a star, since in its central regions the temperature is not sufficient for thermonuclear reactions to begin. The gas pressure inside the ball is not yet able to balance the forces of attraction of its individual parts, so it will continuously compress. Some astronomers previously believed that such “protostars” were observed in individual Nebulae in the form of very dark compact formations, the so-called globules (Fig. 12). The successes of radio astronomy, however, forced us to abandon this rather naive point of view (see below). Usually, not one protostar is formed at the same time, but a more or less numerous group of them. Subsequently, these groups become stellar associations and clusters, well known to astronomers. It is very likely that at this very early stage of the star’s evolution, clumps of lower mass form around it, which then gradually turn into planets (see Chapter 9).Rice. 12. Globules in a diffusion nebula
When a protostar contracts, its temperature rises and a significant part of the released potential energy is radiated into the surrounding space. Since the dimensions of the collapsing gas ball are very large, the radiation per unit of its surface will be insignificant. Since the radiation flux per unit surface is proportional to the fourth power of temperature (Stefan-Boltzmann law), the temperature of the surface layers of the star is relatively low, while its luminosity is almost the same as that of an ordinary star with the same mass. Therefore, on the spectrum-luminosity diagram, such stars will be located to the right of the main sequence, i.e., they will fall into the region of red giants or red dwarfs, depending on the values of their initial masses. Subsequently, the protostar continues to contract. Its dimensions become smaller, and the surface temperature increases, as a result of which the spectrum becomes more and more “early”. Thus, moving along the spectrum-luminosity diagram, the protostar will rather quickly “sit down” on the main sequence. During this period, the temperature of the stellar interior is already sufficient for thermonuclear reactions to begin there. In this case, the gas pressure inside the future star balances the attraction and the gas ball stops compressing. A protostar becomes a star. It takes relatively little time for protostars to go through this earliest stage of their evolution. If, for example, the mass of the protostar is greater than that of the Sun, it takes only a few million years; if it is less, it takes several hundred million years. Since the evolutionary time of protostars is relatively short, this earliest phase of star development is difficult to detect. Nevertheless, stars in such a stage are apparently observed. We mean very interesting stars type T Tauri, usually immersed in dark nebulae. In 1966, quite unexpectedly, it became possible to observe protostars on early stages their evolution. We have already mentioned in the third chapter of this book about the discovery by radio astronomy of a number of molecules in the interstellar medium, primarily hydroxyl OH and water vapor H2O. The surprise of radio astronomers was great when, when surveying the sky at a wavelength of 18 cm, corresponding to the OH radio line, bright, extremely compact (i.e., having small angular dimensions) sources. This was so unexpected that at first they refused to even believe that such bright radio lines could belong to a hydroxyl molecule. It was hypothesized that these lines belonged to some unknown substance, which was immediately given the “appropriate” name “mysterium”. However, "mysterium" very soon shared the fate of its optical "brothers" - "nebulia" and "corona". The fact is that for many decades the bright lines of nebulae and the solar corona could not be identified with any known spectral lines. Therefore, they were attributed to certain hypothetical elements unknown on earth - “nebulium” and “crown”. Let us not smile condescendingly at the ignorance of astronomers at the beginning of our century: after all, there was no atomic theory then! The development of physics has not left in periodic table Mendeleev's place for exotic "celestials": in 1927, "nebulium" was debunked, the lines of which were completely reliably identified with the "forbidden" lines of ionized oxygen and nitrogen, and in 1939 -1941. It was convincingly shown that the mysterious "coronium" lines belong to multiply ionized atoms of iron, nickel and calcium. If it took decades to “debunk” “nebulium” and “codonia,” then within a few weeks after the discovery it became clear that the “mysterium” lines belong to ordinary hydroxyl, but only under unusual conditions. Further observations, first of all, revealed that the sources of the “mysterium” have extremely small angular dimensions. This was shown using the then new, very effective method research, called "radio interferometry at very long baselines." The essence of the method comes down to simultaneous observations of sources on two radio telescopes located at distances of several thousand km from each other. As it turns out, the angular resolution is determined by the ratio of the wavelength to the distance between the radio telescopes. In our case, this value can be ~3x10 -8 rad or several thousandths of an arcsecond! Note that in optical astronomy such angular resolution is still completely unattainable. Such observations have shown that there are at least three classes of sources of "mysterium". Here we will be interested in 1st class sources. All of them are located inside gaseous ionized nebulae, such as the famous Orion Nebula. As already mentioned, their sizes are extremely small, many thousands of times smaller sizes nebulae. The most interesting thing is that they have a complex spatial structure. Consider, for example, a source located in a nebula called W3.
Rice. 13. Profiles of the four components of the hydroxyl line
In Fig. Figure 13 shows the profile of the OH line emitted by this source. As you can see, it consists of a large number of narrow bright lines. Each line corresponds to a certain speed of movement along the line of sight of the cloud emitting this line. The magnitude of this speed is determined by the Doppler effect. The difference in velocities (along the line of sight) between different clouds reaches ~10 km/s. The interferometric observations mentioned above showed that the clouds emitting each line are not spatially aligned. The picture turns out like this: inside an area approximately 1.5 seconds in size, the arcs move with at different speeds about 10 compact clouds. Each cloud emits one specific (frequency) line. The angular dimensions of the clouds are very small, on the order of several thousandths of an arcsecond. Since the distance to the W3 nebula is known (about 2000 pc), the angular dimensions can easily be converted to linear ones. It turns out that the linear dimensions of the region in which the clouds move are of the order of 10 -2 pc, and the dimensions of each cloud are only an order of magnitude greater than the distance from the Earth to the Sun. Questions arise: what kind of clouds are these and why do they emit so much in hydroxyl radio lines? The answer to the second question was received quite quickly. It turned out that the radiation mechanism is quite similar to that observed in laboratory masers and lasers. So, the sources of “mysterium” are giant, natural cosmic masers operating at the wave of the hydroxyl line, the length of which is 18 cm. It is in masers (and at optical and infrared frequencies - in lasers) that enormous brightness in the line is achieved, and its spectral width is small . As is known, amplification of radiation in lines due to this effect is possible when the medium in which the radiation propagates is “activated” in some way. This means that some “external” energy source (the so-called “pumping”) makes the concentration of atoms or molecules at the initial (upper) level abnormally high. Without a constantly operating "pumping" a maser or laser is impossible. The question of the nature of the mechanism for “pumping” cosmic masers has not yet been completely resolved. However, most likely the “pumping” is provided by fairly powerful infrared radiation. Another possible pumping mechanism could be certain chemical reactions. It is worth interrupting our story about cosmic masers in order to think about what amazing phenomena astronomers encounter in space. One of the greatest technical inventions of our turbulent century, which plays a significant role in the scientific and technological revolution we are now experiencing, is easily realized in natural conditions and, moreover, on a huge scale! The flux of radio emission from some cosmic masers is so great that it could have been detected even at the technical level of radio astronomy 35 years ago, i.e. even before the invention of masers and lasers! To do this, you “only” needed to know the exact wavelength of the OH radio link and be interested in the problem. By the way, this is not the first time that the most important scientific and technical problems facing humanity have been realized in natural conditions. Thermonuclear reactions that support the radiation of the Sun and stars (see below) stimulated the development and implementation of projects to produce nuclear “fuel” on Earth, which in the future should solve all our energy problems. Alas, we are still far from solving this most important problem, which nature solved “easily.” A century and a half ago, the founder of the wave theory of light, Fresnel, remarked (on a different occasion, of course): “Nature laughs at our difficulties.” As we see, Fresnel's remark is even more true today. Let us return, however, to cosmic masers. Although the mechanism for “pumping” these masers is not yet entirely clear, it is still possible to get a rough idea of the physical conditions in the clouds emitting the 18 cm line using the maser mechanism. First of all, it turns out that these clouds are quite dense: per cubic centimeter there are at least 10 8 -10 9 particles, and a significant (and perhaps most) part of them are molecules. The temperature is unlikely to exceed two thousand Kelvin, most likely it is about 1000 Kelvin. These properties are sharply different from the properties of even the densest clouds of interstellar gas. Considering still relatively small sizes clouds, we involuntarily come to the conclusion that they rather resemble the extended, rather cold atmospheres of supergiant stars. It is very likely that these clouds are nothing more than an early stage in the development of protostars, immediately following their condensation from the interstellar medium. Other facts also support this statement (which the author of this book expressed back in 1966). In nebulae where cosmic masers are observed, young, hot stars are visible (see below). Consequently, the star formation process there recently ended and, most likely, continues at the present time. Perhaps the most curious thing is that, as radio astronomy observations show, cosmic masers of this type are, as it were, “immersed” in small, very dense clouds of ionized hydrogen. These clouds contain a lot of cosmic dust, which makes them unobservable in the optical range. Such "cocoons" are ionized by the young, hot star located inside them. Infrared astronomy has proven to be very useful in studying star formation processes. Indeed, for infrared rays, interstellar absorption of light is not so significant. We can now imagine the following picture: from the cloud of the interstellar medium, through its condensation, several clumps are formed different weights, evolving into protostars. The rate of evolution is different: for more massive clumps it will be greater (see Table 2 below). Therefore, the most massive clump will turn into a hot star first, while the rest will linger more or less long at the protostar stage. We observe them as sources of maser radiation in the immediate vicinity of a “newborn” hot star, ionizing the “cocoon” hydrogen that has not condensed into clumps. Of course, this rough scheme will be further refined, and, of course, significant changes will be made to it. But the fact remains: it unexpectedly turned out that for some time (most likely a relatively short time) newborn protostars, figuratively speaking, “scream” about their birth, using using the latest methods quantum radiophysics (i.e. masers)... 2 years after the discovery of cosmic masers on hydroxyl (18 cm line) - it was found that the same sources simultaneously emit (also by a maser mechanism) a line of water vapor, the wavelength of which is 1, 35 cm. The intensity of the “water” maser is even greater than that of the “hydroxyl” one. Clouds emitting the H2O line, although located in the same small volume as “hydroxyl” clouds, move at different speeds and are much more compact. It cannot be ruled out that other maser lines* will be discovered in the near future. Thus, quite unexpectedly, radio astronomy turned the classical problem of star formation into a branch of observational astronomy**. Once on the main sequence and having stopped contracting, the star radiates for a long time, practically without changing its position on the spectrum-luminosity diagram. Its radiation is supported by thermonuclear reactions occurring in the central regions. Thus, the main sequence is, as it were, a geometric location of points on the spectrum-luminosity diagram where a star (depending on its mass) can emit for a long time and steadily due to thermonuclear reactions. A star's place on the main sequence is determined by its mass. It should be noted that there is one more parameter that determines the position of the equilibrium emitting star on the spectrum-luminosity diagram. This parameter is the initial chemical composition of the star. If the relative abundance of heavy elements decreases, the star will "fall" in the diagram below. It is this circumstance that explains the presence of a sequence of subdwarfs. As mentioned above, the relative abundance of heavy elements in these stars is tens of times less than in main sequence stars. The time a star stays on the main sequence is determined by its initial mass. If the mass is large, the star’s radiation has enormous power and it quickly uses up its reserves of hydrogen “fuel”. For example, main sequence stars with a mass several tens of times greater than the Sun (these are hot blue giants of spectral class O) can emit steadily while remaining on this sequence for only a few million years, while stars with a mass close to solar, have been on the main sequence for 10-15 billion years. Below is the table. 2, giving the calculated duration of gravitational compression and stay on the main sequence for stars of different spectral classes. The same table shows the values of the masses, radii and luminosities of stars in solar units.
table 2
| years | |||||
|
Spectral class |
Luminosity |
gravitational compression |
stay on the main sequence | ||
|
G2 (Sun) |
|||||
Rice. 14. Evolutionary tracks for stars of different masses on the luminosity-temperature diagram
Rice. 15. Hertzsprung-Russell diagram for the star cluster NGC 2254
Rice. 16. Hertzsprung - Russell diagram for the globular cluster M 3. Along the vertical axis - relative magnitude
The corresponding diagram clearly shows the entire main sequence, including its upper left part, where hot massive stars are located (a color index of 0.2 corresponds to a temperature of 20 thousand K, i.e., a class B spectrum). The globular cluster M3 is an “old” object. It is clearly visible that there are almost no stars in the upper part of the main sequence diagram constructed for this cluster. But the red giant branch of M 3 is very richly represented, while NGC 2254 has very few red giants. This is understandable: the old cluster has M 3 big number stars have already “left” the main sequence, while in the young cluster NGC 2254 this happened only with a small number of relatively massive, rapidly evolving stars. It is noteworthy that the giant branch for M 3 goes quite steeply upward, while for NGC 2254 it is almost horizontal. From a theoretical point of view, this can be explained by the significantly lower content of heavy elements in M 3. And indeed, in stars of globular clusters (as well as in other stars that concentrate not so much towards the galactic plane as towards the galactic center), the relative abundance of heavy elements is insignificant . In the “color index - luminosity” diagram for M 3, another almost horizontal branch is visible. There is no similar branch in the diagram constructed for NGC 2254. The theory explains the appearance of this branch as follows. After the temperature of the contracting dense helium core of the star - a red giant - reaches 100-150 million K, a new nuclear reaction will begin to take place there. This reaction consists of the formation of a carbon nucleus from three helium nuclei. As soon as this reaction begins, the compression of the nucleus will stop. Subsequently, the surface layers
stars increase their temperature and the star on the spectrum-luminosity diagram will move to the left. It is from such stars that the third horizontal branch of the diagram for M 3 is formed.
Rice. 17. Summary Hertzsprung-Russell diagram for 11 star clusters
In Fig. Figure 17 schematically shows a summary “color-luminosity” diagram for 11 clusters, two of which (M 3 and M 92) are globular. It is clearly visible how the main sequences of different clusters “bend” to the right and upward in full agreement with the theoretical concepts that have already been discussed. From Fig. 17 one can immediately determine which clusters are young and which are old. For example, the “double” cluster X and h Perseus is young. It "preserved" a significant part of the main sequence. The M 41 cluster is older, the Hyades cluster is even older, and the M 67 cluster is very old, the color-luminosity diagram for which is very similar to the similar diagram for the globular clusters M 3 and M 92. Only the giant branch of the globular clusters is higher in agreement with differences in chemical composition, which were mentioned earlier. Thus, observational data fully confirm and justify the conclusions of the theory. It would seem difficult to expect observational verification of the theory of processes in stellar interiors, which are hidden from us by a huge thickness of stellar matter. And yet the theory here is constantly monitored by the practice of astronomical observations. It should be noted that the compilation of a large number of color-luminosity diagrams required enormous work by observing astronomers and a radical improvement in observation methods. On the other hand, advances in the theory of the internal structure and evolution of stars would have been impossible without modern computing technology based on the use of high-speed electronic calculating machines. Research in the field of nuclear physics also provided an invaluable service to the theory, which made it possible to obtain quantitative characteristics of those nuclear reactions that occur in the interior of stars. Without exaggeration, we can say that the development of the theory of the structure and evolution of stars is one of the largest achievements in astronomy of the second half of the 20th century. The development of modern physics opens up the possibility of direct observational testing of the theory of the internal structure of stars, and in particular the Sun. We are talking about the possibility of detecting a powerful stream of neutrinos, which should be emitted by the Sun if nuclear reactions take place in its depths. It is well known that neutrinos interact extremely weakly with other elementary particles. For example, a neutrino can fly through the entire thickness of the Sun almost without absorption, while X-ray radiation can pass through only a few millimeters of matter in the solar interior without absorption. If we imagine that a powerful beam of neutrinos with the energy of each particle in
Stars are known to get their energy from thermonuclear fusion reactions, and every star sooner or later comes to a point when its thermonuclear fuel runs out. The higher the mass of a star, the faster it burns up everything it can and enters the final stage of its existence. Further events may follow different scenarios, which one depends primarily on the masses.
While the hydrogen in the center of the star “burns out”, a helium core is released in it, compressing and releasing energy. Subsequently, combustion reactions of helium and subsequent elements may begin in it (see below). The outer layers expand many times under the influence of increased pressure coming from the heated core, the star becomes a red giant.
Depending on the mass of the star, different reactions can occur in it. This determines what composition the star will have by the time the fusion dies out.
White dwarfs
For stars with masses up to about 10 MC, the core weighs less than 1.5 MC. After the completion of thermonuclear reactions, the radiation pressure ceases, and the core begins to shrink under the influence of gravity. It contracts until the pressure of the degenerate electron gas, caused by the Pauli principle, begins to interfere. The outer layers shed and dissipate, forming a planetary nebula. The first such nebula was discovered by the French astronomer Charles Messier in 1764 and cataloged it under the number M27.
What emerges from the core is called a white dwarf. White dwarfs have a density greater than 10 7 g/cm 3 and a surface temperature of the order of 10 4 K. The luminosity is 2-4 orders of magnitude lower than the luminosity of the Sun. Thermonuclear fusion does not occur in it; all the energy emitted by it was accumulated earlier. Thus, white dwarfs slowly cool down and cease to be visible.
A white dwarf still has a chance to be active if it is part of a binary star and pulls the mass of its companion onto itself (for example, the companion became a red giant and filled its entire Roche lobe with its mass). In this case, either the synthesis of hydrogen in the CNO cycle can begin with the help of carbon contained in the white dwarf, ending with the release of the outer hydrogen layer (a “new” star). Or the mass of the white dwarf could grow so large that its carbon-oxygen component ignites in a wave of explosive combustion coming from the center. As a result, heavy elements are formed with the release of large amounts of energy:
12 C + 16 O → 28 Si + 16.76 MeV
28 Si + 28 Si → 56 Ni + 10.92 MeV
The star's luminosity increases strongly for 2 weeks, then rapidly decreases over another 2 weeks, after which it continues to decrease by approximately 2 times in 50 days. The main energy (about 90%) is emitted in the form of gamma rays from the decay chain of the nickel isotope. This phenomenon is called a type 1 supernova.
There are no white dwarfs with a mass of 1.5 or more solar masses. This is explained by the fact that for the existence of a white dwarf it is necessary to balance the gravitational compression with the pressure of the electron gas, but this happens at masses of no more than 1.4 M C, this limitation is called the Chandrasekhar limit. The value can be obtained as the condition of equality of pressure forces to the forces of gravitational compression under the assumption that the electron momenta are determined by the uncertainty relation for the volume they occupy, and they move at a speed close to the speed of light.
Neutron stars
In the case of more massive (> 10 M C) stars, everything happens a little differently. High temperature in the core activates energy absorption reactions, such as the knocking of protons, neutrons and alpha particles from the cores, as well as the e-capture of high-energy electrons, compensating for the mass difference two cores. The second reaction creates an excess of neutrons in the nucleus. Both reactions lead to its cooling and general compression of the star. When the nuclear fusion energy runs out, the compression turns into an almost free fall of the shell onto the collapsing core. At the same time, the rate of thermonuclear fusion in the outer falling layers sharply accelerates, which leads to the emission of a huge amount of energy in a few minutes (comparable to the energy that light stars emit during their entire existence).
Due to its high mass, the collapsing core overcomes the pressure of the electron gas and contracts further. In this case, reactions p + e - → n + ν e occur, after which there are almost no electrons remaining in the nucleus that interfere with compression. Compression occurs to sizes of 10 − 30 km, corresponding to the density established by the pressure of the neutron degenerate gas. The matter falling onto the core receives a shock wave reflected from the neutron core and part of the energy released during its compression, which leads to a rapid ejection of the outer shell to the sides. The resulting object is called a neutron star. Most (90%) of the energy released from gravitational compression is carried away by neutrinos in the first seconds after the collapse. The above process is called a type 2 supernova explosion. The energy of the explosion is such that some of them are (rarely) visible to the naked eye even in daytime. The first supernova was recorded by Chinese astronomers in 185 AD. Currently, several hundred outbreaks are recorded per year.
The resulting neutron star has a density of ρ ~ 10 14 − 10 15 g/cm 3 . Conservation of angular momentum during star compression leads to very short orbital periods, usually ranging from 1 to 1000 ms. For ordinary stars such periods are impossible, because Their gravity will not be able to counteract the centrifugal forces of such rotation. A neutron star has a very large magnetic field, reaching 10 12 -10 13 Gauss at the surface, which leads to strong electromagnetic radiation. A magnetic axis that does not coincide with the rotation axis leads to the fact that the neutron star sends periodic (with a rotation period) pulses of radiation in a given direction. Such a star is called a pulsar. This fact aided their experimental discovery and is used for detection. Detecting a neutron star using optical methods is much more difficult due to its low luminosity. The orbital period gradually decreases due to the transition of energy into radiation.
The outer layer of a neutron star consists of crystalline matter, mainly iron and its neighboring elements. Most of the rest of the mass is neutrons; pions and hyperons can be found in the very center. The density of the star increases towards the center and can reach values noticeably greater than the density of nuclear matter. The behavior of matter at such densities is poorly understood. There are theories about free quarks, including not only the first generation, at such extreme densities of hadronic matter. Superconducting and superfluid states of neutron matter are possible.
There are 2 mechanisms for cooling a neutron star. One of them is the emission of photons, as everywhere else. The second mechanism is neutrino. It prevails as long as the core temperature is above 10 8 K. This usually corresponds to a surface temperature above 10 6 K and lasts 10 5 −10 6 years. There are several ways to emit neutrinos:
Black holes
If the mass of the original star exceeded 30 solar masses, then the core formed in the supernova explosion will be heavier than 3 M C. At this mass, the pressure of the neutron gas can no longer hold back gravity, and the core does not stop at the neutron star stage, but continues to collapse (however, experimentally detected neutron stars have masses of no more than 2 solar masses, not three). This time nothing will prevent the collapse, and a black hole is formed. This object has a purely relativistic nature and cannot be explained without general relativity. Despite the fact that matter, according to theory, has collapsed into a point - a singularity, the black hole has a non-zero radius, called the Schwarzschild radius:
R Ш = 2GM/s 2.
The radius marks the boundary of the black hole's gravitational field, which is insurmountable even for photons, called the event horizon. For example, the Schwarzschild radius of the Sun is only 3 km. Outside the event horizon, the gravitational field of a black hole is the same as that of an ordinary object of its mass. A black hole can only be observed by indirect effects, since it itself does not emit any noticeable energy.
Even though nothing can escape the event horizon, a black hole can still create radiation. In the quantum physical vacuum, virtual particle-antiparticle pairs are constantly being born and disappearing. The strongest gravitational field of a black hole can interact with them before they disappear and absorb the antiparticle. If the total energy of the virtual antiparticle was negative, the black hole loses mass, and the remaining particle becomes real and receives energy sufficient to fly away from the field of the black hole. This radiation is called Hawking radiation and has a black body spectrum. A certain temperature can be attributed to it:
The effect of this process on the mass of most black holes is negligible compared to the energy they receive even from the cosmic microwave background radiation. The exception is relic microscopic black holes, which could have formed in the early stages of the evolution of the Universe. Small sizes speed up the evaporation process and slow down the process of mass gain. The final stages of evaporation of such black holes should end in an explosion. No explosions matching the description were ever recorded.
Matter falling into a black hole heats up and becomes a source of X-rays, which serves as an indirect sign of the presence of a black hole. When matter with high angular momentum falls onto a black hole, it forms a rotating accretion disk around it, in which particles lose energy and angular momentum before falling into the black hole. In the case of a supermassive black hole, two distinct directions appear along the axis of the disk, in which the pressure of the emitted radiation and electromagnetic effects accelerate particles ejected from the disk. This creates powerful jets of substance in both directions, which can also be registered. According to one theory, this is how active galactic nuclei and quasars are structured.
A spinning black hole is a more complex object. With its rotation, it “captures” a certain region of space beyond the event horizon (“Lense-Thirring effect”). This area is called the ergosphere, its boundary is called the limit of staticity. The static limit is an ellipsoid that coincides with the event horizon at the two poles of the black hole's rotation.
Rotating black holes have an additional mechanism of energy loss through the transfer of energy to particles trapped in the ergosphere. This loss of energy is accompanied by a loss of angular momentum and slows down the rotation.
Bibliography
- S.B.Popov, M.E.Prokhorov "Astrophysics of single neutron stars: radio-quiet neutron stars and magnetars" SAI MSU, 2002
- William J. Kaufman "The Cosmic Frontiers of Relativity" 1977
- Other Internet sources
December 20 10 g.
Stellar evolution in astronomy is the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years while it emits light and heat. Over such enormous periods of time, the changes are quite significant.
The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in a galaxy actually contains between 0.1 and 1 molecule per cm 3 . A molecular cloud has a density of about a million molecules per cm 3 . The mass of such a cloud exceeds the mass of the Sun by 100,000–10,000,000 times due to its size: from 50 to 300 light years across.
The evolution of a star begins in a giant molecular cloud, also called a stellar cradle.
While the cloud rotates freely around the center of its home galaxy, nothing happens. However, due to the inhomogeneity of the gravitational field, disturbances may arise in it, leading to local concentrations of mass. Such disturbances cause gravitational collapse of the cloud. One of the scenarios leading to this is the collision of two clouds. Another event that causes collapse could be the passage of a cloud through the dense arm of a spiral galaxy. Also a critical factor could be the explosion of a nearby supernova, the shock wave of which will collide with the molecular cloud at enormous speed. It is also possible that galaxies collide, which could cause a burst of star formation as the gas clouds in each galaxy are compressed by the collision. In general, any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.
any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.
During this process, the inhomogeneities of the molecular cloud will compress under the influence of their own gravity and gradually take the shape of a ball. When compressed, gravitational energy turns into heat, and the temperature of the object increases.
When the temperature in the center reaches 15–20 million K, thermonuclear reactions begin and compression stops. The object becomes a full-fledged star.
Subsequent stages of a star's evolution depend almost entirely on its mass, and only at the very end of a star's evolution can its chemical composition play a role.
The first stage of a star's life is similar to the sun's - it is dominated by hydrogen cycle reactions.
It remains in this state for most of its life, being on the main sequence of the Hertzsprung–Russell diagram, until the fuel reserves in its core run out. When all the hydrogen in the center of the star is converted to helium, a helium core is formed, and thermonuclear burning of hydrogen continues at the periphery of the core.
Small, cool red dwarfs slowly burn up their hydrogen reserves and remain on the main sequence for tens of billions of years, while massive supergiants leave the main sequence within a few tens of millions (and some just a few million) years after formation.
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 13.8 billion years, which is not enough for the hydrogen fuel supply in such stars to be depleted, modern theories are based on computer modeling of the processes occurring in such stars.
According to theoretical concepts, some of the light stars, losing their matter (stellar wind), will gradually evaporate, becoming smaller and smaller. Others, red dwarfs, will slowly cool over billions of years while continuing to emit faint emissions in the infrared and microwave ranges of the electromagnetic spectrum.
Medium-sized stars like the Sun remain on the main sequence for an average of 10 billion years.
It is believed that the Sun is still on it as it is in the middle of its life cycle. Once a star runs out of hydrogen in its core, it leaves the main sequence.
Once a star runs out of hydrogen in its core, it leaves the main sequence.
Without the pressure that arose during thermonuclear reactions and balanced the internal gravity, the star begins to shrink again, as it had previously during the process of its formation.
Temperature and pressure rise again, but, unlike the protostar stage, to a much higher level.
The collapse continues until, at a temperature of approximately 100 million K, thermonuclear reactions involving helium begin, during which helium is converted into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally – silicon to iron).
The collapse continues until thermonuclear reactions involving helium begin at a temperature of approximately 100 million K
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.
The star becomes a red giant, and the helium burning phase lasts about several million years.
What happens next also depends on the mass of the star.
At the stars average size the reaction of thermonuclear combustion of helium can lead to the explosive release of the outer layers of the star with the formation of planetary nebula. The core of the star, in which thermonuclear reactions stop, cools down and 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.
For massive and supermassive stars (with a mass of five solar masses or more), the processes occurring in their core as gravitational compression increases lead to an explosion supernova with the release of enormous energy. The explosion is accompanied by the ejection of a significant mass of star matter into interstellar space. This substance subsequently participates in the formation of new stars, planets or satellites. It is thanks to supernovae that the Universe as a whole, and each galaxy in particular, chemically evolves. The stellar core remaining after the explosion may end up evolving as a neutron star (pulsar) if the star's late-stage mass exceeds the Chandrasekhar limit (1.44 Solar masses), or as a black hole if the star's mass exceeds the Oppenheimer–Volkoff limit (estimated values of 2 .5-3 Solar masses).
The process of stellar evolution in the Universe is continuous and cyclical - old stars fade away and new ones light up to replace them.
According to modern scientific concepts, the elements necessary for the emergence of planets and life on Earth were formed from stellar matter. Although there is no single generally accepted point of view on how life arose.
