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 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.
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 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 - rotation speed 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 already emits 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 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 bowels 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 consists of 75% 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. 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 follow 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;
- go into the category of white dwarfs, explode as a supernova and turn into 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.
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. The energy released 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. 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 compression processes 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 the listed 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, 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.
Evolution of Stars of Different Masses
Astronomers cannot observe the life of one star from beginning to end, because even the shortest-lived stars exist for millions of years - longer than the life of all humanity. Changes in the physical characteristics and chemical composition of stars over time, i.e. Astronomers study stellar evolution by comparing the characteristics of many stars at different stages of evolution.
Physical patterns connecting the observed characteristics of stars are reflected in the color-luminosity diagram - the Hertzsprung - Russell diagram, on which the stars form separate groups - sequences: the main sequence of stars, sequences of supergiants, bright and faint giants, subgiants, subdwarfs and white dwarfs.
For most of its life, any star is on the so-called main sequence of the color-luminosity diagram. All other stages of the star's evolution before the formation of a compact remnant take no more than 10% of this time. This is why most of the stars observed in our Galaxy are modest red dwarfs with the mass of the Sun or less. The main sequence contains about 90% of all observed stars.
The lifespan of a star and what it turns into at the end of its life is entirely determined by its mass. Stars with masses greater than the Sun live much less than the Sun, and the lifetime of the most massive stars is only millions of years. For the vast majority of stars, the lifetime is about 15 billion years. After a star exhausts its energy sources, it begins to cool and contract. The end product of stellar evolution is compact, massive objects whose density is many times greater than that of ordinary stars.
Stars of different masses end up in one of three states: white dwarfs, neutron stars or black holes. If the mass of the star is small, then the gravitational forces are relatively weak and the compression of the star (gravitational collapse) stops. It transitions to a stable white dwarf state. If the mass exceeds a critical value, compression continues. At very high densities, electrons combine with protons to form neutrons. Soon, almost the entire star consists of only neutrons and has such an enormous density that the huge stellar mass is concentrated in a very small ball with a radius of several kilometers and the compression stops - a neutron star is formed. If the mass of the star is so great that even the formation of a neutron star will not stop the gravitational collapse, then the final stage of the star’s evolution will be a black hole.
Our Sun has been shining for more than 4.5 billion years. At the same time, it constantly consumes hydrogen. It is absolutely clear that no matter how large its reserves are, they will someday be exhausted. And what will happen to the luminary? There is an answer to this question. The life cycle of a star can be studied from other similar cosmic formations. After all, there are real patriarchs in space, whose age is 9-10 billion years. And there are very young stars. They are no more than several tens of millions of years old.
Consequently, by observing the state of the various stars with which the Universe is “strewn”, one can understand how they behave over time. Here we can draw an analogy with an alien observer. He flew to Earth and began to study people: children, adults, old people. Thus, in a very short period of time, he understood what changes happen to people throughout life.
The Sun is currently a yellow dwarf - 1
Billions of years will pass, and it will become a red giant - 2
And then it will turn into a white dwarf - 3
Therefore, we can say with all confidence that when the hydrogen reserves in the central part of the Sun are exhausted, the thermonuclear reaction will not stop. The zone where this process will continue will begin to shift towards the surface of our star. But at the same time, gravitational forces will no longer be able to influence the pressure that is formed as a result of the thermonuclear reaction.
Consequently, the star will begin to grow in size and gradually turn into a red giant. This is a space object of a late stage of evolution. But it also happens at an early stage during star formation. Only in the second case does the red giant shrink and turn into main sequence star. That is, one in which the reaction of synthesis of helium from hydrogen takes place. In a word, where the life cycle of a star begins is where it ends.
Our Sun will increase in size so much that it will engulf nearby planets. These are Mercury, Venus and Earth. But don't be afraid. The star will begin to die in a few billion years. During this time, dozens, and maybe hundreds of civilizations will change. A person will pick up a club more than once, and after thousands of years he will sit down at a computer again. This is the usual cyclicity on which the entire Universe is based.
But becoming a red giant doesn't mean the end. The thermonuclear reaction will throw the outer shell into space. And in the center there will remain an energy-deprived helium core. Under the influence of gravitational forces, it will compress and, ultimately, turn into an extremely dense cosmic formation with a large mass. Such remnants of extinct and slowly cooling stars are called white dwarfs.
Our white dwarf will have a radius 100 times smaller than the radius of the Sun, and its luminosity will decrease by 10 thousand times. In this case, the mass will be comparable to the current solar one, and the density will be a million times greater. There are a lot of such white dwarfs in our Galaxy. Their number is 10% of the total number of stars.
It should be noted that white dwarfs are hydrogen and helium. But we will not go into the wilds, but will only note that with strong compression, gravitational collapse can occur. And this is fraught with a colossal explosion. In this case, a supernova explosion is observed. The term "supernova" does not describe the age, but the brightness of the flash. It’s just that the white dwarf was not visible for a long time in the cosmic abyss, and suddenly a bright glow appeared.
Most of the exploding supernova scatters through space at tremendous speed. And the remaining central part is compressed into an even denser formation and is called neutron star. It is the end product of stellar evolution. Its mass is comparable to that of the sun, and its radius reaches only a few tens of kilometers. One cube cm neutron star can weigh millions of tons. There are quite a lot of such formations in space. Their number is about a thousand times less than the usual suns with which the Earth's night sky is strewn.
It must be said that the life cycle of a star is directly related to its mass. If it matches the mass of our Sun or is less than it, then a white dwarf appears at the end of its life. However, there are luminaries that are tens and hundreds of times larger than the Sun.
When such giants shrink as they age, they distort space and time so much that instead of a white dwarf a white dwarf appears. black hole. Its gravitational attraction is so strong that even those objects that move at the speed of light cannot overcome it. The dimensions of the hole are characterized by gravitational radius. This is the radius of the sphere bounded by event horizon. It represents a space-time limit. Any cosmic body, having overcome it, disappears forever and never returns back.
There are many theories about black holes. All of them are based on the theory of gravity, since gravity is one of the most important forces in the Universe. And its main quality is versatility. At least, today not a single space object has been discovered that lacks gravitational interaction.
There is an assumption that through a black hole you can get into a parallel world. That is, it is a channel to another dimension. Anything is possible, but any statement requires practical evidence. However, no mortal has yet been able to carry out such an experiment.
Thus, the life cycle of a star consists of several stages. In each of them, the luminary appears in a certain capacity, which is radically different from previous and future ones. This is the uniqueness and mystery of outer space. Getting to know him, you involuntarily begin to think that a person also goes through several stages in his development. And the shell in which we exist now is only a transitional stage to some other state. But this conclusion again requires practical confirmation..
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 the masses of the four initial protons, which means that free energy is released during the reaction ( 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 the helium itself - a kind of “ash” of the fading primary nucleosynthesis reaction - enters into a new thermonuclear fusion reaction: from three helium nuclei one carbon nucleus is formed. This process of secondary thermonuclear fusion reaction, fueled by the products of the primary reaction, is one of the key moments in the life cycle of stars.
During the secondary combustion of helium in the star's core, 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 that powers 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 moving freely 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 beginning of each new reaction in the core of the star, the previous one continues in its shell. In fact, all the chemical elements, including iron, that make 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 bounces off at tremendous 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
Formed by condensation of the interstellar medium. Through observations, it was possible to determine that stars arose at different times and still appear to this day.
The main problem in the evolution of stars is the question of the origin of their energy, thanks to which they glow and emit huge amounts of energy. Previously, many theories were put forward that were designed to identify the sources of energy of stars. It was believed that a continuous source of stellar energy was continuous compression. This source is certainly good, but cannot maintain appropriate radiation for a long time. In the middle of the 20th century, the answer to this question was found. The source of radiation is thermonuclear fusion reactions. As a result of these reactions, hydrogen turns into helium, and the released energy passes through the bowels of the star, is transformed and emitted into outer space (it is worth noting that the higher the temperature, the faster these reactions occur; this is why hot massive stars leave the main sequence faster).
Now imagine the emergence of a star...
A cloud of interstellar gas and dust medium began to condense. From this cloud a rather dense ball of gas is formed. The pressure inside the ball is not yet able to balance the forces of attraction, so it will shrink (perhaps at this time clumps with less mass will form around the star, which will eventually turn into planets). When compressed, the temperature rises. Thus, the star gradually sets on the main sequence. Then the pressure of the gas inside the star balances the gravity and the protostar turns into a star.
The early stage of the star's evolution is very small and the star at this time is immersed in a nebula, so the protostar is very difficult to detect.
The conversion of hydrogen into helium occurs only in the central regions of the star. In the outer layers, the hydrogen content remains practically unchanged. Since the amount of hydrogen is limited, sooner or later it burns out. The release of energy in the center of the star stops and the core of the star begins to shrink and the shell begins to swell. Further, if the star is less than 1.2 solar masses, it sheds its outer layer (formation of a planetary nebula).
After the envelope separates from the star, its inner, very hot layers are exposed, and meanwhile the envelope moves further and further away. After several tens of thousands of years, the shell will disintegrate and only a very hot and dense star will remain; gradually cooling, it will turn into a white dwarf. Gradually cooling, they turn into invisible black dwarfs. Black dwarfs are very dense and cool stars, slightly larger than the Earth, but with a mass comparable to the mass of the sun. The cooling process of white dwarfs lasts several hundred million years.
If the mass of a star is from 1.2 to 2.5 solar, then such a star will explode. This explosion is called supernova explosion. The flaring star increases its luminosity hundreds of millions of times in a few seconds. Such outbreaks occur extremely rarely. In our Galaxy, a supernova explosion occurs approximately once every hundred years. After such an outbreak, a nebula remains, which has a lot of radio emission and also scatters very quickly, and a so-called neutron star (more on this a little later). In addition to the enormous radio emission, such a nebula will also be a source of X-ray radiation, but this radiation is absorbed by the earth’s atmosphere, and therefore can only be observed from space.
There are several hypotheses about the cause of star explosions (supernovae), but there is no generally accepted theory yet. There is an assumption that this is due to the too rapid decline of the inner layers of the star towards the center. The star quickly contracts to a catastrophically small size of the order of 10 km, and its density in this state is 10 17 kg/m 3, which is close to the density of the atomic nucleus. This star consists of neutrons (at the same time, electrons are pressed into protons), which is why it is called "NEUTRON". Its initial temperature is about a billion Kelvin, but in the future it will quickly cool down.
This star, due to its small size and rapid cooling, was long considered impossible to observe. But after some time, pulsars were discovered. These pulsars turned out to be neutron stars. They are named so because of the short-term emission of radio pulses. Those. the star seems to “blink.” This discovery was made completely by accident and not so long ago, namely in 1967. These periodic impulses are due to the fact that during very rapid rotation, the cone of the magnetic axis constantly flashes past our gaze, which forms an angle with the axis of rotation.
A pulsar can only be detected for us under the conditions of orientation of the magnetic axis, and this is approximately 5% of their total number. Some pulsars are not located in radio nebulae, since nebulae dissipate relatively quickly. After a hundred thousand years, these nebulae cease to be visible, and the age of pulsars is tens of millions of years.
If the mass of a star exceeds 2.5 solar, then at the end of its existence it will seem to collapse in on itself and be crushed by its own weight. In a matter of seconds it will turn into a dot. This phenomenon was called “gravitational collapse”, and this object was also called a “black hole”.
From all that has been said above, it is clear that the final stage of the evolution of a star depends on its mass, but it is also necessary to take into account the inevitable loss of this very mass and rotation.
