The evolution of stars is the change over time in the physical characteristics, internal structure and chemical composition of stars. The modern theory of stellar evolution is capable of explaining the general course of stellar development in satisfactory agreement with the data of astronomical observations. The evolution of a star depends on its mass and initial chemical composition. The stars of the first generation were formed from matter whose composition was determined by cosmological conditions (about 70% hydrogen, 30% helium, negligible admixture of deuterium and lithium). During the evolution of the first generation of stars, heavy elements were formed that were ejected into interstellar space as a result of the outflow of matter from stars or during star explosions. The stars of subsequent generations were formed from matter containing 3–4% of heavy elements.
The birth of a star is the formation of an object whose radiation is maintained by its own energy sources. The process of star formation continues uninterruptedly, it is happening at the present time.
To explain the structure of the mega world, the most important is the gravitational interaction. In gas and dust nebulae, under the action of gravitational forces, unstable inhomogeneities are formed, due to which diffuse matter breaks up into a number of clumps. If such clumps persist long enough, they turn into stars over time. It is important to note that the process of the birth of not a single star, but of stellar associations takes place. The resulting gaseous bodies are attracted to each other, but do not necessarily combine into one huge body. They usually begin to rotate relative to each other, and the centrifugal forces of this movement counteract the forces of attraction, leading to further concentration.
Young stars are those that are still in the stage of initial gravitational contraction. The temperature at the center of such stars is still insufficient for thermonuclear reactions to take place. The glow of stars occurs only due to the conversion of gravitational energy into heat. Gravitational contraction is the first stage in the evolution of stars. It leads to the heating of the central zone of the star to the temperature of the beginning of a thermonuclear reaction (10 - 15 million K) - the conversion of hydrogen into helium.
The huge energy radiated by stars is formed as a result of nuclear processes occurring inside stars. The energy generated inside a star allows it to radiate light and heat for millions and billions of years. For the first time, the assumption that the source of stellar energy is thermonuclear reactions of helium synthesis from hydrogen was put forward in 1920 by the English astrophysicist A.S. Eddington. In the interiors of stars, two types of thermonuclear reactions involving hydrogen are possible, called hydrogen (proton-proton) and carbon (carbon-nitrogen) cycles. In the first case, only hydrogen is required for the reaction to proceed, in the second, the presence of carbon, which serves as a catalyst, is also necessary. The starting material is protons, from which helium nuclei are formed as a result of nuclear fusion.
Since two neutrinos are born during the transformation of four protons into a helium nucleus, 1.8∙10 38 neutrinos are generated every second in the depths of the Sun. The neutrino weakly interacts with matter and has a high penetrating power. Having passed through the huge thickness of the solar matter, neutrinos retain all the information that they received in thermonuclear reactions in the bowels of the Sun. The flux density of solar neutrinos incident on the Earth's surface is 6.6∙10 10 neutrinos per 1 cm 2 in 1 s. Measuring the flux of neutrinos incident on the Earth makes it possible to judge the processes occurring inside the Sun.
Thus, the source of energy for most stars is hydrogen thermonuclear reactions in the central zone of the star. As a result of a thermonuclear reaction, an outward flow of energy arises in the form of radiation in a wide range of frequencies (wavelengths). The interaction between radiation and matter leads to a steady equilibrium: the pressure of the outward radiation is balanced by the pressure of gravity. Further contraction of the star stops as long as enough energy is produced in the center. This state is fairly stable and the size of the star remains constant. Hydrogen is the main component of cosmic matter and the most important type of nuclear fuel. A star has enough hydrogen reserves for billions of years. This explains why stars are stable for such a long time. Until all the hydrogen in the central zone burns out, the properties of the star change little.
The field of hydrogen burnout in the central zone of the star forms a helium core. Hydrogen reactions continue to take place, but only in a thin layer near the surface of the nucleus. Nuclear reactions move to the periphery of the star. The structure of the star at this stage is described by models with a layered energy source. The burnt-out core begins to shrink, and the outer shell expands. The shell swells to colossal proportions, the external temperature becomes low. The star becomes a red giant. From this moment on, the life of a star begins to decline. Red giants are characterized by low temperatures and huge sizes (from 10 to 1000 R s). The average density of matter in them does not even reach 0.001 g/cm 3 . Their luminosity is hundreds of times higher than the luminosity of the Sun, but the temperature is much lower (about 3000 - 4000 K).
It is believed that our Sun, during the transition to the stage of a red giant, can increase so much that it fills the orbit of Mercury. True, the Sun will become a red giant in 8 billion years.
A red giant is characterized by a low external temperature, but a very high internal temperature. With its increase, ever heavier nuclei are included in thermonuclear reactions. At a temperature of 150 million K, helium reactions begin, which are not only a source of energy, but during them the synthesis of heavier chemical elements is carried out. After the formation of carbon in the helium core of a star, the following reactions are possible:
It should be noted that the synthesis of the next heavier nucleus requires higher and higher energies. By the time magnesium is formed, all the helium in the core of the star is depleted, and in order for further nuclear reactions to become possible, a new compression of the star and an increase in its temperature are necessary. However, this is not possible for all stars, only for sufficiently large ones, the mass of which exceeds the mass of the Sun by more than 1.4 times (the so-called Chandrasekhar limit). In stars of smaller mass, the reactions end at the stage of magnesium formation. In stars whose mass exceeds the Chandrasekhar limit, due to gravitational contraction, the temperature rises to 2 billion degrees, the reactions continue, forming heavier elements - up to iron. Elements heavier than iron are formed when stars explode.
As a result of increasing pressure, pulsations, and other processes, the red giant continuously loses matter, which is ejected into interstellar space in the form of a stellar wind. When the internal thermonuclear energy sources are completely depleted, the further fate of the star depends on its mass.
With a mass less than 1.4 solar masses, the star passes into a stationary state with a very high density (hundreds of tons per 1 cm3). Such stars are called white dwarfs. In the process of turning a red giant into a white dwarf, the race can shed its outer layers like a light shell, exposing the core. The gaseous envelope glows brightly under the influence of powerful radiation from the star. This is how planetary nebulae are formed. At high densities of matter inside a white dwarf, the electron shells of atoms are destroyed, and the matter of the star is an electron-nuclear plasma, and its electronic component is a degenerate electron gas. White dwarfs are in equilibrium due to the equality of forces between gravity (compression factor) and the pressure of degenerate gas in the interior of the star (expansion factor). White dwarfs can exist for billions of years.
The thermal reserves of the star are gradually depleted, the star is slowly cooling, which is accompanied by ejections of the stellar envelope into interstellar space. The star gradually changes its color from white to yellow, then to red, and finally it ceases to radiate, becomes a small lifeless object, a dead cold star, the size of which is smaller than the size of the Earth, and its mass is comparable to the mass of the Sun. The density of such a star is billions of times greater than the density of water. Such stars are called black dwarfs. This is how most stars end their lives.
When the mass of the star is more than 1.4 solar masses, the stationary state of the star without internal energy sources becomes impossible, because The pressure inside the star cannot balance the force of gravity. Gravitational collapse begins - compression of matter towards the center of the star under the influence of gravitational forces.
If the repulsion of particles and other causes stop the collapse, then a powerful explosion occurs ─ a supernova explosion with the ejection of a significant part of the matter into the surrounding space and the formation of gaseous nebulae. The name was proposed by F. Zwicky in 1934. A supernova explosion is one of the intermediate stages in the evolution of stars before they turn into white dwarfs, neutron stars or black holes. An explosion releases energy of 10 43 ─ 10 44 J at a radiation power of 10 34 W. In this case, the brightness of the star increases by tens of magnitudes in a few days. The luminosity of a supernova can exceed the luminosity of the entire galaxy in which it burst.
The gaseous nebula formed during a supernova explosion consists partly of the upper layers of the star ejected by the explosion, and partly of interstellar matter, compacted and heated by the expanding products of the explosion. The most famous gaseous nebula is the Crab Nebula in the constellation Taurus - the remnant of a supernova of 1054. Young supernova remnants are expanding at speeds of 10-20 thousand km / s. The collision of the expanding shell with the stationary interstellar gas generates a shock wave in which the gas heats up to millions of Kelvin and becomes a source of X-rays. The propagation of a shock wave in a gas leads to the appearance of fast charged particles (cosmic rays), which, moving in an interstellar magnetic field compressed and enhanced by the same wave, radiate in the radio range.
Astronomers recorded supernova explosions in 1054, 1572, 1604. In 1885, a supernova was observed in the Andromeda Nebula. Its brightness exceeded the brightness of the entire Galaxy and turned out to be 4 billion times more intense than the brightness of the Sun.
Already by 1980, more than 500 supernova explosions had been discovered, but not a single one was observed in our Galaxy. Astrophysicists have calculated that supernovae in our Galaxy flare with a period of 10 million years in the immediate vicinity of the Sun. On average, a supernova explosion occurs in the Metagalaxy every 30 years.
In this case, doses of cosmic radiation on Earth can exceed the normal level by 7000 times. This will lead to the most serious mutations in living organisms on our planet. Some scientists explain the sudden death of dinosaurs in this way.
Part of the mass of an exploded supernova may remain in the form of a superdense body - a neutron star or a black hole. The mass of neutron stars is (1.4 - 3) M s, the diameter is about 10 km. The density of a neutron star is very high, higher than the density of atomic nuclei ─ 10 15 g/cm 3 . With an increase in compression and pressure, the reaction of absorption of electrons by protons becomes possible 
As a result, all the matter of the star will consist of neutrons. The neutronization of a star is accompanied by a powerful burst of neutrino radiation. During the burst of supernova SN1987A, the duration of the neutrino flash was 10 s, and the energy carried away by all neutrinos reached 3∙10 46 J. The temperature of a neutron star reaches 1 billion K. Neutron stars cool very quickly, their luminosity weakens. But they intensely radiate radio waves in a narrow cone in the direction of the magnetic axis. Stars whose magnetic axis does not coincide with the axis of rotation are characterized by radio emission in the form of repetitive pulses. Therefore, neutron stars are called pulsars. The first pulsars were discovered in 1967. The frequency of radiation pulsations, determined by the speed of rotation of the pulsar, is from 2 to 200 Hz, which indicates their small size. For example, the pulsar in the Crab Nebula has a pulse period of 0.03 s. There are currently hundreds of neutron stars known. A neutron star may appear as a result of the so-called "silent collapse". If a white dwarf enters a binary system of closely spaced stars, then the phenomenon of accretion occurs, when matter from a neighboring star flows onto a white dwarf. The mass of the white dwarf grows and at some point exceeds the Chandrasekhar limit. A white dwarf turns into a neutron star.
If the final mass of the white dwarf exceeds 3 solar masses, then the degenerate neutron state is unstable, and gravitational contraction continues until the formation of an object called a black hole. The term "black hole" was introduced by J. Wheeler in 1968. However, the concept of such objects arose several centuries earlier, after the discovery by I. Newton in 1687 of the law of universal gravitation. In 1783, J. Mitchell suggested that dark stars must exist in nature, the gravitational field of which is so strong that light cannot escape from them. In 1798 the same idea was expressed by P. Laplace. In 1916, the physicist Schwarzschild, solving Einstein's equations, came to the conclusion about the possibility of the existence of objects with unusual properties, later called black holes. A black hole is a region of space in which the gravitational field is so strong that the second cosmic velocity for bodies located in this region must exceed the speed of light, i.e. nothing can escape from a black hole, neither particles nor radiation. In accordance with the general theory of relativity, the characteristic size of a black hole is determined by the gravitational radius: R g =2GM/c 2 , where M is the object's mass, c is the speed of light in vacuum, and G is the gravitational constant. The gravitational radius of the Earth is 9 mm, the Sun is 3 km. The boundary of the region beyond which no light escapes is called the event horizon of a black hole. Rotating black holes have an event horizon radius smaller than the gravitational radius. Of particular interest is the possibility of capture by a black hole of bodies arriving from infinity.
The theory allows the existence of black holes with a mass of 3–50 solar masses, which are formed in the late stages of the evolution of massive stars with a mass of more than 3 solar masses, supermassive black holes in the nuclei of galaxies with a mass of millions and billions of solar masses, primordial (relic) black holes formed in the early stages of the evolution of the universe. To this day, relic black holes weighing more than 10 15 g (the mass of an average mountain on Earth) should have survived due to the mechanism of quantum evaporation of black holes proposed by S. W. Hawking.
Astronomers detect black holes by powerful x-rays. An example of this type of star is the powerful X-ray source Cygnus X-1, whose mass exceeds 10 M s. Often black holes are found in X-ray binary star systems. Dozens of stellar-mass black holes have already been discovered in such systems (m black holes = 4-15 M s). Based on the effects of gravitational lensing, several single stellar-mass black holes (m black holes = 6-8 M s) have been discovered. In the case of a close binary star, the phenomenon of accretion is observed - the flow of plasma from the surface of an ordinary star under the influence of gravitational forces onto a black hole. Matter flowing into a black hole has an angular momentum. Therefore, the plasma forms a rotating disk around the black hole. The temperature of the gas in this rotating disk can reach 10 million degrees. At this temperature, the gas emits in the X-ray range. From this radiation, you can determine the presence of a black hole in a given place.
Of particular interest are supermassive black holes in the cores of galaxies. Based on the study of the X-ray image of the center of our Galaxy, obtained with the help of the CHANDRA satellite, the presence of a supermassive black hole, the mass of which is 4 million times greater than the mass of the Sun, has been established. As a result of recent research, American astronomers have discovered a unique superheavy black hole located in the center of a very distant galaxy, the mass of which is 10 billion times the mass of the Sun. In order to reach such an unimaginably huge size and density, a black hole had to form over many billions of years, continuously attracting and absorbing matter. Scientists estimate its age at 12.7 billion years, i.e. it began to form about one billion years after the Big Bang. To date, more than 250 supermassive black holes have been discovered in the nuclei of galaxies (m black holes = (10 6 – 10 9) M s).
The question of the origin of chemical elements is closely related to the evolution of stars. If hydrogen and helium are elements left over from the early stages of the evolution of the expanding universe, then heavier chemical elements could only be formed in the interiors of stars during thermonuclear reactions. Inside stars during thermonuclear reactions, up to 30 chemical elements (including iron) can be formed.
According to their physical state, stars can be divided into normal and degenerate. The former consist mainly of low-density matter; thermonuclear fusion reactions take place in their depths. Degenerate stars include white dwarfs and neutron stars, they represent the final stage of stellar evolution. The fusion reactions in them have ended, and the equilibrium is maintained by the quantum-mechanical effects of degenerate fermions: electrons in white dwarfs and neutrons in neutron stars. White dwarfs, neutron stars, and black holes are collectively referred to as "compact remnants".
At the end of the evolution, depending on the mass, the star either explodes or releases more calmly matter already enriched in heavy chemical elements. In this case, the rest of the elements of the periodic system are formed. From the interstellar medium enriched with heavy elements, the stars of the next generations are formed. For example, the Sun is a second-generation star formed from matter that has already been in the interiors of stars and enriched with heavy elements. Therefore, the age of stars can be judged from their chemical composition determined by spectral analysis.
> Life cycle of a star
Description life and death of stars: evolutionary stages with photo, molecular clouds, protostar, T Taurus, main sequence, red giant, white dwarf.
Everything in this world is evolving. Any cycle begins with birth, growth and ends with death. Of course, the stars have these cycles in a special way. Let us recall, for example, that they have a larger time frame and are measured in millions and billions of years. In addition, their death carries certain consequences. What does it look like life cycle of stars?
The first life cycle of a star: Molecular clouds
Let's start with the birth of a star. Imagine a huge cloud of cold molecular gas that can easily exist in the universe without any changes. But suddenly a supernova explodes not far from it, or it collides with another cloud. Because of this push, the process of destruction is activated. It is divided into small parts, each of which is drawn into itself. As you already understood, all these bunches are preparing to become stars. Gravity heats up the temperature, and the stored momentum keeps the rotation going. The lower diagram clearly demonstrates the cycle of stars (life, stages of development, transformation options and death of a celestial body with a photo).
The second life cycle of a star: protostar
The material condenses more densely, heats up and is repelled by gravitational collapse. Such an object is called a protostar, around which a disk of material is formed. The part is attracted to the object, increasing its mass. The rest of the debris will be grouped and create a planetary system. Further development of the star all depends on the mass.
Third life cycle of a star: T Taurus
When material hits a star, a huge amount of energy is released. The new stellar stage was named after the prototype, T Taurus. This is a variable star located 600 light years away (not far from).
It can reach great brightness because the material breaks down and releases energy. But in the central part there is not enough temperature to support nuclear fusion. This phase lasts 100 million years.
The fourth life cycle of a star:Main sequence
At a certain moment, the temperature of the celestial body rises to the required level, activating nuclear fusion. All stars go through this. Hydrogen is transformed into helium, releasing a huge thermal reserve and energy.
The energy is released as gamma rays, but due to the star's slow motion, it falls off with wavelength. Light is pushed outward and confronts gravity. We can assume that a perfect balance is created here.
How long will she be in the main sequence? You need to start from the mass of the star. Red dwarfs (half the solar mass) are capable of spending hundreds of billions (trillions) of years on their fuel supply. Average stars (like) live 10-15 billion. But the largest ones are billions or millions of years old. See how the evolution and death of stars of various classes looks like in the diagram.
Fifth life cycle of a star: red giant
During the melting process, hydrogen ends and helium accumulates. When there is no hydrogen left at all, all nuclear reactions stop, and the star begins to shrink due to gravity. The hydrogen shell around the core heats up and ignites, causing the object to grow 1000-10000 times. At a certain moment, our Sun will repeat this fate, having increased to the earth's orbit.
Temperature and pressure reach a maximum, and helium fuses into carbon. At this point, the star contracts and ceases to be a red giant. With greater massiveness, the object will burn other heavy elements.
The sixth life cycle of a star: white dwarf
A solar-mass star doesn't have enough gravitational pressure to fuse carbon. Therefore, death occurs with the end of helium. The outer layers are ejected and a white dwarf appears. At first it is hot, but after hundreds of billions of years it will cool down.
It occupies a point in the upper right corner: it has a high luminosity and a low temperature. The main radiation occurs in the infrared range. Radiation from the cold dust shell reaches us. In the process of evolution, the position of the star on the diagram will change. The only source of energy at this stage is gravitational contraction. Therefore, the star moves quite quickly parallel to the y-axis.
The surface temperature does not change, but the radius and luminosity decrease. The temperature in the center of the star rises, reaching a value at which reactions begin with light elements: lithium, beryllium, boron, which quickly burn out, but manage to slow down the compression. The track turns parallel to the y-axis, the temperature on the surface of the star rises, and the luminosity remains almost constant. Finally, in the center of the star, the reactions of the formation of helium from hydrogen (hydrogen combustion) begin. The star enters the main sequence.
The duration of the initial stage is determined by the mass of the star. For stars like the Sun, it is about 1 million years, for a star with a mass of 10 M☉ about 1000 times smaller, and for a star with a mass of 0.1 M☉ thousands of times more.
Young low mass stars
At the beginning of its evolution, a low-mass star has a radiant core and a convective envelope (Fig. 82, I).
At the main sequence stage, the star shines due to the release of energy in the nuclear reactions of the conversion of hydrogen into helium. The supply of hydrogen ensures the luminosity of a star of mass 1 M☉ Approximately within 10 10 years. Stars of greater mass consume hydrogen faster: for example, a star with a mass of 10 M☉ will use up hydrogen in less than 10 7 years (luminosity is proportional to the fourth power of mass).
low mass stars
As the hydrogen burns out, the central regions of the star are strongly compressed.
Stars of high mass
After entering the main sequence, the evolution of a large-mass star (>1.5 M☉) is determined by the conditions of combustion of nuclear fuel in the interior of the star. At the main sequence stage, this is the combustion of hydrogen, but unlike low-mass stars, reactions of the carbon-nitrogen cycle dominate in the core. In this cycle, the C and N atoms play the role of catalysts. The rate of energy release in the reactions of such a cycle is proportional to T 17 . Therefore, a convective core is formed in the core, surrounded by a zone in which energy transfer is carried out by radiation.
The luminosity of large mass stars is much higher than the luminosity of the Sun, and hydrogen is consumed much faster. This is due to the fact that the temperature in the center of such stars is also much higher.
As the proportion of hydrogen in the substance of the convective core decreases, the rate of energy release decreases. But since the rate of release is determined by the luminosity, the core begins to shrink, and the rate of energy release remains constant. At the same time, the star expands and passes into the region of red giants.
low mass stars
By the time the hydrogen is completely burned out, a small helium core is formed in the center of a low-mass star. In the core, the matter density and temperature reach 10 9 kg/m and 10 8 K, respectively. Hydrogen combustion occurs on the surface of the nucleus. As the temperature in the core rises, the rate of hydrogen burning increases, and the luminosity increases. The radiant zone gradually disappears. And because of the increase in the speed of convective flows, the outer layers of the star swell. Its size and luminosity increase - the star turns into a red giant (Fig. 82, II).
Stars of high mass
When the hydrogen of a large mass star is completely exhausted, a triple helium reaction begins in the core and at the same time the reaction of oxygen formation (3He => C and C + He => 0). At the same time, hydrogen begins to burn on the surface of the helium core. The first layer source appears.
The supply of helium is exhausted very quickly, since in the described reactions in each elementary act, relatively little energy is released. The picture repeats itself, and two layer sources appear in the star, and the C + C => Mg reaction begins in the core.
The evolutionary track in this case turns out to be very complex (Fig. 84). In the Hertzsprung-Russell diagram, the star moves along the sequence of giants or (with a very large mass in the supergiant region) periodically becomes a cephei.
Old low-mass stars
In a star of low mass, in the end, the speed of the convective flow at some level reaches the second cosmic velocity, the shell comes off, and the star turns into a white dwarf, surrounded by a planetary nebula.
The evolutionary track of a low-mass star on the Hertzsprung-Russell diagram is shown in Figure 83.
Death of high mass stars
At the end of evolution, a large mass star has a very complex structure. Each layer has its own chemical composition, nuclear reactions take place in several layer sources, and an iron core is formed in the center (Fig. 85).
Nuclear reactions with iron do not proceed, since they require the expenditure (and not release) of energy. Therefore, the iron core is rapidly compressed, the temperature and density in it increase, reaching fantastic values - a temperature of 10 9 K and a pressure of 10 9 kg / m 3. material from the site
At this moment, two most important processes begin, going on in the nucleus simultaneously and very quickly (apparently, in minutes). The first is that during the collision of nuclei, iron atoms decay into 14 helium atoms, the second is that electrons are “pressed” into protons, forming neutrons. Both processes are associated with the absorption of energy, and the temperature in the core (also pressure) drops instantly. The outer layers of the star begin to fall towards the center.
The fall of the outer layers leads to a sharp increase in temperature in them. Hydrogen, helium, carbon begin to burn. This is accompanied by a powerful stream of neutrons that comes from the central core. As a result, a powerful nuclear explosion occurs, throwing off the outer layers of the star, which already contain all the heavy elements, up to californium. According to modern views, all atoms of heavy chemical elements (i.e., heavier than helium) were formed in the Universe precisely in flares
The 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 a human life, this incomprehensible time span is enormous. On a cosmic scale, these changes are rather fleeting. The stars that we now observe 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 heavenly bodies did not stop for a second. Stars are born, live and certainly grow old - 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 of 100,000 years ago - our time and after 100 thousand years
Interpretation of the evolution of stars from the point of view of the layman
For the layman, space appears to be a world of calm and silence. In fact, the Universe is a gigantic physical laboratory, where grandiose transformations take 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 is not eternal. A 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 the 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, thanks to which a star exists, radiating 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 sunset of a brilliant stellar career, this balance is disturbed. There comes a series of irreversible processes, the result of which is the destruction of a star or collapse - a grandiose process of instantaneous and brilliant death of a heavenly body.
A supernova explosion is a bright end to the life of a star born in the early years of the Universe
The change in the physical characteristics of stars is due to their mass. The rate of evolution of objects is influenced by their chemical composition and, to some extent, existing astrophysical parameters - the speed of rotation and the state of the magnetic field. It is not possible to say exactly how everything actually happens due to the huge duration of the described processes. The rate of evolution, the stages of transformation depend on the time of the 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 clot 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 a gaseous substance does not stop even for a moment, accompanied by a colossal release of thermal energy. The temperature of the new formation rises until thermonuclear fusion is launched. From that moment on, the compression of stellar matter ceases, and a balance is reached between the hydrostatic and thermal state of the object. The universe was replenished with a new full-fledged star.
The main stellar fuel is a 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 the cooling of the inner layers of the celestial body. Constantly occurring thermonuclear reactions and gravitational contraction in the interior of the star make up for the loss. As long as there is enough nuclear fuel in the depths of the star, the star glows brightly and radiates heat. As soon as the process of thermonuclear fusion slows down or stops altogether, the mechanism of internal compression of the star is launched to maintain thermal and thermodynamic equilibrium. At this stage, the object is already emitting thermal energy that is only visible in the infrared.
Based on the described processes, we can conclude that the evolution of stars is a successive change in the sources of stellar energy. In modern astrophysics, the processes of transformation of stars can be arranged in accordance with three scales:
- nuclear timeline;
- thermal segment of the life of a star;
- dynamic segment (final) of the life of the 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 radiates energy that is the product of nuclear reactions. The estimate of the duration of this stage is calculated by determining the amount of hydrogen that will turn into helium in the process of 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 supergiant to red dwarf
The thermal time scale defines the stage of evolution during which the star consumes all thermal energy. This process begins from the moment when the last reserves of hydrogen have been used up and nuclear reactions have ceased. To maintain the balance of the object, the compression process is started. The stellar matter falls towards the center. In this case, there is a transition of kinetic energy into thermal energy spent on maintaining the necessary temperature balance inside the star. Part 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.
Star on the way to the main sequence
Star formation occurs according to a dynamic timeline. Stellar gas falls freely inward towards 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 interior of the future star. The free fall of molecules and atoms stops, the process of compression of the stellar gas stops. This state of an object is usually called a protostar. The object is 90% molecular hydrogen. Upon reaching a temperature of 1800K, hydrogen passes into the atomic state. In the process of decay, energy is consumed, the temperature increase slows down.
The universe is 75% molecular hydrogen, which in the process of formation of protostars turns into atomic hydrogen - the nuclear fuel of the star
In such a state, the pressure inside the gas ball decreases, thereby giving freedom to the compressive force. This sequence is repeated each time when all the hydrogen is first ionized, and then it is the turn of the helium ionization. At a temperature of 10⁵ K, the gas is completely ionized, the compression of the star stops, and the hydrostatic equilibrium of the object occurs. The further evolution of the star will occur in accordance with the thermal time scale, much more slowly and more consistently.
The radius of a protostar has been shrinking from 100 AU since the start 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 rise, accompanying the process of convection - the transfer of energy from the inner layers of the star to its outer edge. Subsequently, with an increase in temperature in the interior of a celestial body, convection is replaced by radiative transport, moving towards the surface of the star. At this moment, the luminosity of the object is rapidly increasing, and the temperature of the surface layers of the stellar ball is also growing.
Convection processes and radiative transport in a newly formed star before the onset of thermonuclear fusion reactions
For example, for stars whose mass is identical to that 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 an object, the condensation of stellar matter has been stretched out for millions of years. The sun is moving towards the main sequence quite quickly, and this path will take a hundred million 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 15 M will move along the path to the main sequence for much longer - about 60 thousand years.
Main sequence phase
Although some fusion reactions start at lower temperatures, the main phase of hydrogen combustion starts at 4 million degrees. From this point on, the main sequence phase begins. A new form of reproduction of stellar energy, nuclear, comes into play. The kinetic energy released during the compression of the object fades into the background. The achieved equilibrium ensures a long and quiet life of a star that finds itself in the initial phase of the main sequence.
The fission and decay of hydrogen atoms in the process of a thermonuclear reaction occurring in the interior of a star
From this point on, the 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 the nuclear fuel is consumed, only the chemical composition of the object changes. The stay of the Sun in the phase of the main sequence will last approximately 10 billion years. So much time will be required for our native luminary to use up the entire supply of hydrogen. As for massive stars, their evolution is faster. Radiating more energy, a massive star stays in the main sequence phase for only 10-20 million years.
Less massive stars burn much longer in the night sky. So, a star with a mass of 0.25 M will stay in the main sequence phase for tens of billions of years.
Hertzsprung–Russell diagram estimating 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 phases of giants and white dwarfs.
To imagine the evolution of stars, it is enough to look at the diagram that characterizes the path of the celestial body in the main sequence. The upper part of the graph looks less crowded 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.
The celestial bodies, whose mass is less than 0.08M, do not have the ability to overcome the critical mass necessary for the start of thermonuclear fusion and remain cold all their lives. The smallest protostars shrink and form planet-like dwarfs.
A planetary brown dwarf compared to a normal star (our Sun) and the planet Jupiter
In the lower part of the sequence, objects are concentrated, dominated by stars with a mass equal to the mass of our Sun and a little 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 takes place. However, the Universe is a multifaceted and complex mechanism, so the evolution of stars can go in other ways.
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;
- go into the red giant phase and age slowly;
- go into the category of white dwarfs, burst into 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 interior of the star are completely exhausted, the central region of the object is a helium core, and thermonuclear reactions are shifted to the surface of the object. Under the influence of thermonuclear fusion, the shell expands, but the mass of the helium core grows. An ordinary star turns into a red giant.
The giant phase and its features
In stars with a small mass, the core density becomes colossal, turning stellar matter into a degenerate relativistic gas. If the mass of the star is slightly more than 0.26M, the increase in pressure and temperature leads to the start of helium fusion, covering the entire central region of the object. Since then, the temperature of the star has been rising 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. There is a transition of the star to a new state, where all thermodynamic processes occur in the helium core and in the rarefied 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 sustainable. Stellar matter is constantly mixed, while a significant part of it is ejected into the surrounding space, forming a planetary nebula. A hot core remains in the center, which is called a white dwarf.
For high-mass stars, these processes are not so catastrophic. The helium combustion is replaced by the nuclear fission reaction of carbon and silicon. Eventually the stellar core will turn into stellar iron. The phase of a giant 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 start a 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. Stopped nuclear reactions lead to a drop in pressure, the nucleus goes into a state of collapse. The energy released in this case is spent on the decay of iron to helium atoms, which further decays into protons and neutrons. The launched process is developing at a rapid pace. The collapse of a star characterizes the dynamic section of the scale and takes a fraction of a second in time. The ignition of the remaining nuclear fuel occurs in an explosive manner, releasing a colossal amount of energy in a fraction of a 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 core of the star begins to collapse (left). The collapse forms a neutron star and creates a flow of energy into the outer layers of the star (center). The energy released as a result of the ejection of the outer layers of a star during a supernova explosion (right).
The remaining superdense core will be a cluster of protons and electrons that collide with each other to form neutrons. The universe was replenished with a new object - a neutron star. Due to the high density, the nucleus becomes degenerate, and the process of collapse of the nucleus stops. If the mass of the star were large enough, the collapse could continue until the remnants of stellar matter finally fall into the center of the object, forming a black hole.
Explanation of the final part of the evolution of stars
For normal equilibrium stars, the described processes of evolution are unlikely. However, the existence of white dwarfs and neutron stars proves the real existence of processes of compression of stellar matter. A 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 - ejection of outer layers - white dwarf;
- massive star - red supergiant - supernova explosion - neutron star or black hole - non-existence.
Scheme of the evolution of stars. Options for the continuation of the life of stars outside the main sequence.
It is rather difficult to explain the ongoing processes from the point of view of science. Nuclear scientists agree that in the case of the final stage of stellar evolution, we are dealing with matter fatigue. As a result of prolonged mechanical, thermodynamic impact, matter changes its physical properties. The fatigue of stellar matter, exhausted by long-term nuclear reactions, can explain the appearance of a degenerate electron gas, its subsequent neutronization and annihilation. If all the listed processes go from beginning to end, stellar matter ceases to be a physical substance - the star disappears into space, leaving nothing behind.
Interstellar bubbles and gas and dust clouds, which are the birthplace of stars, cannot be replenished only at the expense of disappeared and exploded stars. The universe and galaxies are in equilibrium. 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 has disappeared in one place, in another place of the Universe the same amount of matter has appeared in a different form.
Finally
Studying the evolution of stars, we come to the conclusion that the Universe is a giant rarefied solution in which part of the matter is transformed into hydrogen molecules, which are the building material for stars. The other part dissolves into space, disappearing from the sphere of material sensations. A black hole in this sense is the transition point 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, relying only on the laws of nuclear, quantum physics and thermodynamics. The theory of relative probability should be connected to the study of this issue, which allows for the curvature of space, which allows one energy to be transformed into another, one state into another.
Thermonuclear fusion in the interior of stars
At this time, for stars with a mass greater than 0.8 solar masses, the core becomes transparent to radiation, and radiative energy transfer in the core will prevail, while the shell at the top remains convective. No one knows for sure what kind of stars of smaller mass arrive on the main sequence, since the time these stars spend in the category of young ones exceeds the age of the Universe. All our ideas about the evolution of these stars are based on numerical calculations.
As the star shrinks, the pressure of the degenerate electron gas begins to increase, and at some radius of the star, this pressure stops the growth of the central temperature, and then begins to lower it. And for stars less than 0.08, this turns out to be fatal: the energy released during nuclear reactions will never be enough to cover the cost of radiation. Such sub-stars are called brown dwarfs, and their fate is constant contraction until the pressure of the degenerate gas stops it, and then gradual cooling with a stop to all nuclear reactions.
Young stars of intermediate mass
Young stars of intermediate mass (from 2 to 8 solar masses) qualitatively evolve in exactly the same way as their smaller sisters, with the exception that they do not have convective zones until the main sequence.
Objects of this type are associated with the so-called. Ae\Be Herbit stars are irregular variables of spectral type B-F5. They also have bipolar jet disks. The exhaust velocity, luminosity, and effective temperature are substantially greater than for τ Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.
Young stars with a mass greater than 8 solar masses
In fact, these are already normal stars. While the mass of the hydrostatic core was accumulating, the star managed to skip all the intermediate stages and heat up the nuclear reactions to such an extent that they compensate for the losses due to radiation. For these stars, the outflow of mass and luminosity is so high that it not only stops the collapse of the remaining outer regions, but pushes them back. Thus, the mass of the formed star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars more than 100-200 solar masses.
mid-life cycle of a star
Among the formed stars there is a huge variety of colors and sizes. They range in spectral type from hot blues to cool reds, and in mass from 0.08 to more than 200 solar masses. The luminosity and color of a star depends on the temperature of its surface, which, in turn, is determined by its mass. All new stars "take their place" on the main sequence according to their chemical composition and mass. We are not talking about the physical movement of the star - only about its position on the indicated diagram, which depends on the parameters of the star. That is, we are talking, in fact, only about changing the parameters of the star.
What happens next depends again on the mass of the star.
Later years and the death of stars
Old stars with low mass
To date, it is not known for certain what happens to light stars after the depletion of the hydrogen supply. Since the universe is 13.7 billion years old, which is not enough to deplete the supply of hydrogen fuel, current theories are based on computer simulations of the processes occurring in such stars.
Some stars can only fuse helium in certain active regions, which causes instability and strong solar winds. In this case, the formation of a planetary nebula does not occur, and the star only evaporates, becoming even smaller than a brown dwarf.
But a star with a mass of less than 0.5 solar mass will never be able to synthesize helium even after reactions involving hydrogen cease in the core. Their stellar shell is not massive enough to overcome the pressure produced by the core. Such stars include red dwarfs (such as Proxima Centauri), whose main sequence lifetimes are hundreds of billions of years. After the termination of thermonuclear reactions in their core, they, gradually cooling down, will continue to weakly radiate in the infrared and microwave ranges of the electromagnetic spectrum.
medium sized stars
When a star reaches an average size (from 0.4 to 3.4 solar masses) of the red giant phase, its outer layers continue to expand, the core contracts, and reactions of carbon synthesis from helium begin. The fusion releases a lot of energy, giving the star a temporary reprieve. For a star similar in size to the Sun, this process can take about a billion years.
Changes in the amount of energy emitted cause the star to go through periods of instability, including changes in size, surface temperature, and energy release. The release of energy is shifted towards low-frequency radiation. All this is accompanied by an increasing mass loss due to strong solar winds and intense pulsations. The stars in this phase are called late-type stars, OH-IR stars or Mira-like stars, depending on their exact characteristics. The ejected gas is relatively rich in heavy elements produced in the interior of the star, such as oxygen and carbon. The gas forms an expanding shell and cools as it moves away from the star, allowing the formation of dust particles and molecules. With strong infrared radiation from the central star, ideal conditions are formed in such shells for the activation of masers.
Helium combustion reactions are very sensitive to temperature. Sometimes this leads to great instability. Violent pulsations occur, which eventually impart enough kinetic energy to the outer layers to be ejected and become a planetary nebula. In the center of the nebula, the core of the star remains, which, cooling down, turns into a helium white dwarf, as a rule, having a mass of up to 0.5-0.6 solar and a diameter of the order of the diameter of the Earth.
white dwarfs
The vast majority of stars, including the Sun, end their evolution by shrinking until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a factor of a hundred and the density becomes a million times that of water, the star is called a white dwarf. It is deprived of sources of energy and, gradually cooling down, becomes dark and invisible.
In stars more massive than the Sun, the pressure of degenerate electrons cannot hold back the contraction of the core, and it continues until most of the particles turn into neutrons, packed so densely that the size of the star is measured in kilometers, and the density is 100 million times greater than the density water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.
supermassive stars
After the outer layers of the star, with a mass greater than five solar masses, have scattered to form a red supergiant, the core begins to shrink due to gravitational forces. As the compression increases, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, heavy elements are synthesized, which temporarily restrains the collapse of the nucleus.
Ultimately, as more and more heavy elements of the periodic system are formed, iron -56 is synthesized from silicon. Up to this point, the synthesis of elements released a large amount of energy, but it is the iron-56 nucleus that has the maximum mass defect and the formation of heavier nuclei is unfavorable. Therefore, when the iron core of a star reaches a certain value, the pressure in it is no longer able to withstand the colossal force of gravity, and an immediate collapse of the core occurs with the neutronization of its matter.
What happens next is not entirely clear. But whatever it is, in a matter of seconds, it leads to the explosion of a supernova of incredible force.
The accompanying burst of neutrinos provokes a shock wave. Strong neutrino jets and a rotating magnetic field push out most of the material accumulated by the star - the so-called seating elements, including iron and lighter elements. The expanding matter is bombarded by neutrons escaping from the nucleus, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and possibly even California). Thus, supernova explosions explain the presence of elements heavier than iron in the interstellar matter.
The blast wave and jets of neutrinos carry material away from the dying star and into interstellar space. Subsequently, moving through space, this supernova material may collide with other space debris, and possibly participate in the formation of new stars, planets or satellites.
The processes that take place during the formation of a supernova are still being studied, and so far this issue is not clear. It is also questionable what actually remains of the original star. However, two options are being considered:
neutron stars
In some supernovae, the strong gravity in the interior of the supergiant is known to cause electrons to fall into the atomic nucleus, where they fuse with protons to form neutrons. The electromagnetic forces separating nearby nuclei disappear. The core of a star is now a dense ball of atomic nuclei and individual neutrons.
Such stars, known as neutron stars, are extremely small - no larger than a major city - and have unimaginably high densities. Their orbital period becomes extremely short as the size of the star decreases (due to conservation of angular momentum). Some make 600 revolutions per second. When the axis connecting the north and south magnetic poles of this rapidly rotating star points to the Earth, it is possible to fix a radiation pulse that repeats at intervals equal to the period of rotation of the star. Such neutron stars were called "pulsars", and became the first discovered neutron stars.
Black holes
Not all supernovae become neutron stars. If the star has a large enough mass, then the collapse of the star will continue and the neutrons themselves will begin to fall inward until its radius becomes less than the Schwarzschild radius. The star then becomes a black hole.
The existence of black holes was predicted by the general theory of relativity. According to general relativity, matter and information cannot leave a black hole under any circumstances. However, quantum mechanics makes exceptions to this rule possible.
A number of open questions remain. Chief among them: "Are there any black holes at all?" Indeed, in order to say for sure that a given object is a black hole, it is necessary to observe its event horizon. All attempts to do so ended in failure. But there is still hope, since some objects cannot be explained without involving accretion, moreover, accretion onto an object without a solid surface, but the very existence of black holes does not prove this.
Questions are also open: is it possible for a star to collapse directly into a black hole, bypassing a supernova? Are there supernovae that will eventually become black holes? What is the exact effect of the initial mass of a star on the formation of objects at the end of its life cycle?