What determines the final stage of a star's evolution? The nature of planetary nebulae. Episode I. Protostars

Each of us has looked at the starry sky at least once in our lives. Someone looked at this beauty, experiencing romantic feelings, another tried to understand where all this beauty comes from. Life in space, unlike life on our planet, flows at a different speed. Time in outer space lives in its own categories; distances and sizes in the Universe are colossal. We rarely think about the fact that the evolution of galaxies and stars is constantly happening before our eyes. Every object in the vast space is the result of certain physical processes. Galaxies, stars and even planets have main phases of development.

Our planet and we all depend on our star. How long will the Sun delight us with its warmth, breathing life into the Solar System? What awaits us in the future after millions and billions of years? In this regard, it is interesting to learn more about what are the stages of evolution of astronomical objects, where stars come from and how the life of these wonderful luminaries in the night sky ends.

Origin, birth and evolution of stars

The evolution of the stars and planets that inhabit our Milky Way galaxy and the entire Universe has, for the most part, been well studied. In space, the laws of physics are unshakable and help to understand the origin of space objects. In this case, it is customary to rely on the Big Bang theory, which is now the dominant doctrine about the process of the origin of the Universe. The event that shook the universe and led to the formation of the universe is, by cosmic standards, lightning fast. For the cosmos, moments pass from the birth of a star to its death. Vast distances create the illusion of the constancy of the Universe. A star that flares up in the distance shines on us for billions of years, at which time it may no longer exist.

The theory of evolution of the galaxy and stars is a development of the Big Bang theory. The doctrine of the birth of stars and the emergence star systems differs in the scale of what is happening and the time frame, which, unlike the Universe as a whole, can be observed modern means Sciences.

When studying the life cycle of stars, you can use the example of the closest star to us. The Sun is one of hundreds of trillions of stars in our field of vision. In addition, the distance from the Earth to the Sun (150 million km) provides a unique opportunity to study the object without leaving the solar system. The information obtained will make it possible to understand in detail how other stars are structured, how quickly these gigantic heat sources are depleted, what are the stages of development of a star, and what will be the ending of this brilliant life - quiet and dim or sparkling, explosive.

After big bang The smallest particles formed interstellar clouds, which became the “maternity hospital” for trillions of stars. It is characteristic that all stars were born at the same time as a result of compression and expansion. Compression in the clouds of cosmic gas occurred under the influence of its own gravity and similar processes in new stars in the neighborhood. The expansion arose as a result of the internal pressure of interstellar gas and under the influence of magnetic fields within the gas cloud. At the same time, the cloud rotated freely around its center of mass.

The gas clouds formed after the explosion consist of 98% atomic and molecular hydrogen and helium. Only 2% of this massif consists of dust and solid microscopic particles. Previously it was believed that at the center of any star lies a core of iron, heated to a temperature of a million degrees. It was this aspect that explained the gigantic mass of the star.

In the opposition of physical forces, compression forces prevailed, since the light resulting from the release of energy does not penetrate into the gas cloud. The light, along with part of the released energy, spreads outward, creating a dense accumulation of gas inside minus temperature and a low pressure area. Being in this state, the cosmic gas rapidly contracts, the influence of gravitational attraction forces leads to the fact that particles begin to form stellar matter. When a collection of gas is dense, the intense compression causes a star cluster to form. When the size of the gas cloud is small, compression leads to the formation of a single star.

A brief description of what is happening is that the future star goes through two stages - fast and slow compression to the state of a protostar. To put it simply and in clear language, rapid compression is the fall of stellar matter towards the center of the protostar. Slow compression occurs already against the background of the formed center of the protostar. Over the next hundreds of thousands of years, the new formation shrinks in size, and its density increases millions of times. Gradually, the protostar becomes opaque due to the high density of stellar matter, and the ongoing compression triggers the mechanism of internal reactions. An increase in internal pressure and temperature leads to the formation of the future star’s own center of gravity.

The protostar remains in this state for millions of years, slowly giving off heat and gradually shrinking, decreasing in size. As a result, the contours of the new star emerge, and the density of its matter becomes comparable to the density of water.

On average, the density of our star is 1.4 kg/cm3 - almost the same as the density of water in the salty Dead Sea. At the center, the Sun has a density of 100 kg/cm3. Stellar matter is not in liquid state, but exists in the form of plasma.

Under the influence of enormous pressure and temperature of approximately 100 million K, thermonuclear reactions of the hydrogen cycle begin. The compression stops, the mass of the object increases when the gravitational energy transforms into thermonuclear combustion of hydrogen. From this moment on, the new star, emitting energy, begins to lose mass.

The above-described version of star formation is just a primitive diagram that describes the initial stage of the evolution and birth of a star. Today, such processes in our galaxy and throughout the Universe are practically invisible due to the intense depletion of stellar material. In the entire conscious history of observations of our Galaxy, only isolated appearances of new stars have been noted. On the scale of the Universe, this figure can be increased hundreds and thousands of times.

For most of their lives, protostars are hidden from the human eye by a dusty shell. The radiation from the core can only be observed in the infrared, which is the only way to see the birth of a star. For example, in the Orion Nebula in 1967, astrophysicists discovered a new star in the infrared range, the radiation temperature of which was 700 degrees Kelvin. Subsequently, it turned out that the birthplace of protostars are compact sources that exist not only in our galaxy, but also in other distant corners of the Universe. In addition to infrared radiation, the birthplaces of new stars are marked by intense radio signals.

The process of studying and the evolution of stars

The entire process of knowing the stars can be divided into several stages. At the very beginning, you should determine the distance to the star. Information about how far the star is from us and how long the light has been coming from it gives an idea of ​​what happened to the star throughout this time. After man learned to measure the distance to distant stars, it became clear that stars are the same as suns, only different sizes and with different fates. Knowing the distance to the star, by the level of light and the amount of emitted energy, you can trace the process of thermal nuclear fusion stars.

After determining the distance to the star, you can use spectral analysis to calculate the chemical composition of the star and find out its structure and age. Thanks to the advent of the spectrograph, scientists have the opportunity to study the nature of starlight. This device can determine and measure the gas composition of stellar matter that a star possesses on different stages of its existence.

By studying the spectral analysis of the energy of the Sun and other stars, scientists came to the conclusion that the evolution of stars and planets has common roots. All cosmic bodies have the same type, similar chemical composition and originated from the same matter, which arose as a result of the Big Bang.

Stellar matter consists of the same chemical elements (even iron) as our planet. The only difference is in the amount of certain elements and in the processes occurring on the Sun and inside the earth's solid surface. This is what distinguishes stars from other objects in the Universe. The origin of stars should also be considered in the context of another physical discipline: quantum mechanics. According to this theory, the matter that defines stellar matter consists of constantly dividing atoms and elementary particles creating their own microcosm. In this light, the structure, composition, structure and evolution of stars is of interest. As it turned out, the bulk of the mass of our star and many other stars consists of only two elements - hydrogen and helium. A theoretical model describing the structure of stars will allow us to understand their structure and the main difference from other space objects.

main feature is that many objects in the Universe have a certain size and shape, while a star can change size as it develops. A hot gas is a combination of atoms loosely bound to each other. Millions of years after the formation of a star, the surface layer of stellar matter begins to cool. The star gives off most of its energy into outer space, decreasing or increasing in size. Heat and energy are transferred from the interior of the star to the surface, affecting the intensity of radiation. In other words, the same star in different periods its existence looks different. Thermonuclear processes based on reactions of the hydrogen cycle contribute to the transformation of light hydrogen atoms into heavier elements - helium and carbon. According to astrophysicists and nuclear scientists, such a thermonuclear reaction is the most efficient in terms of the amount of heat generated.

Why doesn’t thermonuclear fusion of the nucleus end with the explosion of such a reactor? The thing is that the forces of the gravitational field in it can hold stellar matter within a stabilized volume. From this we can draw an unambiguous conclusion: any star is a massive body that maintains its size due to the balance between gravitational forces and thermal energy nuclear reactions. The result of this ideal natural model is a heat source that can operate for a long time. It is assumed that the first forms of life on Earth appeared 3 billion years ago. The sun in those distant times warmed our planet just as it does now. Consequently, our star has changed little, despite the fact that the scale of the emitted heat and solar energy colossal - more than 3-4 million tons every second.

It is not difficult to calculate how much weight our star has lost over the years of its existence. This will be a huge figure, but due to its enormous mass and high density, such losses on the scale of the Universe look insignificant.

Stages of star evolution

The fate of the star depends on the initial mass of the star and its chemical composition A. While the main reserves of hydrogen are concentrated in the core, the star remains in the so-called main sequence. As soon as there is a tendency for the size of the star to increase, it means that the main source for thermonuclear fusion has dried up. The long final path of transformation of the celestial body has begun.

The luminaries formed in the Universe are initially divided into three most common types:

• normal stars (yellow dwarfs);
• dwarf stars;
• giant stars.

Low-mass stars (dwarfs) slowly burn up their hydrogen reserves and live their lives quite calmly.

Such stars are the majority in the Universe, and our star, a yellow dwarf, is one of them. With the onset of old age, a yellow dwarf becomes a red giant or supergiant.

Based on the theory of the origin of stars, the process of star formation in the Universe has not ended. The brightest stars in our galaxy are not only the largest, compared to the Sun, but also the youngest. Astrophysicists and astronomers call such stars blue supergiants. In the end, they will suffer the same fate as trillions of other stars. First there is a rapid birth, a brilliant and ardent life, after which comes a period of slow decay. Stars the size of the Sun have a long life cycle, being in the main sequence (in its middle part).

Using data on the mass of a star, we can assume its evolutionary path of development. A clear illustration of this theory is the evolution of our star. Nothing lasts forever. As a result of thermonuclear fusion, hydrogen is converted into helium, therefore, its original reserves are consumed and reduced. Someday, not very soon, these reserves will run out. Judging by the fact that our Sun continues to shine for more than 5 billion years, without changing in its size, the mature age of the star can still last about the same period.

The depletion of hydrogen reserves will lead to the fact that, under the influence of gravity, the core of the sun will begin to rapidly shrink. The density of the core will become very high, as a result of which thermonuclear processes will move to the layers adjacent to the core. This state is called collapse, which can be caused by the passage of thermonuclear reactions in upper layers stars. As a result of high pressure, thermonuclear reactions involving helium are triggered.

The reserves of hydrogen and helium in this part of the star will last for millions of years. It will not be long before the depletion of hydrogen reserves will lead to an increase in the intensity of radiation, to an increase in the size of the shell and the size of the star itself. As a result, our Sun will become very large. If you imagine this picture tens of billions of years from now, then instead of a dazzling bright disk, a hot red disk of gigantic proportions will hang in the sky. Red giants are a natural phase in the evolution of a star, its transition state into the category of variable stars.

As a result of this transformation, the distance from the Earth to the Sun will decrease, so that the Earth will fall into the zone of influence of the solar corona and begin to “fry” in it. The temperature on the surface of the planet will increase tenfold, which will lead to the disappearance of the atmosphere and the evaporation of water. As a result, the planet will turn into a lifeless rocky desert.

The final stages of stellar evolution

Having reached the red giant phase, a normal star becomes a white dwarf under the influence of gravitational processes. If the mass of a star is approximately equal to the mass of our Sun, all the main processes in it will occur calmly, without impulses or explosive reactions. The white dwarf will die for a long time, burning out to the ground.

In cases where the star initially had a mass greater than 1.4 times the Sun, the white dwarf will not be the final stage. With a large mass inside the star, processes of compaction of stellar matter begin at the atomic and molecular level. Protons turn into neutrons, the density of the star increases, and its size rapidly decreases.

Neutron stars known to science have a diameter of 10-15 km. With such a small size, a neutron star has a colossal mass. One cubic centimeter of stellar matter can weigh billions of tons.

In the event that we were initially dealing with a high-mass star, the final stage of evolution takes other forms. The fate of a massive star is a black hole - an object with an unexplored nature and unpredictable behavior. The huge mass of the star contributes to an increase in gravitational forces, driving compression forces. It is not possible to pause this process. The density of matter increases until it becomes infinite, forming a singular space (Einstein's theory of relativity). The radius of such a star will eventually become zero, becoming a black hole in outer space. There would be significantly more black holes if massive and supermassive stars occupied most of the space.

It should be noted that when a red giant transforms into a neutron star or a black hole, the Universe can experience a unique phenomenon - the birth of a new cosmic object.

The birth of a supernova is the most spectacular final stage in the evolution of stars. A natural law of nature operates here: the cessation of the existence of one body gives rise to a new life. The period of such a cycle as the birth of a supernova mainly concerns massive stars. The exhausted reserves of hydrogen lead to the inclusion of helium and carbon in the process of thermonuclear fusion. As a result of this reaction, the pressure increases again, and an iron core is formed in the center of the star. Under the influence of strong gravitational forces, the center of mass shifts to the central part of the star. The core becomes so heavy that it is unable to resist its own gravity. As a result, rapid expansion of the core begins, leading to an instant explosion. The birth of a supernova is an explosion, a shock wave of monstrous force, a bright flash in the vast expanses of the Universe.

It should be noted that our Sun is not a massive star, so a similar fate does not threaten it, and our planet should not be afraid of such an ending. In most cases, supernova explosions occur in distant galaxies, which is why they are rarely detected.

Finally

The evolution of stars is a process that extends over tens of billions of years. Our idea of ​​the processes taking place is just a mathematical and physical model, a theory. Earthly time is only a moment in the huge time cycle in which our Universe lives. We can only observe what happened billions of years ago and imagine what subsequent generations of earthlings may face.

If you have any questions, leave them in the comments below the article. We or our visitors will be happy to answer them

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 prevails, while the shell at the top remains convective. No one knows for sure how stars of lower mass arrive on the main sequence, since the time these stars spend in the young category exceeds the age of the Universe. All our ideas about the evolution of these stars are based on numerical calculations.

As the star contracts, the pressure of the degenerate electron gas begins to increase, and at a certain radius of the star, this pressure stops the increase in the central temperature, and then begins to lower it. And for stars smaller than 0.08, this turns out to be fatal: the energy released during nuclear reactions will never be enough to cover the costs of radiation. Such sub-stars are called brown dwarfs, and their fate is constant compression until the pressure of the degenerate gas stops it, and then gradual cooling with the stop of all nuclear reactions.

Young intermediate mass stars

Young stars of intermediate mass (from 2 to 8 times the mass of the Sun) evolve qualitatively in exactly the same way as their smaller sisters, except 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 with irregular variables of spectral type B-F5. They also have bipolar jet disks. The outflow velocity, luminosity and effective temperature are significantly higher 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 jump through all the intermediate stages and heat up nuclear reactions to such an extent that they compensated for losses due to radiation. For these stars, the outflow of mass and luminosity is so great that it not only stops the collapse of the remaining outer regions, but pushes them back. Thus, the mass of the resulting star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars more than 100-200 times the mass of the Sun.

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 blue to cool red, 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, depending on the parameters of the star. That is, we are talking, in fact, only about changing the parameters of the star.

What happens next again depends on the mass of the star.

Later years and death of stars

Old stars with low mass

To date, it is not known for certain what happens to light stars after their hydrogen supply is depleted. Since the universe is 13.7 billion years old, which is not long enough to exhaust its supply of hydrogen fuel, modern theories are based on computer modeling of the processes occurring in such stars.

Some stars can only synthesize helium in some active areas, causing 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 less than 0.5 solar will never be able to synthesize helium even after reactions involving hydrogen cease in the core. Their stellar envelope is not massive enough to overcome the pressure generated by the core. These stars include red dwarfs (such as Proxima Centauri), which have been on the main sequence for hundreds of billions of years. After the cessation of thermonuclear reactions in their core, they, gradually cooling, will continue to weakly emit in the infrared and microwave ranges of the electromagnetic spectrum.

Medium sized stars

When the star reaches average size(from 0.4 to 3.4 solar masses) red giant phase, its outer layers continue to expand, the core contracts, and reactions begin to synthesize carbon from helium. 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 output. Energy output shifts towards low frequency radiation. All this is accompanied by increasing mass loss due to strong solar winds and intense pulsations. 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 star's interior, 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 for the activation of masers are formed in such shells.

Helium combustion reactions are very temperature sensitive. 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, as it cools, turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar and a diameter on the order of the diameter of the Earth.

White dwarfs

The vast majority of stars, including the Sun, end their evolution by contracting until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a hundred times, and the density becomes a million times higher than the density of water, the star is called a white dwarf. It is deprived of energy sources and, gradually cooling down, becomes dark and invisible.

In stars more massive than the Sun, the pressure of degenerate electrons cannot contain the compression of the core, and it continues until most of the particles are converted into neutrons, packed so tightly that the size of the star is measured in kilometers and is 100 million times denser 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 a star with a mass greater than five solar masses have scattered to form a red supergiant, the core begins to compress due to gravitational forces. As compression increases, 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 heavier and heavier elements of the periodic table are formed, iron-56 is synthesized from silicon. Up until 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 immediate collapse of the core occurs with neutronization of its matter.

What happens next is not entirely clear. But whatever it is, it causes a supernova explosion of incredible power in a matter of seconds.

The accompanying burst of neutrinos provokes a shock wave. Strong jets of neutrinos and a rotating magnetic field push out much of the star's accumulated material - the so-called seed elements, including iron and lighter elements. The exploding matter is bombarded by neutrons emitted from the nucleus, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and perhaps even californium). Thus, supernova explosions explain the presence of elements heavier than iron in interstellar matter.

The blast wave and jets of neutrinos carry material away from the dying star 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 occurring during the formation of a supernova are still being studied, and so far there is no clarity on this issue. It is also questionable what actually remains of the original star. However, two options are being considered:

Neutron stars

It is known that in some supernovae, strong gravity in the depths of the supergiant causes electrons to fall into the atomic nucleus, where they fuse with protons to form neutrons. The electromagnetic forces separating nearby nuclei disappear. The star's core is now a dense ball of atomic nuclei and individual neutrons.

Such stars, known as neutron stars, are extremely small - no more than the size of a large city - and have an unimaginably high density. Their orbital period becomes extremely short as the size of the star decreases (due to 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 toward Earth, a pulse of radiation can be detected repeating at intervals equal to the star's orbital period. Such neutron stars were called “pulsars”, and became the first neutron stars to be discovered.

Black holes

Not all supernovae become neutron stars. If the star has a sufficiently large 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. After this, the star 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 black hole no way. However, quantum mechanics makes exceptions to this rule possible.

A number of open questions remain. Chief among them: “Are there black holes at all?” After all, 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 this ended in failure. But there is still hope, since some objects cannot be explained without involving accretion, and accretion onto an object without a solid surface, but this does not prove the very existence of black holes.

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 later become black holes? What is the exact influence of a star's initial mass on the formation of objects at the end of its life cycle?

The evolution of stars is a change in physicality. characteristics, internal structures and chemistry composition of stars over time. The most important tasks of the theory of E.Z. - explanation of the formation of stars, changes in their observable characteristics, study of the genetic connection of various groups of stars, analysis of their final states.

Since in the part of the Universe known to us, approx. 98-99% of the mass of the observed matter is contained in stars or has passed the stage of stars, explanation by E.Z. yavl. one of the most important problems in astrophysics.

A star in a stationary state is a gas ball, which is in a hydrostatic state. and thermal equilibrium (i.e., the action of gravitational forces is balanced by internal pressure, and energy losses due to radiation are compensated by the energy released in the bowels of the star, see). The “birth” of a star is the formation of a hydrostatically equilibrium object, the radiation of which is supported by its own. energy sources. The “death” of a star is an irreversible imbalance leading to the destruction of the star or its catastrophe. compression.

Isolation of gravitational energy can play a decisive role only when the temperature of the star’s interior is insufficient for nuclear energy release to compensate for energy losses, and the star as a whole or part of it must contract to maintain equilibrium. The release of thermal energy becomes important only after nuclear energy reserves have been exhausted. T.o., E.z. can be represented as a consistent change in the energy sources of stars.

Characteristic time E.z. too large for all evolution to be traced directly. Therefore the main E.Z. research method yavl. construction of sequences of star models describing changes in internal structures and chemistry composition of stars over time. Evolution. the sequences are then compared with the results of observations, for example, with (G.-R.d.), summing up the observations large number stars at different stages of evolution. A particularly important role is played by comparison with G.-R.d. for star clusters, since all stars in a cluster have the same initial chemical. composition and formed almost simultaneously. According to G.-R.d. clusters of different ages, it was possible to establish the direction of the E.Z. Evolution in detail. sequences are calculated by numerically solving a system of differential equations describing the distribution of mass, density, temperature and luminosity over a star, to which are added the laws of energy release and opacity of stellar matter and equations describing changes in chemical properties. star composition over time.

The course of a star's evolution depends mainly on its mass and initial chemistry. composition. The rotation of the star and its magnetic field can play a certain, but not fundamental, role. field, however, the role of these factors in E.Z. has not yet been sufficiently researched. Chem. The composition of a star depends on the time at which it was formed and on its position in the Galaxy at the time of formation. Stars of the first generation were formed from matter, the composition of which was determined by cosmology. conditions. Apparently, it contained approximately 70% by mass hydrogen, 30% helium and an insignificant admixture of deuterium and lithium. During the evolution of first-generation stars, heavy elements (following helium) were formed, which were ejected into interstellar space as a result of the outflow of matter from stars or during stellar explosions. Stars of subsequent generations were formed from matter containing up to 3-4% (by mass) of heavy elements.

The most direct indication that star formation in the Galaxy is still ongoing is the phenomenon. existence of massive bright stars spectrum. classes O and B, the lifetime of which cannot exceed ~ 10 7 years. The rate of star formation in modern times. era is estimated at 5 per year.

2. Star formation, stage of gravitational compression

According to the most common point of view, stars are formed as a result of gravitational forces. condensation of matter in the interstellar medium. The necessary division of the interstellar medium into two phases - dense cold clouds and a rarefied medium with a higher temperature - can occur under the influence of Rayleigh-Taylor thermal instability in the interstellar magnetic field. field. Gas-dust complexes with mass , characteristic size (10-100) pc and particle concentration n~10 2 cm -3 . are actually observed due to their emission of radio waves. Compression (collapse) of such clouds requires certain conditions: gravity. particles of the cloud must exceed the sum of the energy of thermal motion of the particles, the rotational energy of the cloud as a whole and the magnetic field. cloud energy (Jeans criterion). If only the energy of thermal motion is taken into account, then, accurate to a factor of the order of unity, the Jeans criterion is written in the form: align="absmiddle" width="205" height="20">, where is the mass of the cloud, T- gas temperature in K, n- number of particles per 1 cm3. With typical modern interstellar clouds temperature K can only collapse clouds with a mass not less than . The Jeans criterion indicates that for the formation of stars of the actually observed mass spectrum, the concentration of particles in collapsing clouds must reach (10 3 -10 6) cm -3, i.e. 10-1000 times higher than observed in typical clouds. However, such concentrations of particles can be achieved in the depths of clouds that have already begun to collapse. It follows from this that it happens through a sequential process, carried out in several steps. stages, fragmentation of massive clouds. This picture naturally explains the birth of stars in groups - clusters. At the same time, questions related to the thermal balance in the cloud, the velocity field in it, and the mechanism determining the mass spectrum of fragments still remain unclear.

Collapsed stellar mass objects are called protostars. Collapse of a spherically symmetric non-rotating protostar without a magnetic field. fields includes several. stages. At the initial moment of time, the cloud is homogeneous and isothermal. It is transparent to its own. radiation, so the collapse comes with volumetric energy losses, Ch. arr. due to thermal radiation dust, the cut transmits its kinetic. energy of a gas particle. In a homogeneous cloud there is no pressure gradient and compression begins in free fall with a characteristic time , where G- , - cloud density. With the beginning of compression, a rarefaction wave appears, moving towards the center at the speed of sound, and since collapse occurs faster where the density is higher, the protostar is divided into a compact core and an extended shell, into which the matter is distributed according to the law. When the concentration of particles in the core reaches ~ 10 11 cm -3 it becomes opaque to the IR radiation of dust grains. The energy released in the core slowly seeps to the surface due to radiative thermal conduction. The temperature begins to increase almost adiabatically, this leads to an increase in pressure, and the core becomes hydrostatic. balance. The shell continues to fall onto the core, and it appears at its periphery. The parameters of the core at this time weakly depend on the total mass of the protostar: K. As the mass of the core increases due to accretion, its temperature changes almost adiabatically until it reaches 2000 K, when the dissociation of H 2 molecules begins. As a result of energy consumption for dissociation, and not an increase in kinetic. particle energy, the adiabatic index value becomes less than 4/3, pressure changes are not able to compensate for gravitational forces and the core collapses again (see). A new core with parameters is formed, surrounded by a shock front, onto which the remnants of the first core accrete. A similar rearrangement of the nucleus occurs with hydrogen.

Further growth of the core at the expense of the shell matter continues until all the matter falls onto the star or is scattered under the influence of or, if the core is sufficiently massive (see). Protostars with a characteristic time of shell matter t a >t kn, therefore their luminosity is determined by the energy release of the collapsing nuclei.

A star, consisting of a core and an envelope, is observed as an IR source due to the processing of radiation in the envelope (the dust of the envelope, absorbing photons of UV radiation from the core, emits in the IR range). When the shell becomes optically thin, the protostar begins to be observed as an ordinary object of stellar nature. The most massive stars retain their shells until thermonuclear burning of hydrogen begins at the center of the star. Radiation pressure limits the mass of stars to probably . Even if more massive stars are formed, they turn out to be pulsationally unstable and can lose their power. part of the mass at the stage of hydrogen combustion in the core. The duration of the stage of collapse and scattering of the protostellar shell is of the same order as the free fall time for the parent cloud, i.e. 10 5 -10 6 years. Illuminated by the core, clumps of dark matter from the remnants of the shell, accelerated by the stellar wind, are identified with Herbig-Haro objects (stellar clumps with an emission spectrum). Low-mass stars, when they become visible, are in the G.-R.D. region occupied by T Tauri stars (dwarf), more massive ones are in the region where Herbig emission stars are located (irregular early spectral classes with emission lines in spectra).

Evolution. tracks of protostar cores with constant mass at the hydrostatic stage. compressions are shown in Fig. 1. For stars of low mass, at the moment when hydrostatic is established. equilibrium, the conditions in the nuclei are such that energy is transferred to them. Calculations show that the surface temperature of a fully convective star is almost constant. The radius of the star is continuously decreasing, because she continues to shrink. With a constant surface temperature and a decreasing radius, the luminosity of the star should also fall on the G.-R.D. This stage of evolution corresponds to vertical sections of tracks.

As the compression continues, the temperature in the interior of the star increases, the matter becomes more transparent, and stars with align="absmiddle" width="90" height="17"> have radiant cores, but the shells remain convective. Less massive stars remain completely convective. Their luminosity is controlled by a thin radiant layer in the photosphere. The more massive the star and the higher its effective temperature, the larger its radiative core (in stars with align="absmiddle" width="74" height="17"> the radiative core appears immediately). In the end, almost the entire star (with the exception of the surface convective zone for stars with a mass) goes into a state of radiative equilibrium, in which all the energy released in the core is transferred by radiation.

3. Evolution based on nuclear reactions

At a temperature in the nuclei of ~ 10 6 K, the first nuclear reactions begin - deuterium, lithium, boron burn out. The primary quantity of these elements is so small that their burnout practically does not withstand compression. The compression stops when the temperature at the center of the star reaches ~ 10 6 K and hydrogen ignites, because The energy released during thermonuclear combustion of hydrogen is sufficient to compensate for radiation losses (see). Homogeneous stars, in the cores of which hydrogen burns, form on the G.-R.D. initial main sequence (IMS). Massive stars reach the NGP faster than low-mass stars, because their rate of energy loss per unit mass, and therefore the rate of evolution, is higher than that of low-mass stars. Since entering the NGP E.z. occurs on the basis of nuclear combustion, the main stages of which are summarized in table. Nuclear combustion can occur before the formation of iron group elements, which have the highest binding energy among all nuclei. Evolution. tracks of stars on G.-R.D. are shown in Fig. 2. The evolution of the central values ​​of temperature and density of stars is shown in Fig. 3. At K main. source of energy yavl. reaction of the hydrogen cycle, at large T- reactions of the carbon-nitrogen (CNO) cycle (see). A side effect of the CNO cycle is. establishing equilibrium concentrations of nuclides 14 N, 12 C, 13 C - 95%, 4% and 1% by weight, respectively. The predominance of nitrogen in the layers where hydrogen combustion occurred is confirmed by the results of observations, in which these layers appear on the surface as a result of the loss of external. layers. In stars in the center of which the CNO cycle is realized ( align="absmiddle" width="74" height="17">), a convective core appears. The reason for this is the very strong dependence of energy release on temperature: . The flow of radiant energy ~ T 4(see), therefore, it cannot transfer all the energy released, and convection must occur, which is more efficient than radiative transfer. In the most massive stars, more than 50% of the stellar mass is covered by convection. The importance of the convective core for evolution is determined by the fact that nuclear fuel is uniformly depleted in a region much larger than the region of effective combustion, while in stars without a convective core it initially burns out only in a small vicinity of the center, where the temperature is quite high. The hydrogen burnout time ranges from ~ 10 10 years for to years for . The time of all subsequent stages of nuclear combustion does not exceed 10% of the time of hydrogen combustion, therefore stars at the stage of hydrogen combustion form on the G.-R.D. densely populated region - (GP). In stars with a temperature in the center that never reaches the values ​​necessary for the combustion of hydrogen, they shrink indefinitely, turning into “black” dwarfs. Burnout of hydrogen leads to an increase in avg. molecular weight of the core substance, and therefore to maintain hydrostatic. equilibrium, the pressure in the center must increase, which entails an increase in the temperature in the center and the temperature gradient across the star, and consequently, the luminosity. An increase in luminosity also results from a decrease in the opacity of matter with increasing temperature. The core contracts to maintain nuclear energy release conditions with a decrease in hydrogen content, and the shell expands due to the need to transfer the increased energy flow from the core. On G.-R.d. the star moves to the right of the NGP. A decrease in opacity leads to the death of convective cores in all but the most massive stars. The rate of evolution of massive stars is the highest, and they are the first to leave the MS. The lifetime on the MS is for stars with ca. 10 million years, from ca. 70 million years, and from ca. 10 billion years.

When the hydrogen content in the core decreases to 1%, the expansion of the shells of stars with align="absmiddle" width="66" height="17"> is replaced by a general contraction of the star necessary to maintain energy release. Compression of the shell causes heating of hydrogen in the layer adjacent to the helium core to the temperature of its thermonuclear combustion, and a layer source of energy release arises. In stars with mass , in which it depends less on temperature and the region of energy release is not so strongly concentrated towards the center, there is no stage of general compression.

E.z. after hydrogen burns out depends on their mass. The most important factor, influencing the course of evolution of stars with mass , yavl. degeneracy of electron gas at high densities. Due to the high density, the number of quantum states with low energy is limited due to the Pauli principle and electrons fill quantum levels with high energy, significantly exceeding the energy of their thermal motion. The most important feature of a degenerate gas is that its pressure p depends only on the density: for non-relativistic degeneracy and for relativistic degeneracy. The gas pressure of electrons is much greater than the pressure of ions. This follows what is fundamental for E.Z. conclusion: since the gravitational force acting on a unit volume of a relativistically degenerate gas depends on density in the same way as the pressure gradient, there must be a limiting mass (see), such that at align="absmiddle" width="66" height ="15"> electron pressure cannot counteract gravity and compression begins. Limit weight align="absmiddle" width="139" height="17">. The boundary of the region in which the electron gas is degenerate is shown in Fig. 3. In low-mass stars, degeneracy plays a noticeable role already in the process of formation of helium nuclei.

The second factor determining E.z. at later stages, these are neutrino energy losses. In the depths of the stars T~10 8 K main. a role in the birth is played by: photoneutrino process, decay of plasma oscillation quanta (plasmons) into neutrino-antineutrino pairs (), annihilation of electron-positron pairs () and (see). The most important feature of neutrinos is that the star’s matter is almost transparent to them and neutrinos freely carry energy away from the star.

The helium core, in which conditions for helium combustion have not yet arisen, is compressed. The temperature in the layered source adjacent to the core increases, and the rate of hydrogen combustion increases. The need to transfer an increased energy flow leads to expansion of the shell, for which part of the energy is wasted. Since the luminosity of the star does not change, the temperature of its surface drops, and on the G.-R.D. the star moves to the region occupied by red giants. The star's restructuring time is two orders of magnitude less than the time it takes for hydrogen to burn out in the core, so there are few stars between the MS strip and the region of red supergiants. With a decrease in the temperature of the shell, its transparency increases, as a result of which an external appearance appears. convective zone and the luminosity of the star increases.

The removal of energy from the core through the thermal conductivity of degenerate electrons and neutrino losses in stars delays the moment of helium combustion. The temperature begins to increase noticeably only when the core becomes almost isothermal. Combustion of 4 He determines the E.Z. from the moment when the energy release exceeds the energy loss through thermal conductivity and neutrino radiation. The same condition applies to the combustion of all subsequent types of nuclear fuel.

A remarkable feature of stellar cores made of degenerate gas, cooled by neutrinos, is “convergence” - the convergence of tracks, which characterize the relationship between density and temperature Tc in the center of the star (Fig. 3). The rate of energy release during compression of the core is determined by the rate of addition of matter to it through a layer source, and depends only on the mass of the core for a given type of fuel. A balance of inflow and outflow of energy must be maintained in the core, therefore the same distribution of temperature and density is established in the cores of stars. By the time 4 He ignites, the mass of the nucleus depends on the content of heavy elements. In nuclei of degenerate gas, the combustion of 4 He has the character of a thermal explosion, because the energy released during combustion goes to increase the energy of thermal motion of electrons, but the pressure remains almost unchanged with increasing temperature until thermal energy electrons is not equal to the energy of the degenerate gas of electrons. Then the degeneracy is removed and the core rapidly expands - a helium flash occurs. Helium flares are likely accompanied by the loss of stellar matter. In , where massive stars have long finished evolution and red giants have masses, stars at the helium burning stage are on the horizontal branch of the G.-R.D.

In the helium cores of stars with align="absmiddle" width="90" height="17"> the gas is not degenerate, 4 He ignites quietly, but the cores also expand due to increasing Tc. In the most massive stars, the combustion of 4 He occurs even when they are active. blue supergiants. Expansion of the core leads to a decrease T in the region of the hydrogen layer source, and the luminosity of the star after the helium burst decreases. To maintain thermal equilibrium, the shell contracts, and the star leaves the region of red supergiants. When the 4 He in the core is depleted, compression of the core and expansion of the shell begin again, the star again becomes a red supergiant. A layered combustion source of 4 He is formed, which dominates the energy release. External appears again. convective zone. As helium and hydrogen burn out, the thickness of the layer sources decreases. A thin layer of helium combustion turns out to be thermally unstable, because with a very strong sensitivity of energy release to temperature (), the thermal conductivity of the substance is insufficient to extinguish thermal disturbances in the combustion layer. During thermal outbreaks, convection occurs in the layer. If it penetrates into layers rich in hydrogen, then as a result of a slow process ( s-process, see) elements are synthesized with atomic masses from 22 Ne to 209 B.

Radiation pressure on dust and molecules formed in the cold, extended shells of red supergiants leads to continuous loss of matter at a rate of up to a year. Continuous mass loss can be supplemented by losses caused by instability of layer combustion or pulsations, which can lead to the release of one or more. shells. When the amount of substance above the carbon-oxygen core becomes less than a certain limit, the shell, in order to maintain the temperature in the combustion layers, is forced to compress until the compression is capable of maintaining combustion; star on G.-R.D. moves almost horizontally to the left. At this stage, the instability of the combustion layers can also lead to expansion of the shell and loss of matter. While the star is hot enough, it is observed as a core with one or more. shells. When layer sources shift toward the surface of the star so much that the temperature in them becomes lower than that required for nuclear combustion, the star cools, turning into a white dwarf with , radiating due to the consumption of thermal energy of the ionic component of its matter. The characteristic cooling time of white dwarfs is ~ 10 9 years. The lower limit on the masses of single stars turning into white dwarfs is unclear, it is estimated at 3-6. In c stars, the electron gas degenerates at the stage of growth of carbon-oxygen (C,O-) stellar cores. As in the helium cores of stars, due to neutrino energy losses, a “convergence” of conditions occurs in the center and at the moment of combustion of carbon in the C,O core. The combustion of 12 C under such conditions most likely has the nature of an explosion and leads to the complete destruction of the star. Complete destruction may not occur if . Such a density is achievable when the core growth rate is determined by the accretion of satellite matter in a close binary system.

The Universe is a constantly changing macrocosm, where every object, substance or matter is in a state of transformation and change. These processes last for billions of years. Compared to duration human life this incomprehensible period of time is enormous. On a cosmic scale, these changes are quite fleeting. The stars that we now see in the night sky were the same thousands of years ago, when 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 age - the evolution of stars goes on as usual.

The position of the stars of the constellation Ursa Major in different historical periods in the interval 100,000 years ago - our time and after 100 thousand years

Interpretation of the evolution of stars from the point of view of the average person

For the average person, space appears to be a world of calm and silence. In fact, the Universe is a giant physical laboratory where enormous transformations are taking place, during which the chemical composition, physical characteristics and structure of stars change. The life of a star lasts as long as it shines and gives off heat. However, such a brilliant state does not last forever. The bright birth is followed by a period of star maturity, which inevitably ends with the aging of the celestial body and its death.

Formation of a protostar from a gas and dust cloud 5-7 billion years ago

All our information about stars today fits within the framework of science. Thermodynamics gives us an explanation of the processes of hydrostatic and thermal equilibrium in which stellar matter resides. Nuclear and quantum physics provide insight into difficult process nuclear fusion, thanks to which a star exists, emitting heat and giving light to the surrounding space. At the birth of a star, hydrostatic and thermal equilibrium is formed, maintained by its own energy sources. At the end of a brilliant stellar career, this balance is disrupted. A series of irreversible processes begins, the result of which is the destruction of the star or collapse - a grandiose process of instant and brilliant death of the heavenly body.

A supernova explosion is a bright finale to the life of a star born in the early years of the Universe.

Changes in the physical characteristics of stars are due to their mass. The rate of evolution of objects is influenced by their chemical composition and, to some extent, by existing astrophysical parameters - the speed of rotation and the state of the magnetic field. It is not possible to talk exactly about how everything actually happens due to the enormous duration of the processes described. The rate of evolution and the stages of transformation depend on the time of birth of the star and its location in the Universe at the time of birth.

The evolution of stars from a scientific point of view

Any star is born from a clump of cold interstellar gas, which, under the influence of external and internal gravitational forces, is compressed to the state of a gas ball. The process of compression of the gaseous substance does not stop for a moment, accompanied by a colossal release of thermal energy. The temperature of the new formation increases until thermonuclear fusion starts. From this moment, the compression of stellar matter stops, and a balance is reached between the hydrostatic and thermal states of the object. The Universe has been replenished with a new full-fledged star.

The main stellar fuel is the hydrogen atom as a result of a launched thermonuclear reaction.

In the evolution of stars, their sources of thermal energy are of fundamental importance. The radiant and thermal energy escaping into space from the surface of the star is replenished 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 depths of the future star. The free fall of molecules and atoms stops, and the process of compression of stellar gas stops. This state of an object is usually called a protostar. The object is 90% molecular hydrogen. When the temperature reaches 1800K, hydrogen passes into the atomic state. During the decay process, energy is consumed, and the temperature increase slows down.

The Universe is 75% composed of molecular hydrogen, which during the formation of protostars turns into atomic hydrogen - the nuclear fuel of a star

In this state, the pressure inside the gas ball decreases, thereby giving freedom to the compression force. This sequence is repeated each time all the hydrogen is ionized first, and then the helium is ionized. At a temperature of 10⁵ K, the gas is completely ionized, the compression of the star stops, and hydrostatic equilibrium of the object arises. The further evolution of the star will occur in accordance with the thermal time scale, much slower and more consistent.

The radius of the protostar has been decreasing from 100 AU since the beginning of formation. up to ¼ a.u. The object is in the middle of a gas cloud. As a result of the accretion of particles from the outer regions of the stellar gas cloud, the mass of the star will constantly increase. Consequently, the temperature inside the object will increase, accompanying the process of convection - the transfer of energy from the inner layers of the star to its outer edge. Subsequently, with increasing temperature in the interior of the celestial body, convection is replaced by radiative transfer, moving towards the surface of the star. At this moment, the luminosity of the object rapidly increases, and the temperature of the surface layers of the stellar ball also increases.

Convection processes and radiative transfer in a newly formed star before the onset of thermonuclear fusion reactions

For example, for stars with a mass identical to the mass of our Sun, the compression of the protostellar cloud occurs in just a few hundred years. As for the final stage of the formation of the object, the condensation of stellar matter has been stretching for millions of years. The Sun is moving towards the main sequence quite quickly, and this journey will take hundreds of millions or billions of years. In other words, the greater the mass of the star, the longer the period of time spent on the formation of a full-fledged star. A star with a mass of 15M will move along the path to the main sequence for much longer - about 60 thousand years.

Main sequence phase

Although some fusion reactions are started at more low temperatures, the main phase of hydrogen combustion starts at a temperature of 4 million degrees. From this moment the main sequence phase begins. Comes into play new form reproduction of stellar energy - nuclear. The kinetic energy released during the compression of an object fades into the background. The achieved equilibrium ensures a long and quiet life for a star that finds itself in the initial phase of the main sequence.

The fission and decay of hydrogen atoms during a thermonuclear reaction occurring in the interior of a star

From this moment on, observation of the life of a star is clearly tied to the main sequence phase, which is important part evolution of celestial bodies. It is at this stage that the only source of stellar energy is the result of hydrogen combustion. The object is in a state of equilibrium. As nuclear fuel is consumed, only the chemical composition of the object changes. The Sun's stay in the main sequence phase will last approximately 10 billion years. This is how long it will take for our native star to use up its entire supply of hydrogen. As for massive stars, their evolution occurs faster. By emitting more energy, a massive star remains in the main sequence phase for only 10-20 million years.

Less massive stars burn in the night sky for much longer. Thus, a star with a mass of 0.25 M will remain in the main sequence phase for tens of billions of years.

Hertzsprung–Russell diagram assessing the relationship between the spectrum of stars and their luminosity. The points on the diagram are the locations of known stars. The arrows indicate the displacement of stars from the main sequence into the giant and white dwarf phases.

To imagine the evolution of stars, just look at the diagram characterizing the path of a celestial body in the main sequence. The upper part of the graph looks less saturated with objects, since this is where the massive stars are concentrated. This location is explained by their short duration life cycle. Of the stars known today, some have a mass of 70M. Objects whose mass exceeds the upper limit of 100M may not form at all.

Heavenly bodies whose mass is less than 0.08 M do not have the opportunity to overcome the critical mass required for the onset of thermonuclear fusion and remain cold throughout their lives. The smallest protostars collapse and form planet-like dwarfs.

A planet-like brown dwarf compared to a normal star (our Sun) and the planet Jupiter

At the bottom of the sequence are concentrated objects dominated by stars with a mass equal to the mass of our Sun and slightly more. The imaginary boundary between the upper and lower parts of the main sequence are objects whose mass is – 1.5M.

Subsequent stages of stellar evolution

Each of the options for the development of the state of a star is determined by its mass and the length of time during which the transformation of stellar matter occurs. However, the Universe is a multifaceted and complex mechanism, so the evolution of stars can take other paths.

When traveling along the main sequence, a star with a mass approximately equal to the mass of the Sun has three main route options:

1. live your life calmly and rest peacefully in the vast expanses of the Universe;
2. enter the red giant phase and slowly age;
3. become a white dwarf, explode as a supernova, and become a neutron star.

Possible options for the evolution of protostars depending on time, the chemical composition of objects and their mass

After the main sequence comes the giant phase. By this time, the reserves of hydrogen in the bowels of the star are completely exhausted, the central region of the object is a helium core, and thermonuclear reactions shift to the surface of the object. Under the influence of thermonuclear fusion, the shell expands, but the mass of the helium core increases. An ordinary star turns into a red giant.

Giant phase and its features

In stars with low mass, the core density becomes colossal, turning stellar matter into a degenerate relativistic gas. If the mass of the star is slightly more than 0.26 M, an increase in pressure and temperature leads to the beginning of helium synthesis, covering the entire central region of the object. From this moment on, the temperature of the star increases rapidly. The main feature of the process is that the degenerate gas does not have the ability to expand. Under the influence of high temperature, only the rate of helium fission increases, which is accompanied by an explosive reaction. At such moments we can observe a helium flash. The brightness of the object increases hundreds of times, but the agony of the star continues. The star transitions to a new state, where all thermodynamic processes occur in the helium core and in the discharged outer shell.

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, thermodynamic influence, matter changes its physical properties. The fatigue of stellar matter, depleted by long-term nuclear reactions, can explain the appearance of degenerate electron gas, its subsequent neutronization and annihilation. If all of the above processes take place from beginning to end, stellar matter ceases to be a physical substance - the star disappears in space, leaving nothing behind.

Interstellar bubbles and gas and dust clouds, which are the birthplace of stars, cannot be replenished only by disappeared and exploded stars. The Universe and galaxies are in an equilibrium state. There is a constant loss of mass, the density of interstellar space decreases in one part of outer space. Consequently, in another part of the Universe, conditions are created for the formation of new stars. In other words, the scheme works: if a certain amount of matter was lost in one place, in another place in the Universe the same amount of matter appeared in a different form.

Finally

By studying the evolution of stars, we come to the conclusion that the Universe is a gigantic rarefied solution in which part of the matter is transformed into molecules of hydrogen, which is building material for the stars. The other part dissolves in space, disappearing from the sphere of material sensations. A black hole in this sense is the place of transition of all material into antimatter. It is quite difficult to fully comprehend the meaning of what is happening, especially if, when studying the evolution of stars, we rely only on the laws of nuclear power, quantum physics and thermodynamics. The theory of relative probability should be included in the study of this issue, which allows for the curvature of space, allowing the transformation of one energy into another, one state into another.

Let us briefly consider the main stages of stellar evolution.

Changes in the physical characteristics, internal structure and chemical composition of a star over time.

Fragmentation of matter. .

It is assumed that stars are formed during gravitational compression of fragments of a gas and dust cloud. So, so-called globules can be places of star formation.

A globule is a dense opaque molecular-dust (gas-dust) interstellar cloud that is observed against the background glowing clouds gas and dust in the form of a dark round formation. Consists predominantly of molecular hydrogen (H 2) and helium ( He ) with an admixture of molecules of other gases and solid interstellar dust grains. Gas temperature in the globule (mainly the temperature of molecular hydrogen) T≈ 10 ÷ 50K, average density n~ 10 5 particles/cm 3, which is several orders of magnitude greater than in the densest conventional gas and dust clouds, diameter D~ 0.1 ÷ 1 . Mass of globules M≤ 10 2 × M ⊙ . In some globules, young type T Taurus.

The cloud is compressed by its own gravity due to gravitational instability, which can arise either spontaneously or as a result of the interaction of the cloud with a shock wave from a supersonic stellar wind flow from another nearby source of star formation. There are other possible causes of gravitational instability.

Theoretical studies show that under the conditions that exist in ordinary molecular clouds (T≈ 10 ÷ 30K and n ~ 10 2 particles/cm 3), the initial one can occur in cloud volumes with mass M≥ 10 3 × M ⊙ . In such a collapsing cloud, further disintegration into less massive fragments is possible, each of which will also be compressed under the influence of its own gravity. Observations show that in the Galaxy, during the process of star formation, not one, but a group of stars with different masses, for example, an open star cluster.

When compressed in the central regions of the cloud, the density increases, resulting in a moment when the substance of this part of the cloud becomes opaque to its own radiation. In the depths of the cloud, a stable dense condensation appears, which astronomers call oh.

Fragmentation of matter is the disintegration of a molecular dust cloud into smaller parts, the further part of which leads to the appearance.

- an astronomical object that is in the stage, from which after some time (for the solar mass this time T~ 10 8 years) normal is formed.

With the further fall of matter from the gas shell onto the core (accretion), the mass of the latter, and therefore the temperature, increases so much that the gas and radiant pressure are compared with the forces. Kernel compression stops. The formation is surrounded by a shell of gas and dust, opaque to optical radiation, allowing only infrared and longer wavelength radiation to pass through. Such an object (-cocoon) is observed as a powerful source of radio and infrared radiation.

With a further increase in the mass and temperature of the core, light pressure stops accretion, and the remains of the shell are scattered in outer space. A young one appears, the physical characteristics of which depend on its mass and initial chemical composition.

The main source of energy for a nascent star is apparently the energy released during gravitational compression. This assumption follows from the virial theorem: in stationary system sum of potential energy E p all members of the system and double kinetic energy 2 E to of these terms is equal to zero:

E p + 2 E k = 0. (39)

The theorem is valid for systems of particles moving in a limited region of space under the influence of forces, the magnitude of which is inversely proportional to the square of the distance between the particles. It follows that thermal (kinetic) energy is equal to half of gravitational (potential) energy. When a star contracts, the total energy of the star decreases, while the gravitational energy decreases: half of the change in gravitational energy leaves the star through radiation, and due to the second half, the thermal energy of the star increases.

Young low mass stars(up to three solar masses) that are approaching the main sequence are completely convective; the convection process covers all areas of the star. These are essentially protostars, in the center of which nuclear reactions are just beginning, and all the radiation occurs mainly due to. It has not yet been established that the star wanes at a constant effective temperature. On the Hertzsprung-Russell diagram, such stars form an almost vertical track called the Hayashi track. As compression slows, the young approaches the main sequence.

As the star contracts, the pressure of the degenerate electron gas begins to increase, and when a certain radius of the star is reached, the compression stops, which leads to a stop in the further growth of the central temperature caused by the compression, and then to its decrease. For stars less than 0.0767 solar masses, this does not happen: the energy released during nuclear reactions is never enough to balance internal pressure And . Such “understars” emit more energy than is produced during nuclear reactions, and are classified as so-called; their fate is constant compression until the pressure of the degenerate gas stops it, and then gradual cooling with the cessation of all nuclear reactions that have begun.

Young stars of intermediate mass (from 2 to 8 times the mass of the Sun) evolve qualitatively in exactly the same way as their smaller sisters, except that they do not have convective zones until the main sequence.

Stars with a mass greater than 8 solar massesalready have the characteristics of normal stars, since they have gone through all the intermediate stages and were able to achieve such a rate of nuclear reactions that they compensate for the energy lost to radiation while the core mass accumulates. The outflow of mass from these stars is so great that it not only stops the collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, thaws them away. Thus, the mass of the resulting star is noticeably less than the mass of the protostellar cloud.

Main sequence

The temperature of the star increases until in the central regions it reaches values ​​sufficient to enable thermonuclear reactions, which then become the main source of energy for the star. For massive stars ( M > 1 ÷ 2 × M ⊙ ) is the “combustion” of hydrogen in the carbon cycle; For stars with a mass equal to or less than the mass of the Sun, energy is released in the proton-proton reaction. enters the equilibrium stage and takes its place on the main sequence of the Hertzsprung-Russell diagram: a large-mass star has a very high core temperature ( T ≥ 3 × 10 7 K ), energy production is very intense, - on the main sequence it occupies a place above the Sun in the region of early ( O … A , (F )); a star of small mass has a relatively low core temperature ( T ≤ 1.5 × 10 7 K ), energy production is not so intense, - on the main sequence it occupies a place next to or below the Sun in the region of late (( F), G, K, M).

It spends up to 90% of the time allotted by nature for its existence on the main sequence. The time a star spends at the main sequence stage also depends on its mass. Yes, with mass M ≈ 10 ÷ 20 × M ⊙ O or B is in the main sequence stage for about 10 7 years, while the red dwarf K 5 with mass M ≈ 0.5 × M ⊙ is in the main sequence stage for about 10 11 years, that is, a time comparable to the age of the Galaxy. Massive hot stars quickly move into the next stages of evolution, cool dwarfs are in the main sequence stage throughout the existence of the Galaxy. It can be assumed that red dwarfs are the main type of population of the Galaxy.

Red giant (supergiant).

The rapid burning of hydrogen in the central regions of massive stars leads to the appearance of a helium core. With a fraction of the hydrogen mass of several percent in the core, the carbon reaction of converting hydrogen into helium almost completely stops. The core contracts, causing its temperature to increase. As a result of heating caused by gravitational compression of the helium core, hydrogen “ignites” and energy release begins in thin layer, located between the core and the extended shell of the star. The shell expands, the radius of the star increases, the effective temperature decreases and increases. “leaves” the main sequence and moves to the next stage of evolution - to the stage of a red giant or, if the mass of the star M > 10 × M ⊙ , into the red supergiant stage.

With increasing temperature and density, helium begins to “burn” in the core. At T ~ 2 × 10 8 K and r ~ 10 3 ¸ 10 4 g/cm 3 a thermonuclear reaction begins, which is called a ternary reaction a -process: of three a -particles (helium nuclei 4 He ) one stable carbon 12 C nucleus is formed. At the mass of the star's core M< 1,4 × M ⊙ тройной a -the process leads to an explosive energy release - a helium flare, which for a particular star can be repeated several times.

In the central regions of massive stars in the giant or supergiant stage, an increase in temperature leads to the sequential formation of carbon, carbon-oxygen and oxygen nuclei. After carbon burns out, reactions occur that result in the formation of heavier chemical elements, possibly iron nuclei. Further evolution of a massive star can lead to the ejection of the shell, the outburst of a star as a nova or, with the subsequent formation of objects that are the final stage of the evolution of stars: a white dwarf, a neutron star or a black hole.

The final stage of evolution is the stage of evolution of all normal stars after these stars have exhausted their thermonuclear fuel; cessation of thermonuclear reactions as a source of star energy; the transition of a star, depending on its mass, to the stage of a white dwarf, or black hole.

White dwarfs are the last stage of evolution of all normal stars with mass M< 3 ÷ 5 × M ⊙ after these have exhausted their thermonuclear fuel. Having passed the stage of a red giant (or subgiant), it sheds its shell and exposes the core, which, as it cools, becomes a white dwarf. Small radius (R b.k ~ 10 -2 × R ⊙ ) and white or white-blue color (T b.k ~ 10 4 K) determined the name of this class of astronomical objects. The mass of a white dwarf is always less than 1.4×M⊙ - it has been proven that white dwarfs with large masses cannot exist. With a mass comparable to the mass of the Sun and sizes comparable to the sizes of the large planets of the Solar System, white dwarfs have a huge average density: ρ b.k ~ 10 6 g/cm 3 , that is, a weight with a volume of 1 cm 3 of white dwarf matter weighs a ton! Gravity acceleration on surface g b.k ~ 10 8 cm/s 2 (compare with acceleration on the Earth’s surface - g ≈980 cm/s 2). With such a gravitational load on the inner regions of the star, the equilibrium state of the white dwarf is maintained by the pressure of the degenerate gas (mainly degenerate electron gas, since the contribution of the ion component is small). Let us recall that a gas in which there is no Maxwellian velocity distribution of particles is called degenerate. In such a gas, at certain values ​​of temperature and density, the number of particles (electrons) having any speed in the range from v = 0 to v = v max will be the same. v max is determined by the density and temperature of the gas. With a white dwarf mass M b.k > 1.4 × M ⊙ maximum speed electrons in the gas is comparable to the speed of light, the degenerate gas becomes relativistic and its pressure is no longer able to withstand gravitational compression. The radius of the dwarf tends to zero - it “collapses” into a point.

The thin, hot atmospheres of white dwarfs consist either of hydrogen, with virtually no other elements detectable in the atmosphere; or from helium, while the hydrogen in the atmosphere is hundreds of thousands of times less than in the atmospheres of normal stars. According to the type of spectrum, white dwarfs belong to spectral classes O, B, A, F. To “distinguish” white dwarfs from normal stars, the letter D is placed in front of the designation (DOVII, DBVII, etc. D is the first letter in English word Degenerate - degenerate). The source of radiation from a white dwarf is the reserve of thermal energy that the white dwarf received as the core of the parent star. Many white dwarfs inherited from their parents a strong magnetic field, the intensity of which H ~ 10 8 E. It is believed that the number of white dwarfs is about 10% of the total number of stars in the Galaxy.

In Fig. 15 shows a photograph of Sirius - the brightest star in the sky (α Canis Major; m v = -1 m .46; class A1V). The disk visible in the image is a consequence of photographic irradiation and diffraction of light on the telescope lens, that is, the disk of the star itself is not resolved in the photograph. The rays coming from the photographic disk of Sirius are traces of distortion of the wave front of the light flux on the elements of the telescope optics. Sirius is located at a distance of 2.64 from the Sun, the light from Sirius takes 8.6 years to reach the Earth - thus, it is one of the closest stars to the Sun. Sirius is 2.2 times more massive than the Sun; its M v = +1 m .43, that is, our neighbor emits 23 times more energy than the Sun.

Figure 15.

The uniqueness of the photograph lies in the fact that, together with the image of Sirius, it was possible to obtain an image of its satellite - the satellite “glows” with a bright dot to the left of Sirius. Sirius - telescopically: Sirius itself is designated by the letter A, and its satellite by the letter B. The apparent magnitude of Sirius is B m v = +8 m .43, that is, it is almost 10,000 times weaker than Sirius A. The mass of Sirius B is almost exactly equal to the mass of the Sun, the radius is about 0.01 of the radius of the Sun, the surface temperature is about 12000K, but Sirius B emits 400 times less than the Sun . Sirius B is a typical white dwarf. Moreover, this is the first white dwarf, discovered, by the way, by Alfven Clarke in 1862 during visual observation through a telescope.

Sirius A and Sirius B orbit around the same with a period of 50 years; the distance between components A and B is only 20 AU.

According to the apt remark of V.M.Lipunov, “they “ripe” inside massive stars (with a mass of more than 10×M⊙ )". The cores of stars evolving into a neutron star have 1.4× M ⊙ ≤ M ≤ 3 × M ⊙ ; after the sources of thermonuclear reactions dry up and the parent ejects a significant part of the matter in a flare, these nuclei will become independent objects of the stellar world, possessing very specific characteristics. The compression of the core of the parent star stops at a density comparable to the nuclear density (ρ n. h ~ 10 14 ÷ 10 15 g/cm 3). With such mass and density, the radius of the birth is only 10 and consists of three layers. The outer layer (or outer crust) is formed crystal lattice from iron atomic nuclei ( Fe ) with a possible small admixture of atomic nuclei of other metals; The thickness of the outer crust is only about 600 m with a radius of 10 km. Beneath the outer crust is another inner hard crust made up of iron atoms ( Fe ), but these atoms are over-enriched with neutrons. The thickness of this bark≈ 2 km. The inner crust borders on the liquid neutron core, the physical processes in which are determined by the remarkable properties of the neutron liquid - superfluidity and, in the presence of free electrons and protons, superconductivity. It is possible that in the very center the substance may contain mesons and hyperons.

They rotate quickly around an axis - from one to hundreds of revolutions per second. Such rotation in the presence of a magnetic field ( H ~ 10 13 ÷ 10 15 Oe) often leads to the observed effect of pulsation of star radiation in different ranges electromagnetic waves. We saw one of these pulsars inside the Crab Nebula.

Total number the rotation speed is no longer sufficient for particle ejection, so it cannot be a radio pulsar. However, it is still large, and the surrounding neutron star, captured by the magnetic field, cannot fall, that is, accretion of matter does not occur.

Accrector (X-ray pulsar). The rotation speed decreases to such an extent that there is now nothing stopping the matter from falling onto such a neutron star. The plasma, falling, moves along the magnetic field lines and hits a solid surface in the region of the poles, heating up to tens of millions of degrees. Matter heated to such high temperatures glows in the X-ray range. The region in which the falling matter interacts with the surface of the star is very small - only about 100 meters. Due to the rotation of the star, this hot spot periodically disappears from view, which the observer perceives as pulsations. Such objects are called X-ray pulsars.

Georotator. The rotation speed of such neutron stars is low and does not prevent accretion. But the size of the magnetosphere is such that the plasma is stopped by the magnetic field before it is captured by gravity.

If is a component of close dual system, then there is a “pumping” of matter from the normal star (the second component) to the neutron star. The mass may exceed critical (M > 3×M⊙ ), then the gravitational stability of the star is violated, nothing can resist gravitational compression, and “goes” under its gravitational radius

r g = 2 × G × M/c 2 , (40)

turning into a “black hole”. In the given formula for r g: M is the mass of the star, c is the speed of light, G is the gravitational constant.

A black hole is an object whose gravitational field is so strong that neither a particle, nor a photon, nor any material body can reach the second cosmic speed and escape into outer space.

A black hole is a singular object in the sense that the nature of the physical processes inside it is not yet accessible to theoretical description. The existence of black holes follows from theoretical considerations; in reality, they can be located in the central regions of globular clusters, quasars, giant galaxies, including in the center of our galaxy.