What is the process of evolution of stars of different masses. How stars die

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, the 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 endless space is a consequence of a 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 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 stars and planets inhabiting our galaxy Milky Way and the entire Universe, for the most part 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 science.

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 solar system. The information obtained will make it possible to understand in detail how other stars are structured, how quickly these giant heat sources are depleted, what are the stages of star development and what will be the finale of this brilliant life- quiet and dim or sparkling, explosive.

After the Big Bang, tiny 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 subzero temperature and zone inside the dense accumulation of gas low pressure. 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. In simple and understandable language, rapid compression is the fall of stellar matter towards the center of the protostar. Slow compression occurs 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 temperatures leads to the formation of a future star own center 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 a 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 in the infrared range new star, 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, the level of light and the amount of energy emitted can be used to trace the process of thermonuclear fusion of the star.

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 has on different stages of its existence.

Studying spectral analysis energy of the Sun and other stars, scientists have come 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(down to 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 firmament. 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.

The 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. The transfer of heat and energy occurs from internal regions stars 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 effective 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 whole point is that strength gravitational field it can contain 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 such an ideal natural model is a heat source capable of operating 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 emitted heat and solar energy is 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. 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 journey of transformation has begun celestial body.

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 most bright stars in our galaxy are not only the largest in comparison with 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 size, mature age the stars may still last approximately 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 thermonuclear reactions in the upper layers of the star. As a result 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 the increase 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. A supernova is an explosion shock wave monstrous power, 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.

In conclusion

The evolution of stars is a process that extends over tens of billions of years. Our idea of ​​the processes taking place is just mathematical and physical model, 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 future 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

The internal life of a star is regulated by the influence of two forces: the force of gravity, which counteracts the star and holds it, and the force released during nuclear reactions occurring in the core. On the contrary, it tends to “push” the star into distant space. During the formation stages, a dense and compressed star is under strong impact gravity. As a result, strong heating occurs, the temperature reaches 10-20 million degrees. This is enough to start nuclear reactions, as a result of which hydrogen is converted into helium.

Then, over a long period, the two forces balance each other, the star is in a stable state. When the nuclear fuel in the core gradually runs out, the star enters an instability phase, two forces opposing each other. A critical moment comes for the star, the most various factors– temperature, density, chemical composition. The mass of the star comes first; the future of this celestial body depends on it - either the star will explode like a supernova, or turn into a white dwarf, a neutron star or a black hole.

How hydrogen runs out

Only the very largest among celestial bodies (about 80 times the mass of Jupiter) become stars, the smaller ones (about 17 times smaller than Jupiter) become planets. There are also bodies average weight, they are too large to belong to the class of planets, and too small and cold for nuclear reactions characteristic of stars to occur in their depths.

These dark-colored celestial bodies have low luminosity and are quite difficult to distinguish in the sky. They are called “brown dwarfs”.

So, a star is formed from clouds of interstellar gas. As already noted, the star remains in a balanced state for quite a long time. Then comes a period of instability. Further fate stars depends on various factors. Consider a hypothetical small star whose mass is between 0.1 and 4 solar masses. Characteristic feature stars with low mass is the absence of convection in the inner layers, i.e. The substances that make up the star do not mix, as happens in stars with a large mass.

This means that when the hydrogen in the core runs out, there are no new reserves of this element in the outer layers. Hydrogen burns and turns into helium. Little by little the core heats up, the surface layers destabilize their own structure, and the star, as can be seen from the H-R diagram, slowly leaves the Main Sequence phase. In the new phase, the density of matter inside the star increases, the composition of the core “degenerates,” and as a result, a special consistency appears. It is different from normal matter.

Modification of matter

When matter changes, pressure depends only on the density of the gases, not on temperature.

In the Hertzsprung–Russell diagram, the star moves to the right and then upward, approaching the red giant region. Its dimensions increase significantly, and because of this, the temperature of the outer layers drops. The diameter of a red giant can reach hundreds of millions of kilometers. When ours enters this phase, it will “swallow” or Venus, and if it cannot capture the Earth, it will heat it up to such an extent that life on our planet will cease to exist.

During the evolution of a star, the temperature of its core increases. First, nuclear reactions occur, then, upon reaching the optimal temperature, helium begins to melt. When this happens, a sudden increase in core temperature causes a flare and the star quickly moves to the left G-R diagrams. This is the so-called “helium flash”. At this time, the core containing helium burns together with hydrogen, which is part of the shell surrounding the core. On the H-R diagram, this stage is recorded by moving to the right along a horizontal line.

Last phases of evolution

When helium is transformed into carbon, the nucleus is modified. Its temperature rises until (if the star is large) until the carbon begins to burn. A new outbreak occurs. In any case, during the last phases of the star's evolution, a significant loss of its mass is noted. This can happen gradually or suddenly, during an outburst, when the outer layers of the star burst like a large bubble. In the latter case, a planetary nebula is formed - a spherical shell, spreading in outer space at a speed of several tens or even hundreds of km/sec.

The final fate of a star depends on the mass remaining after everything that happens in it. If during all transformations and flares it ejected a lot of matter and its mass does not exceed 1.44 solar masses, the star turns into a white dwarf. This figure is called the “Chandra-sekhar limit” in honor of the Pakistani astrophysicist Subrahmanyan Chandrasekhar. This is the maximum mass of a star at which a catastrophic end may not occur due to the pressure of electrons in the core.

After the explosion of the outer layers, the core of the star remains, and its surface temperature is very high - about 100,000 °K. The star moves to the left edge of the H-R diagram and goes down. Its luminosity decreases as its size decreases.

The star is slowly reaching the white dwarf zone. These are stars of small diameter (like ours), but characterized by a very high density, one and a half million times the density of water. A cubic centimeter of the material that makes up a white dwarf would weigh about one ton on Earth!

A white dwarf represents the final stage of star evolution, without outbursts. She is gradually cooling down.

Scientists believe that the end of the white dwarf is very slow, at least since the beginning of the Universe, it seems that not a single white dwarf has suffered from “thermal death”.

If the star is large and its mass is greater than the Sun, it will explode like a supernova. During a flare, a star may collapse completely or partially. In the first case, what will be left behind is a cloud of gas with residual matter from the star. In the second, a celestial body of the highest density remains - a neutron star or a black hole.

> Life cycle of a star

Description life and death of stars: stages of development with photos, molecular clouds, protostar, T Tauri, main sequence, red giant, white dwarf.

Everything in this world is evolving. Any cycle begins with birth, growth and ends with death. Of course, stars have these cycles in a special way. Let us at least remember that their time frames are larger 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 quietly exist in the Universe without any changes. But suddenly a supernova explodes not far from it or it collides with another cloud. Due to such a push, the destruction process is activated. It is divided into small parts, each of which is retracted into itself. As you already understand, all these groups are preparing to become stars. Gravity heats up the temperature, and the stored momentum maintains the rotation process. The lower diagram clearly demonstrates the cycle of stars (life, stages of development, transformation options and death of a celestial body with a photo).

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 forms. The part is attracted to the object, increasing its mass. The remaining debris will group and create a planetary system. Further development of the star all depends on 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 Tauri. It is a variable star located 600 light years away (near).

It can reach great brightness because the material breaks down and releases energy. But the central part does not have enough temperature to support nuclear fusion. This phase lasts 100 million years.

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 transforms into helium, releasing enormous heat and energy.

The energy is released as gamma rays, but due to the slow motion of the star, it falls with the same wavelength. Light is pushed out and comes into conflict with gravity. We can assume that an ideal 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 mass of the sun) can burn through their fuel supply for hundreds of billions (trillions) of years. Average stars (like ) live 10-15 billion. But the largest ones are billions or millions of years old. See what the evolution and death of stars of different classes looks like in the diagram.

Fifth life cycle of a star: Red giant

During the melting process, hydrogen runs out 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 1,000 to 10,000 times larger. At a certain moment, our Sun will repeat this fate, increasing to the Earth’s orbit.

Temperature and pressure reach a maximum and helium fuses into carbon. At this point the star shrinks and ceases to be a red giant. With greater massiveness, the object will burn other heavy elements.

Sixth life cycle of a star: White dwarf

A solar-mass star doesn't have enough gravitational pressure to fuse the carbon. Therefore, death occurs with the end of helium. The outer layers are ejected and a white dwarf appears. It starts out hot, but after hundreds of billions of years it cools down.

Studying stellar evolution is impossible by observing just one star - many changes in stars occur too slowly to be noticed even after many centuries. Therefore, scientists study many stars, each of which is at a certain stage life cycle. Over the past few decades, modeling of the structure of stars using computer technology has become widespread in astrophysics.

Encyclopedic YouTube

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Subtitles

Thermonuclear fusion in the interior of stars

Young stars

The process of star formation can be described in a unified way, but the subsequent stages of a star's evolution depend almost entirely on its mass, and only at the very end of the star's evolution can its chemical composition play a role.

Young low mass stars

Young low-mass stars (up to three solar masses) [ ], which are approaching the main sequence, are completely convective - the convection process covers the entire body of the star. These are essentially protostars, in the centers of which nuclear reactions are just beginning, and all radiation occurs mainly due to gravitational compression. Until hydrostatic equilibrium is established, the star's luminosity decreases at a constant effective temperature. On the Hertzsprung-Russell diagram, such stars form an almost vertical track called the Hayashi track. As the compression slows, the young star approaches the main sequence. Objects of this type are associated with T Tauri 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 becomes predominant, since convection is increasingly hampered by the increasing compaction of stellar matter. In the outer layers of the star’s body, convective energy transfer prevails.

It is not known for certain what characteristics do stars of lower mass have at the moment they enter the main sequence, since the time these stars spent in the young category exceeds the age of the Universe [ ] . All ideas about the evolution of these stars are based only on numerical calculations and mathematical modeling.

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 increase in temperature in the star’s core caused by the compression, and then to its decrease. For stars smaller than 0.0767 solar masses, this does not happen: the energy released during nuclear reactions is never enough to balance the internal pressure and gravitational compression. Such “understars” emit more energy than is produced during thermonuclear reactions, and are classified as so-called brown dwarfs. Their fate is constant compression until the pressure of the degenerate gas stops it, and then gradual cooling with the cessation of all thermonuclear reactions that have begun.

Young intermediate mass stars

Young stars of intermediate mass (from 2 to 8 solar masses) [ ] evolve qualitatively in exactly the same way as their smaller sisters and brothers, with the exception that they do not have convective zones up to the main sequence.

Objects of this type are associated with the so-called. Ae\Be Herbig stars with irregular variables of spectral class B-F0. They also exhibit disks and bipolar jets. The rate of outflow of matter from the surface, luminosity and effective temperature are significantly higher than for T Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.

Young stars with a mass greater than 8 solar masses

Stars with such masses already have the characteristics of normal stars, since they went through all the intermediate stages and were able to achieve such a rate of nuclear reactions that compensated for the energy lost to radiation while mass accumulated to achieve hydrostatic equilibrium of the core. For these stars, the outflow of mass and luminosity are so great that they not only stop the gravitational collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, disperse them away. 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 with a mass greater than about 300 solar masses.

Mid-life cycle of a star

Stars come in a wide variety of colors and sizes. By spectral class they range from hot blue to cold red, by mass - from 0.0767 to about 300 Solar masses according to the latest estimates. The luminosity and color of a star depend on its surface temperature, 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. Naturally, 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. In fact, the movement of a star along the diagram corresponds only to a change in the parameters of the star.

The thermonuclear “burning” of matter, resumed at a new level, causes a monstrous expansion of the star. The star "swells", becoming very "loose", and its size increases approximately 100 times. So the star becomes a red giant, and the helium burning phase lasts about several million years. Almost all red giants are variable stars.

Final stages of stellar evolution

Old stars with low mass

At present, it is not known for certain what happens to light stars after the supply of hydrogen in their cores is depleted. Since the age of the Universe is 13.7 billion years, which is not enough for the hydrogen fuel supply in such stars to be depleted, modern theories are based on computer modeling of the processes occurring in such stars.

Some stars can only synthesize helium in certain active zones, causing instability and strong stellar 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 [ ] .

A star with a mass less than 0.5 solar is not able to convert helium even after reactions involving hydrogen stop in its core - the mass of such a star is too small to provide new phase gravitational compression to a degree sufficient to “ignite” helium. Such stars include red dwarfs, such as Proxima Centauri, whose residence time on the main sequence ranges from tens of billions to tens of trillions of years. After the cessation of thermonuclear reactions in their cores, they, gradually cooling, will continue to weakly emit in the infrared and microwave ranges of the electromagnetic spectrum.

Medium sized stars

Upon reaching star average size(from 0.4 to 3.4 solar masses) [ ] of the red giant phase, hydrogen runs out in its core, and reactions of synthesis of carbon from helium begin. This the process is underway with more high temperatures and therefore the flow of energy from the core increases and, as a result, the outer layers of the star begin to expand. The beginning of carbon synthesis marks a new stage in the life of a star and continues for some time. 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. Energy output shifts towards low frequency radiation. All this is accompanied by increasing mass loss due to strong stellar winds and intense pulsations. Stars in this phase are called “late-type stars” (also “retired 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 source stars are formed in such shells ideal conditions to activate cosmic masers.

Thermonuclear combustion reactions of helium are very sensitive to temperature. Sometimes this leads to great instability. Strong pulsations arise, which as a result impart sufficient acceleration to the outer layers to be thrown off and turn into a planetary nebula. In the center of such a nebula, the bare core of the star remains, in which thermonuclear reactions stop, and as it cools, it turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar masses and a diameter on the order of the diameter of the Earth.

The vast majority of stars, including the Sun, complete 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, becomes an invisible black dwarf.

In stars more massive than the Sun, the pressure of degenerate electrons cannot stop further compression of the core, and electrons begin to be “pressed” into atomic nuclei, which turns protons into neutrons, between which there are no electrostatic repulsion forces. This neutronization of matter leads to the fact that the size of the star, which is now, in fact, one huge atomic nucleus, is measured in several kilometers, and its density is 100 million times higher than the density of water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.

Supermassive stars

After a star with a mass greater than five solar masses enters the red supergiant stage, its core begins to shrink under the influence of gravitational forces. As the compression proceeds, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, increasingly heavier elements are synthesized: helium, carbon, oxygen, silicon and iron, which temporarily restrains the collapse of the core.

As a result, as increasingly heavier elements of the Periodic Table are formed, iron-56 is synthesized from silicon. At this stage, further exothermic thermonuclear fusion becomes impossible, since the iron-56 nucleus has a maximum mass defect and the formation of heavier nuclei with the release of energy is impossible. Therefore, when the iron core of a star reaches a certain size, the pressure in it is no longer able to withstand the weight of the overlying layers of the star, and immediate collapse of the core occurs with neutronization of its matter.

What happens next is not yet completely clear, but, in any case, the processes taking place in a matter of seconds lead to a supernova explosion of incredible power.

Strong neutrino jets and a rotating magnetic field push out much of the star's accumulated material. [ ] - so-called seating elements, including iron and lighter elements. The exploding matter is bombarded by neutrons escaping from the stellar core, 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, but this is not the only possible way of their formation, which, for example, is demonstrated by technetium stars.

blast wave And neutrino jets carry matter away from dying star [ ] into interstellar space. Subsequently, as it cools and moves through space, this supernova material can collide with other cosmic “salvage” 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. Also questionable is what actually remains of the original star. However, two options are being considered: neutron stars and black holes.

Neutron stars

It is known that in some supernovae, strong gravity in the depths of the supergiant forces electrons to be absorbed by the atomic nucleus, where they merge with protons to form neutrons. This process is called neutronization. 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 large city, and have an unimaginably high density. Their orbital period becomes extremely short as the size of the star decreases (due to the conservation of angular momentum). Some neutron stars rotate 600 times per second. For some of them, the angle between the radiation vector and the axis of rotation may be such that the Earth falls into the cone formed by this radiation; in this case, it is possible to detect a radiation pulse repeating at time intervals equal to the star’s orbital period. Such neutron stars were called “pulsars”, and became the first to be discovered. neutron stars.

Black holes

Not all stars, having gone through the supernova explosion phase, become neutron stars. If the star has a sufficiently large mass, then the collapse of such a 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 this theory,

If enough matter accumulates somewhere in the Universe, it is compressed into a dense lump, in which a thermonuclear reaction begins. This is how stars light up. The first ones flared up in the darkness of the young Universe 13.7 billion (13.7 * 10 9) years ago, and our Sun - only some 4.5 billion years ago. The lifespan of a star and the processes occurring at the end of this period depend on the mass of the star.

While the thermonuclear reaction of converting hydrogen into helium continues in a star, it is on the main sequence. The time a star spends on the main sequence depends on its mass: the largest and heaviest ones quickly reach the red giant stage, and then leave the main sequence as a result of a supernova explosion or the formation of a white dwarf.

Fate of the Giants

The largest and most massive stars burn quickly and explode as supernovae. After a supernova explosion, a neutron star or black hole remains, and around them is matter ejected by the colossal energy of the explosion, which then becomes material for new stars. Of our closest stellar neighbors, such a fate awaits, for example, Betelgeuse, but it is impossible to calculate when it will explode.

A nebula formed as a result of the ejection of matter during a supernova explosion. At the center of the nebula is a neutron star.

A neutron star is a scary physical phenomenon. The core of an exploding star is compressed, much like gas in an engine. internal combustion, only in a very large and effective way: a ball with a diameter of hundreds of thousands of kilometers turns into a ball from 10 to 20 kilometers in diameter. The compression force is so strong that electrons fall onto atomic nuclei, forming neutrons - hence the name.

NASA Neutron star (artist's vision)

The density of matter during such compression increases by about 15 orders of magnitude, and the temperature rises to an incredible 10 12 K at the center of the neutron star and 1,000,000 K at the periphery. Some of this energy is emitted in the form of photon radiation, while some is carried away by neutrinos produced in the core of a neutron star. But even due to very efficient neutrino cooling, a neutron star cools very slowly: it takes 10 16 or even 10 22 years to completely exhaust its energy. It is difficult to say what will remain in the place of the cooled neutron star, and impossible to observe: the world is too young for that. There is an assumption that a black hole will again form in place of the cooled star.

Black holes arise from the gravitational collapse of very massive objects, such as supernova explosions. Perhaps, after trillions of years, cooled neutron stars will turn into black holes.

The fate of medium-sized stars

Other, less massive stars remain on the main sequence longer than the largest ones, but once they leave it they die much faster than their neutron relatives. More than 99% of the stars in the Universe will never explode and turn into either black holes or neutron stars - their cores are too small for such cosmic dramas. Instead, intermediate-mass stars become red giants at the end of their lives, which, depending on their mass, become white dwarfs, explode and dissipate completely, or become neutron stars.

White dwarfs now make up from 3 to 10% of the stellar population of the Universe. Their temperature is very high - more than 20,000 K, more than three times the temperature of the surface of the Sun - but still less than that of neutron stars, both due to their lower temperature and larger area white dwarfs cool faster - in 10 14 - 10 15 years. This means that in the next 10 trillion years—when the universe will be a thousand times older than it is now—a new type of object will appear in the universe: a black dwarf, a product of the cooling of a white dwarf.

There are no black dwarfs in space yet. Even the oldest cooling stars to date have lost a maximum of 0.2% of their energy; for a white dwarf with a temperature of 20,000 K, this means cooling to 19,960 K.

For the little ones

Science knows even less about what happens when the smallest stars, such as our nearest neighbor, the red dwarf Proxima Centauri, cool down than about supernovae and black dwarfs. Thermonuclear fusion in their cores proceeds slowly, and they remain on the main sequence longer than others - according to some calculations, up to 10 12 years, and after that, presumably, they will continue to live as white dwarfs, that is, they will shine for another 10 14 - 10 15 years before transformation into a black dwarf.