How do stars evolve? Stellar evolution - how it works What is stellar evolution

Like any bodies in nature, stars also cannot remain unchanged. They are born, develop and finally “die”. The evolution of stars takes billions of years, but there is debate about the time of their formation. Previously, astronomers believed that the process of their “birth” from stardust took millions of years, but not so long ago photographs of the sky region from the Great Orion Nebula were obtained. Over the course of several years, a small

Photographs from 1947 showed a small group of star-like objects in this location. By 1954, some of them had already become oblong, and five years later these objects broke up into separate ones. Thus, for the first time, the process of star birth took place literally before the eyes of astronomers.

Let's look in detail at the structure and evolution of stars, where their endless, by human standards, life begins and ends.

Traditionally, scientists assume that stars are formed as a result of the condensation of clouds of gas and dust. Under the influence of gravitational forces, an opaque cloud is formed from the resulting clouds. gas ball, dense in structure. Its internal pressure cannot balance the gravitational forces compressing it. Gradually, the ball contracts so much that the temperature of the stellar interior rises, and the pressure of the hot gas inside the ball balances the external forces. After this, the compression stops. The duration of this process depends on the mass of the star and usually ranges from two to several hundred million years.

The structure of stars implies very high temperatures in their cores, which contributes to continuous thermonuclear processes (the hydrogen that forms them turns into helium). It is these processes that cause intense radiation from stars. The time during which they consume the available supply of hydrogen is determined by their mass. The duration of radiation also depends on this.

When hydrogen reserves are depleted, the evolution of stars approaches the formation stage. This happens as follows. After the release of energy ceases, gravitational forces begin to compress the core. At the same time, the star increases significantly in size. The luminosity also increases as the process continues, but only in a thin layer at the core boundary.

This process is accompanied by an increase in the temperature of the contracting helium core and the transformation of helium nuclei into carbon nuclei.

It is predicted that our Sun could become a red giant in eight billion years. Its radius will increase several tens of times, and its luminosity will increase hundreds of times compared to current levels.

The lifespan of a star, as already noted, depends on its mass. Objects with a mass that is less than the Sun “use up” their reserves very economically, so they can shine for tens of billions of years.

The evolution of stars ends with the formation. This happens to those of them whose mass is close to the mass of the Sun, i.e. does not exceed 1.2 of it.

Giant stars tend to quickly deplete their supply of nuclear fuel. This is accompanied by a significant loss of mass, in particular due to the shedding of outer shells. As a result, only a gradually cooling central part remains, in which nuclear reactions have completely stopped. Over time, such stars stop emitting and become invisible.

But sometimes the normal evolution and structure of stars is disrupted. Most often this concerns massive objects that have exhausted all types of thermonuclear fuel. Then they can be converted into neutrons, or And the more scientists learn about these objects, the more new questions arise.

Stars, like people, can be newborn, young, old. Every moment some stars die and others are formed. Usually the youngest of them are similar to the Sun. They are at the stage of formation and are actually protostars. Astronomers call them T-Taurus stars, after their prototype. In terms of their properties - for example, luminosity - protostars are variable, since their existence has not yet entered a stable phase. Many of them have large amounts of matter around them. Powerful wind currents emanate from T-type stars.

Protostars: the beginning of their life cycle

If matter falls onto the surface of a protostar, it quickly burns and turns into heat. As a consequence, the temperature of protostars is constantly increasing. When it rises so high that nuclear reactions are triggered in the center of the star, the protostar acquires the status of an ordinary one. With the start of nuclear reactions, the star has a constant source of energy that supports its life for a long time. How long a star's life cycle in the Universe will be depends on its original size. However, it is believed that stars the diameter of the Sun have enough energy to exist comfortably for about 10 billion years. Despite this, it also happens that even more massive stars live only a few million years. This is due to the fact that they burn their fuel much faster.

Normal sized stars

Each of the stars is a clump of hot gas. In their depths, the process of generating nuclear energy constantly occurs. However, not all stars are like the Sun. One of the main differences is color. Stars are not only yellow, but also bluish and reddish.

Brightness and Luminosity

They also differ in characteristics such as shine and brightness. How bright a star observed from the Earth's surface will be depends not only on its luminosity, but also on its distance from our planet. Given their distance from Earth, stars can have completely different brightnesses. This indicator ranges from one ten-thousandth of the brilliance of the Sun to a brightness comparable to more than a million Suns.

Most stars are at the lower end of this spectrum, being dim. In many ways, the Sun is an average, typical star. However, compared to others, it has much greater brightness. A large number of dim stars can be observed even with the naked eye. The reason stars vary in brightness is due to their mass. Color, shine and change in brightness over time are determined by the amount of substance.

Attempts to explain the life cycle of stars

People have long tried to trace the life of stars, but the first attempts of scientists were rather timid. The first advance was the application of Lane's law to the Helmholtz-Kelvin hypothesis of gravitational contraction. This brought a new understanding to astronomy: theoretically, the temperature of a star should increase (its indicator is inversely proportional to the radius of the star) until an increase in density slows down the compression processes. Then the energy consumption will be higher than its income. At this moment, the star will begin to rapidly cool down.

Hypotheses about the life of stars

One of the original hypotheses about the life cycle of a star was proposed by astronomer Norman Lockyer. He believed that stars arise from meteoric matter. Moreover, the provisions of his hypothesis were based not only on theoretical conclusions available in astronomy, but also on data from spectral analysis of stars. Lockyer was convinced that the chemical elements that take part in evolution celestial bodies, consist of elementary particles - “protoelements”. Unlike modern neutrons, protons and electrons, they do not have a general, but an individual character. For example, according to Lockyer, hydrogen decays into what is called “protohydrogen”; iron becomes “proto-iron”. Other astronomers also tried to describe the life cycle of a star, for example, James Hopwood, Yakov Zeldovich, Fred Hoyle.

Giant stars and dwarf stars

Larger stars are the hottest and brightest. They are usually white or bluish in appearance. Despite the fact that they are gigantic in size, the fuel inside them burns so quickly that they are deprived of it in just a few million years.

Small stars, as opposed to giant ones, are usually not so bright. They are red in color and live long enough - for billions of years. But among the bright stars in the sky there are also red and orange ones. An example is the star Aldebaran - the so-called “eye of the bull”, located in the constellation Taurus; and also in the constellation Scorpio. Why are these cool stars able to compete in brightness with hot stars like Sirius?

This is due to the fact that they once expanded very much, and their diameter began to exceed huge red stars (supergiants). The huge area allows these stars to emit an order of magnitude more energy than the Sun. This is despite the fact that their temperature is much lower. For example, the diameter of Betelgeuse, located in the constellation Orion, is several hundred times larger than the diameter of the Sun. And the diameter of ordinary red stars is usually not even a tenth the size of the Sun. Such stars are called dwarfs. Each celestial body can go through these types of star life cycles - the same star at different stages of its life can be both a red giant and a dwarf.

As a rule, luminaries like the Sun support their existence due to the hydrogen found inside. It turns into helium inside the star's nuclear core. The sun has a huge amount of fuel, but even it is not infinite - over the past five billion years, half of the supply has been used up.

Lifetime of stars. Life cycle of stars

Once the supply of hydrogen inside a star is depleted, major changes occur. The remaining hydrogen begins to burn not inside its core, but on the surface. At the same time, the lifespan of a star is increasingly shortened. During this period, the cycle of stars, at least most of them, enters the red giant stage. The size of the star becomes larger, and its temperature, on the contrary, decreases. This is how most red giants and supergiants appear. This process is part of the general sequence of changes occurring in stars, which scientists call stellar evolution. The life cycle of a star includes all its stages: ultimately, all stars age and die, and the duration of their existence is directly determined by the amount of fuel. Big stars end their lives with a huge, spectacular explosion. More modest ones, on the contrary, die, gradually shrinking to the size of white dwarfs. Then they just fade away.

How long does the average star live? The life cycle of a star can last from less than 1.5 million years to 1 billion years or more. All this, as has been said, depends on its composition and size. Stars like the Sun live between 10 and 16 billion years. Very bright stars, like Sirius, have relatively short lives - only a few hundred million years. The star life cycle diagram includes the following stages. This is a molecular cloud - gravitational collapse of the cloud - the birth of a supernova - the evolution of a protostar - the end of the protostellar phase. Then follow the stages: the beginning of the young star stage - mid-life - maturity - red giant stage - planetary nebula - white dwarf stage. The last two phases are characteristic of small stars.

The nature of planetary nebulae

So, we briefly looked at the life cycle of a star. But what is Transforming from a huge red giant to a white dwarf, sometimes stars shed their outer layers, and then the core of the star becomes exposed. The gas shell begins to glow under the influence of the energy emitted by the star. This stage got its name due to the fact that luminous gas bubbles in this shell often look like disks around planets. But in reality they have nothing to do with planets. The life cycle of stars for children may not include all the scientific details. One can only describe the main phases of the evolution of celestial bodies.

Star clusters

Astronomers love to explore. There is a hypothesis that all luminaries are born in groups, and not individually. Since stars belonging to the same cluster have similar properties, the differences between them are true and not due to the distance to the Earth. Whatever changes occur to these stars, they originate at the same time and under equal conditions. Especially a lot of knowledge can be obtained by studying the dependence of their properties on mass. After all, the age of the stars in the clusters and their distance from the Earth are approximately equal, so they differ only in this indicator. The clusters will be of interest not only to professional astronomers - every amateur will be happy to take a beautiful photograph and admire their exceptionally beautiful view in the planetarium.

Formed by condensation of the interstellar medium. Through observations, it was possible to determine that stars arose at different times and still appear to this day.

The main problem in the evolution of stars is the question of the origin of their energy, thanks to which they glow and emit huge amounts of energy. Previously, many theories were put forward that were designed to identify the sources of energy of stars. It was believed that a continuous source of stellar energy was continuous compression. This source is certainly good, but cannot maintain appropriate radiation for a long time. In the middle of the 20th century, the answer to this question was found. The source of radiation is thermonuclear fusion reactions. As a result of these reactions, hydrogen turns into helium, and the released energy passes through the bowels of the star, is transformed and emitted into outer space (it is worth noting that the higher the temperature, the faster these reactions occur; this is why hot massive stars leave the main sequence faster).

Now imagine the emergence of a star...

A cloud of interstellar gas and dust medium began to condense. From this cloud a rather dense ball of gas is formed. The pressure inside the ball is not yet able to balance the forces of attraction, so it will shrink (perhaps at this time clumps with less mass will form around the star, which will eventually turn into planets). When compressed, the temperature rises. Thus, the star gradually sets on the main sequence. Then the pressure of the gas inside the star balances the gravity and the protostar turns into a star.

The early stage of the star's evolution is very small and the star at this time is immersed in a nebula, so the protostar is very difficult to detect.

The conversion of hydrogen into helium occurs only in the central regions of the star. In the outer layers, the hydrogen content remains practically unchanged. Since the amount of hydrogen is limited, sooner or later it burns out. The release of energy in the center of the star stops and the core of the star begins to shrink and the shell begins to swell. Further, if the star is less than 1.2 solar masses, it sheds its outer layer (formation of a planetary nebula).

After the envelope separates from the star, its inner, very hot layers are exposed, and meanwhile the envelope moves further and further away. After several tens of thousands of years, the shell will disintegrate and only a very hot and dense star will remain; gradually cooling, it will turn into a white dwarf. Gradually cooling, they turn into invisible black dwarfs. Black dwarfs are very dense and cool stars, slightly larger than the Earth, but with a mass comparable to the mass of the sun. The cooling process of white dwarfs lasts several hundred million years.

If the mass of a star is from 1.2 to 2.5 solar, then such a star will explode. This explosion is called supernova explosion. The flaring star increases its luminosity hundreds of millions of times in a few seconds. Such outbreaks occur extremely rarely. In our Galaxy, a supernova explosion occurs approximately once every hundred years. After such an outbreak, a nebula remains, which has a lot of radio emission and also scatters very quickly, and a so-called neutron star (more on this a little later). In addition to the enormous radio emission, such a nebula will also be a source of X-ray radiation, but this radiation is absorbed by the earth’s atmosphere, and therefore can only be observed from space.

There are several hypotheses about the cause of star explosions (supernovae), but there is no generally accepted theory yet. There is an assumption that this is due to the too rapid decline of the inner layers of the star towards the center. The star quickly contracts to a catastrophically small size of the order of 10 km, and its density in this state is 10 17 kg/m 3, which is close to the density of the atomic nucleus. This star consists of neutrons (at the same time, electrons are pressed into protons), which is why it is called "NEUTRON". Its initial temperature is about a billion Kelvin, but in the future it will quickly cool down.

This star, due to its small size and rapid cooling, was long considered impossible to observe. But after some time, pulsars were discovered. These pulsars turned out to be neutron stars. They are named so because of the short-term emission of radio pulses. Those. the star seems to “blink.” This discovery was made completely by accident and not so long ago, namely in 1967. These periodic impulses are due to the fact that during very rapid rotation, the cone of the magnetic axis constantly flashes past our gaze, which forms an angle with the axis of rotation.

A pulsar can only be detected for us under the conditions of orientation of the magnetic axis, and this is approximately 5% of their total number. Some pulsars are not located in radio nebulae, since nebulae dissipate relatively quickly. After a hundred thousand years, these nebulae cease to be visible, and the age of pulsars is tens of millions of years.

If the mass of a star exceeds 2.5 solar, then at the end of its existence it will seem to collapse in on itself and be crushed by its own weight. In a matter of seconds it will turn into a dot. This phenomenon was called “gravitational collapse”, and this object was also called a “black hole”.

From all that has been said above, it is clear that the final stage of the evolution of a star depends on its mass, but it is also necessary to take into account the inevitable loss of this very mass and rotation.

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 of its 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 it play a role chemical composition.

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 core of the star 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 type they range from hot blue to cool red, and 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 a new phase of 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 a medium-sized star (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 process occurs at higher temperatures and therefore the energy flow 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 from the source star, ideal conditions for the activation of cosmic masers are formed in such shells.

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 the density is 100 million times greater 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 gravity. 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 jets of neutrinos carry matter away from the 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 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 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 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 stars, after going 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 black hole.

The existence of black holes was predicted by the general theory of relativity. According to this theory,

Occupies a point in the upper right corner: it has high luminosity and low temperature. The main radiation occurs in the infrared range. The radiation from the cold dust shell reaches us. During the process of evolution, the position of the star on the diagram will change. The only source of energy at this stage is gravitational compression. Therefore, the star moves quite quickly parallel to the ordinate axis.

The surface temperature does not change, but the radius and luminosity decrease. The temperature in the center of the star rises, reaching a value at which reactions begin with light elements: lithium, beryllium, boron, which quickly burn out, but manage to slow down the compression. The track rotates parallel to the ordinate axis, the temperature on the surface of the star increases, and the luminosity remains almost constant. Finally, in the center of the star, reactions of the formation of helium from hydrogen (hydrogen combustion) begin. The star enters the main sequence.

The duration of the initial stage is determined by the mass of the star. For stars like the Sun it is about 1 million years, for a star with a mass of 10 M☉ about 1000 times less, and for a star with a mass of 0.1 M☉ thousands of times more.

Young low mass stars

At the beginning of evolution, a low-mass star has a radiant core and a convective envelope (Fig. 82, I).

At the main sequence stage, the star shines due to the release of energy in the nuclear reactions of converting hydrogen into helium. The supply of hydrogen ensures the luminosity of a star of mass 1 M☉ approximately within 10 10 years. Stars of greater mass consume hydrogen faster: for example, a star with a mass of 10 M☉ will consume hydrogen in less than 10 7 years (luminosity is proportional to the fourth power of mass).

Low mass stars

As hydrogen burns out, the central regions of the star are greatly compressed.

High mass stars

After reaching the main sequence, the evolution of a high-mass star (>1.5 M☉) is determined by the combustion conditions of nuclear fuel in the bowels of the star. At the main sequence stage, this is the combustion of hydrogen, but unlike low-mass stars, reactions of the carbon-nitrogen cycle dominate in the core. In this cycle, the C and N atoms play the role of catalysts. The rate of energy release in the reactions of such a cycle is proportional to T 17. Therefore, a convective core is formed in the core, surrounded by a zone in which energy transfer is carried out by radiation.

The luminosity of large-mass stars is much higher than the luminosity of the Sun, and hydrogen is consumed much faster. This is also due to the fact that the temperature in the center of such stars is also much higher.

As the proportion of hydrogen in the matter of the convective core decreases, the rate of energy release decreases. But since the rate of release is determined by luminosity, the core begins to compress, and the rate of energy release remains constant. At the same time, the star expands and moves into the region of red giants.

Low mass stars

By the time the hydrogen is completely burned out, a small helium core is formed in the center of a low-mass star. In the core, the density of matter and temperature reach values ​​of 10 9 kg/m and 10 8 K, respectively. Hydrogen combustion occurs on the surface of the core. As the temperature in the core rises, the rate of hydrogen burnout increases and the luminosity increases. The radiant zone gradually disappears. And due to the increase in the speed of convective flows, the outer layers of the star inflate. Its size and luminosity increase - the star turns into a red giant (Fig. 82, II).

High mass stars

When the hydrogen in a large-mass star is completely exhausted, a triple helium reaction begins to occur in the core and at the same time the reaction of oxygen formation (3He=>C and C+He=>0). At the same time, hydrogen begins to burn on the surface of the helium core. The first layer source appears.

The supply of helium is exhausted very quickly, since in the reactions described, relatively little energy is released in each elementary act. The picture repeats itself, and two layer sources appear in the star, and the reaction C+C=>Mg begins in the core.

The evolutionary track turns out to be very complex (Fig. 84). On the Hertzsprung-Russell diagram, the star moves along the sequence of giants or (with a very large mass in the supergiant region) periodically becomes a Cephei.

Old low mass stars

For a low-mass star, eventually the speed of the convective flow at some level reaches the second escape velocity, the shell comes off, and the star turns into a white dwarf surrounded by a planetary nebula.

The evolutionary track of a low-mass star on the Hertzsprung-Russell diagram is shown in Figure 83.

Death of high-mass stars

At the end of its evolution, a large-mass star has a very complex structure. Each layer has its own chemical composition, nuclear reactions occur in several layer sources, and an iron core is formed in the center (Fig. 85).

Nuclear reactions with iron do not occur, since they require the expenditure (and not the release) of energy. Therefore, the iron core quickly contracts, the temperature and density in it increase, reaching fantastic values ​​- a temperature of 10 9 K and a pressure of 10 9 kg/m 3. Material from the site

At this moment, two important processes begin, occurring in the nucleus simultaneously and very quickly (apparently, in minutes). The first is that during nuclear collisions, iron atoms decay into 14 helium atoms, the second is that electrons are “pressed” into protons, forming neutrons. Both processes are associated with the absorption of energy, and the temperature in the core (also pressure) instantly drops. The outer layers of the star begin to fall toward the center.

The fall of the outer layers leads to a sharp increase in temperature in them. Hydrogen, helium, and carbon begin to burn. This is accompanied by a powerful stream of neutrons that comes from the central core. As a result, a powerful nuclear explosion occurs, throwing off the outer layers of the star, already containing all the heavy elements, up to californium. According to modern views, all atoms of heavy chemical elements (i.e., heavier than helium) were formed in the Universe precisely in flares