Each of us at least once in his life looked into the starry sky. 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 dimensions in the Universe are colossal. We rarely think about the fact that the evolution of galaxies and stars is constantly taking place before our eyes. Every object in the vast space is a consequence of certain physical processes. Galaxies, stars, and even planets have major phases of development.

Our planet and we are all dependent 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 in millions and billions of years? In this regard, it is curious to know more about what are the stages of the evolution of astronomical objects, where the 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 is, for the most part, well studied. In space, the laws of physics are unshakable, which 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 on the process of the origin of the Universe. The event that shook the universe and led to the formation of the universe, by cosmic standards, is lightning fast. For space, moments pass from the birth of a star to its death. Great distances create the illusion of the constancy of the universe. A star that has flashed in the distance has been shining for us for billions of years, while it may no longer exist.

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

Studying the life cycle of stars, you can use the example of the closest star to us. The sun is one of a hundred trillion 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 allow us to understand in detail how other stars are arranged, how quickly these gigantic heat sources are depleted, what are the stages of a star's 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 contraction and expansion. Compression of cosmic gas in clouds arose under the influence of its own gravity and similar processes in new stars in the vicinity. The expansion arose from the internal pressure of the interstellar gas and from the magnetic fields inside the gas cloud. The cloud rotated freely around its center of mass.

The gas clouds formed after the explosion are 98% composed of atomic and molecular hydrogen and helium. Only 2% in this massif are dust and solid microscopic particles. Earlier it was believed that in the center of any star lies the 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. Light, together with a part of the released energy, spreads outward, creating a minus temperature and a low pressure zone inside a dense accumulation of gas. While in this state, the cosmic gas is rapidly compressed, the influence of the forces of gravitational attraction leads to the fact that the particles begin to form stellar matter. When a gas accumulation is dense, intense compression causes a star cluster to form. When the size of the gas cloud is small, the compression results in 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 a 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 temperatures leads to the formation of a future star of its own center of gravity.

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

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

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

The above version of the formation of a star 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 a few new stars have been observed. On the scale of the Universe, this figure can be increased hundreds and thousands of times.

For most of their life, protostars are hidden from the human eye by a dusty shell. Radiation from the core can only be observed in the infrared range, 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, whose radiation temperature was 700 degrees Kelvin. Subsequently, it turned out that the birthplace of protostars are compact sources that are available not only in our galaxy, but also in other corners of the Universe far from us. In addition to infrared radiation, the birthplaces of new stars are marked by intense radio signals.

The process of studying and the diagram of the evolution of stars

The whole process of knowing the stars can be roughly 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, how long the light goes from it, gives an idea of \u200b\u200bwhat happened to the star throughout this time. After a person learned to measure the distance to distant stars, it became clear that stars are the same suns, only of different sizes and with different destinies. Knowing the distance to the star, by the level of light and the amount of emitted energy, one can 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 were able to study the nature of starlight. This device can determine and measure the gas composition of stellar matter, which a star possesses at different stages of its existence.

By studying the spectral analysis of the 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 (up to iron) as our planet. The difference is only in the amount of certain elements and in the processes taking place 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 viewed in the context of another physical discipline, quantum mechanics. According to this theory, the matter that determines the stellar matter consists of constantly dividing atoms and elementary particles that create their own microcosm. In this light, the structure, composition, structure and evolution of stars is of interest. As it turned out, the bulk of our star and many other stars are only two elements - hydrogen and helium. A theoretical model describing the structure of a star will make it possible 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. Hot gas is a combination of atoms that are loosely bound to each other. Millions of years after the formation of a star, the cooling of the surface layer of stellar matter begins. The star gives off most of its energy to outer space, decreasing or increasing in size. Heat and energy transfer occurs from the interior of the star to the surface, affecting the intensity of radiation. In other words, one and the same star looks different at different periods of its existence. Thermonuclear processes based on hydrogen cycle reactions promote the conversion 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 does thermonuclear nuclear fusion not end with the explosion of such a reactor? The thing is that the forces of the gravitational field in it can keep the stellar matter within the stabilized volume. From this, an unambiguous conclusion can be drawn: any star is a massive body that retains its size due to the balance between the forces of gravity and the energy of thermonuclear reactions. The result of this ideal natural design is a heat source that can work 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 as it does now. Consequently, our star has changed little, despite the fact that the scale of radiated heat and solar energy is colossal - more than 3-4 million tons every second.

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

Stellar evolution stages

The fate of the star in 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 towards an increase in the size of a star, it means that the main source for thermonuclear synthesis has dried up. The long final path of transformation of the celestial body began.

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 reserves of hydrogen and live their lives quite calmly.

The majority of such stars in the Universe and our star - a yellow dwarf - belongs to them. With the onset of old age, the 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 in comparison with the Sun, but also the youngest. Astrophysicists and astronomers call these stars blue supergiants. In the end, they will face the same fate that trillions of other stars are experiencing. First, a rapid birth, a brilliant and ardent life, after which a period of slow decay sets in. Stars as large as the Sun have a long life cycle in the main sequence (in the middle).

Using data on the mass of a star, one can assume its evolutionary path of development. A clear illustration of this theory is the evolution of our star. Nothing is everlasting. As a result of thermonuclear fusion, hydrogen turns into helium, therefore, its initial reserves are consumed and reduced. Sometime, not very soon, these stocks 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 a star can 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 contract. 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 condition is called collapse, which can be caused by the passage of thermonuclear reactions in the upper layers of the star. As a result of high pressure, thermonuclear reactions are triggered with the participation of helium.

The reserves of hydrogen and helium in this part of the star will last for another millions of years. It will not be very soon that the depletion of hydrogen reserves will lead to an increase in the radiation intensity, to an increase in the size of the envelope and the size of the star itself. As a consequence, our Sun will become very large. If we imagine this picture in tens of billions of years, 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 of a star's evolution, its transitional state into the category of variable stars.

As a result of such a transformation, the distance from the Earth to the Sun will be reduced, so that the Earth will fall into the zone of influence of the solar corona and will begin to "fry" in it. The temperature on the planet's surface will rise tenfold, which will lead to the disappearance of the atmosphere and to the evaporation of water. As a result, the planet will turn into a lifeless rocky desert.

The final stages of star evolution

Having reached the phase of a red giant, 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 proceed calmly, without impulses and explosive reactions. The white dwarf will die for a long time, burning to ashes.

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

The neutron stars known to science have a diameter of 10-15 km. At 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 star of large mass, the final stage of evolution takes on 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 increases the gravitational forces that drive the compressive forces. It is not possible to suspend this process. The density of matter grows until it turns into infinity, 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 much more black holes if massive and supermassive stars occupied most of the space in space.

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

Supernova birth is the most spectacular final stage in stellar evolution. Here the natural law of nature operates: the cessation of the existence of one body gives rise to a new life. The period of such a cycle as a supernova birth mainly concerns massive stars. The spent reserves of hydrogen lead to the fact that helium and carbon are included in the process of thermonuclear fusion. As a result of this reaction, the pressure rises again, and an iron core forms in the center of the star. Under the influence of the strongest gravitational forces, the center of mass shifts to the central part of the star. The core becomes so heavy that it is unable to withstand its own gravity. As a consequence, a rapid expansion of the nucleus 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, therefore, such a 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 explains their rather rare detection.

Finally

The evolution of stars is a process that spans tens of billions of years. Our idea of \u200b\u200bthe ongoing processes is just a mathematical and physical model, theory. Terrestrial time is just a moment in the huge time cycle that our Universe lives on. We can only observe what was happening billions of years ago and guess what the next generations of earthlings might 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 bowels of stars

At this time, for stars with a mass greater than 0.8 times the mass of the Sun, the core becomes transparent to radiation, and radiant energy transfer in the core will prevail, while the upper envelope remains convective. Nobody knows for certain which stars of lower mass arrive on the main sequence, since the time spent by these stars in the category of young ones exceeds the age of the Universe. All our ideas about the evolution of these stars are based on numerical calculations.

As the star shrinks, the pressure of the degenerate electron gas begins to increase, and at some radius of the star, this pressure stops the growth of the central temperature, and then begins to decrease it. And for stars less than 0.08, this turns out to be fatal: the energy released during nuclear reactions will never be enough to cover the cost of radiation. Such under-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 cessation of all nuclear reactions.

Young stars of intermediate mass

Young stars of intermediate mass (from 2 to 8 solar masses) evolve qualitatively in the same way as their smaller sisters, with the exception that they have no convective zones up to the main sequence.

Objects of this type are associated with the so-called. Herbit stars Ae \\ Be as irregular variables of spectral type B-F5. They also have bipolar jet disks. The outflow rate, luminosity, and effective temperature are substantially higher than those for τ Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.

Young stars with masses greater than 8 solar masses

In fact, these are already normal stars. While the mass of the hydrostatic core was accumulating, the star managed to skip all intermediate stages and heat up nuclear reactions to such an extent that they compensated for the radiation losses. These stars have an 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 formed star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars greater than 100-200 solar masses.

Mid-life of a star

Among the formed stars, there is a huge variety of colors and sizes. In spectral type, they range from hot blue to cold red, 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 the death of the stars

Old stars with low mass

To date, it is not known for certain what happens to light stars after the depletion of their hydrogen reserves. Since the age of the universe is 13.7 billion years, which is not enough to deplete the supply of hydrogen fuel, modern theories are based on computer simulations of the processes occurring in such stars.

Some stars can only synthesize helium in some active regions, which causes instability and strong solar winds. In this case, the formation of a planetary nebula does not occur, and the star only evaporates, becoming even smaller than a brown dwarf.

But a star with a mass of less than 0.5 solar will never be able to synthesize helium even after the reactions with the participation of hydrogen in the core cease. Their stellar shell is not massive enough to overcome the pressure produced by the core. These stars include red dwarfs (such as Proxima Centauri), which have lived on the main sequence for hundreds of billions of years. After the termination of thermonuclear reactions in their core, they, gradually cooling down, will continue to emit weakly in the infrared and microwave ranges of the electromagnetic spectrum.

Medium stars

When a star reaches an average size (from 0.4 to 3.4 solar masses) of the red giant phase, its outer layers continue to expand, the core shrinks, and the reactions of carbon synthesis from helium begin. The fusion releases a lot of energy, giving the star a temporary respite. 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, which include changes in size, surface temperature, and energy release. The energy release is shifted towards low frequency radiation. All this is accompanied by an increasing loss of mass due to strong solar winds and intense pulsations. The stars in this phase are named late-type stars, OH -IR stars or World-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 envelope and cools as it moves away from the star, allowing dust particles and molecules to form. The strong infrared radiation of the central star in such envelopes forms ideal conditions for the activation of masers.

Helium combustion reactions are very temperature sensitive. This sometimes leads to great instability. Violent pulsations occur, which ultimately impart enough kinetic energy to the outer layers to be ejected and turn into a planetary nebula. In the center of the nebula, the core of the star remains, which, cooling down, turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar and a diameter of the order of the Earth's diameter.

White dwarfs

The overwhelming majority of stars, including the Sun, end their evolution, contracting until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases a hundred times and the density becomes a million times that of water, the star is called a white dwarf. It is devoid 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 contraction of the core, and it continues until most of the particles turn into neutrons, packed so tightly that the size of the star is measured in kilometers, and the density is 100 million times the density water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.

Supermassive stars

After the outer layers of a star, with a mass greater than five solar masses, scattered to form a red supergiant, the core begins to shrink due to gravitational forces. As the compression proceeds, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, heavy elements are synthesized, which temporarily restrains the collapse of the nucleus.

Ultimately, as more and more heavy elements of the periodic table are formed, iron -56 is synthesized from silicon. Up to this point, the synthesis of elements released a large amount of energy, but it is the iron -56 nucleus that has the maximum mass defect and the formation of heavier nuclei is disadvantageous. Therefore, when the iron core of a star reaches a certain value, the pressure in it is no longer able to withstand the colossal force of gravity, and an immediate collapse of the core occurs with neutronization of its matter.

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

The accompanying neutrino burst provokes a shock wave. Strong jets of neutrinos and a rotating magnetic field expel most of the material accumulated by the star - the so-called seating elements, including iron and lighter elements. The scattering matter is bombarded by neutrons ejected from the nucleus, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and possibly even to 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 can collide with other space debris, and possibly participate in the formation of new stars, planets or satellites.

The processes taking place 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 interior of a supergiant forces electrons to fall onto the atomic nucleus, where they merge 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 period of revolution becomes extremely small as the size of the star decreases (due to the conservation of angular momentum). Some make 600 revolutions per second. When the axis connecting the north and south magnetic poles of this rapidly rotating star points to the Earth, a pulse of radiation can be recorded repeating at intervals equal to the period of the star's revolution. 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 that, the star becomes a black hole.

The existence of black holes was predicted by general relativity. According to general relativity, matter and information cannot leave a black hole under any conditions. However, quantum mechanics makes possible exceptions to this rule.

A number of open questions remain. Chief among them: "Are there black holes at all?" Indeed, in order to say for sure that a given object is a black hole, it is necessary to observe its event horizon. All attempts to do this have ended in failure. But there is still hope, since some objects cannot be explained without attracting accretion, and accretion onto an object without a solid surface, but the very existence of black holes does not prove this.

The 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 effect of the initial mass of a star on the formation of objects at the end of its life cycle?

Although the stars appear to be eternal on the human scale of time, they, like everything in nature, are born, live and die. According to the generally accepted hypothesis of a gas and dust cloud, a star is born as a result of gravitational compression of an interstellar gas and dust cloud. As such a cloud compresses, first protostar,the temperature in its center grows steadily until it reaches the limit necessary for the speed of the thermal motion of particles to exceed the threshold, after which protons are able to overcome the macroscopic forces of mutual electrostatic repulsion ( cm. Coulomb's law) and enter into a thermonuclear fusion reaction ( cm. Nuclear decay and fusion).

As a result of a multistage thermonuclear fusion reaction of four protons, a helium nucleus (2 protons + 2 neutrons) is ultimately formed and a whole fountain of various elementary particles is released. In the final state, the total mass of the formed particles smaller the masses of the four initial protons, which means that free energy is released during the reaction ( cm. Theory of relativity). Because of this, the inner core of a newborn star quickly heats up to ultra-high temperatures, and its excess energy begins to splash out towards its less hot surface - and out. At the same time, the pressure in the center of the star begins to grow ( cm. Ideal gas equation of state). Thus, by “burning” hydrogen in the course of a thermonuclear reaction, the star does not allow the forces of gravitational attraction to compress itself to a superdense state, opposing the continuously renewed internal thermal pressure to gravitational collapse, as a result of which a stable energy equilibrium arises. Stars at the stage of active combustion of hydrogen are said to be in the "main phase" of their life cycle or evolution ( cm. Hertzsprung-Russell diagram). The transformation of some chemical elements into others inside a star is called nuclear fusion or nucleosynthesis.

In particular, the Sun has been in the active stage of burning hydrogen in the process of active nucleosynthesis for about 5 billion years, and the reserves of hydrogen in the core for its continuation for our star should be enough for another 5.5 billion years. The more massive the star, the more hydrogen fuel it has, but to counteract the forces of gravitational collapse, it has to burn hydrogen with an intensity that exceeds the growth rate of hydrogen reserves as the star's mass increases. Thus, the more massive a star, the shorter its life time, determined by the depletion of hydrogen reserves, and the largest stars literally burn out in "some" tens of millions of years. The smallest stars, on the other hand, live “comfortably” for hundreds of billions of years. So on this scale, our Sun belongs to the "strong middle peasants."

Sooner or later, however, any star will use up all the hydrogen available for combustion in its thermonuclear furnace. What's next? It also depends on the mass of the star. The sun (and all stars not exceeding it in mass by more than eight times) end my life in a very banal way. As the reserves of hydrogen in the interior of the star are depleted, the forces of gravitational compression, patiently waiting for this hour from the very moment of the birth of the star, begin to gain the upper hand - and under their influence, the star begins to shrink and thicken. This process leads to a twofold effect: The temperature in the layers directly around the star's core rises to a level at which the hydrogen contained there finally enters into a thermonuclear fusion reaction with the formation of helium. At the same time, the temperature in the core itself, which now consists of almost one helium, rises so much that helium itself - a kind of "ash" of the dying primary nucleosynthesis reaction - enters into a new thermonuclear fusion reaction: one carbon nucleus is formed from three helium nuclei. This secondary reaction of thermonuclear fusion, fueled by the products of the primary reaction, is one of the key moments in the life cycle of stars.

With the secondary combustion of helium in the core of the star, so much energy is released that the star literally begins to swell. In particular, the shell of the Sun at this stage of life will expand beyond the orbit of Venus. In this case, the total radiation energy of the star remains at about the same level as during the main phase of its life, but since this energy is now radiated through a much larger surface area, the outer layer of the star cools down to the red part of the spectrum. The star turns into red giant.

For stars of the class of the Sun, after the depletion of the fuel feeding the secondary reaction of nucleosynthesis, the stage of gravitational collapse begins again - this time the final one. The temperature inside the core is no longer able to rise to the level required for the next level of thermonuclear reaction to begin. Therefore, the star contracts until the forces of gravitational attraction are balanced by the next force barrier. It is played by degenerate electron gas pressure(cm. Chandrasekhar's Limit). Electrons, which up to this stage played the role of unemployed extras in the evolution of the star, without participating in nuclear fusion reactions and freely moving between nuclei in the process of fusion, at a certain stage of compression are deprived of "living space" and begin to "resist" further gravitational compression of the star. The state of the star is stabilized, and it turns into a degenerate white dwarf,which will radiate residual heat into space until it cools down completely.

Stars more massive than the Sun will have a much more spectacular end. After the combustion of helium, their mass during compression turns out to be sufficient to heat the core and shell to temperatures required to trigger the next nucleosynthesis reactions - carbon, then silicon, magnesium - and so on, as the nuclear masses grow. Moreover, at the beginning of each new reaction in the core of the star, the previous one continues in its envelope. In fact, all the chemical elements up to iron that make up the universe were formed precisely as a result of nucleosynthesis in the bowels of dying stars of this type. But iron is the limit; it cannot serve as a fuel for nuclear fusion or decay reactions at any temperature and pressure, since an influx of external energy is required both for its decay and for adding additional nucleons to it. As a result, the massive star gradually accumulates an iron core inside itself, which is not capable of serving as fuel for any further nuclear reactions.

As soon as the temperature and pressure inside the nucleus reach a certain level, the electrons begin to interact with the protons of the iron nuclei, resulting in the formation of neutrons. And in a very short period of time - some theorists believe that it takes only a few seconds - electrons free throughout the previous evolution of the star literally dissolve in the protons of iron nuclei, all the matter of the star's core turns into a continuous bunch of neutrons and begins to rapidly contract in gravitational collapse , since the opposing pressure of the degenerate electron gas drops to zero. The outer shell of the star, from under which any support is knocked out, collapses towards the center. The collision energy of the collapsed outer shell with the neutron core is so high that it bounces off and scatters in all directions from the core with great speed - and the star literally explodes in a blinding flash supernova stars... In a matter of seconds, during a supernova explosion, more energy can be released into space than all the stars of the galaxy combined during the same time.

After a supernova explosion and the expansion of the envelope in stars with a mass of about 10-30 solar masses, the ongoing gravitational collapse leads to the formation of a neutron star, the substance of which is compressed until it begins to make itself felt degenerate neutron pressure -in other words, now neutrons (just as electrons did earlier) begin to resist further compression, requiring myselfliving space. This usually occurs when the star reaches about 15 km in diameter. The result is a rapidly rotating neutron star that emits electromagnetic pulses at its rotational frequency; such stars are called pulsars. Finally, if the mass of the star's core exceeds 30 solar masses, nothing can stop its further gravitational collapse, and as a result of a supernova explosion,

Combustion of hydrogen is the longest stage in the life of a star, which is associated with the initial large abundance of hydrogen (70 by mass) and high calorific value () of the conversion of hydrogen into helium, which is about 70 energy obtained in a chain of successive thermonuclear transformations of hydrogen into an element with the highest energy bonds per nucleon (MeV / nucleon). The photon luminosity of stars on the main sequence, where hydrogen burns, is, as a rule, less than at subsequent stages of evolution, and their neutrino luminosity is much lower, because central temperatures do not exceed K. Therefore, most of the stars in the Galaxy and the Universe are main sequence stars.

After the end of hydrogen burning in the core, the star moves to the right of the main sequence on the effective temperature - luminosity diagram (Hertzsprung-Russell diagram), its effective temperature decreases, and the star moves to the region of red giants. This is due to the convective energy transfer from a layered hydrogen source located directly near the helium core. In the core itself, the temperature gradually increases due to gravitational compression, and at a temperature and density g / cm, helium begins to burn. ( Comment: since there are no stable elements with atomic numbers 5 and 8 in nature, a reaction is impossible, and beryllium-8 decays into 2 alpha particles

The release of energy per gram in the combustion of helium is about an order of magnitude less than in the combustion of hydrogen. Therefore, the lifetime and number of stars at this stage of evolution are much shorter than those of the main sequence stars. But due to their high luminosity (the stage of a red giant or supergiant), these stars are well studied.

The most important reaction is - - process: The energy of the sum of the three alpha particles is 7.28 MeV higher than the rest energy of the carbon-12 nucleus. Therefore, for the reaction to proceed effectively, a "suitable" energy level of the carbon-12 nucleus is needed. The nucleus has such a level (with an energy of 7.656 MeV); therefore, the 3-reaction in stars is of a resonant nature and therefore proceeds at a sufficient rate. Two alpha particles form the corticosteroid nucleus:. The lifetime is about c, but there is a possibility of attaching one more alpha particle to form an excited carbon-12 nucleus:. The excitement is removed by the birth of a pair, not by a photon, since photon transition from this level is prohibited by the selection rules:. Note that the resulting atom basically immediately "breaks down" into Be and He and ultimately into 3 alpha particles, and only in one case out of 2500 there is a transition to the ground level with the release of 7.65 MeV of energy carried away by the pair.

Further reaction rate

strongly depends on temperature (determined by the mass of the star), therefore the final result of helium burning in massive stars is the formation of a carbon, carbon-oxygen or purely oxygen core.

At the subsequent stages of evolution of massive stars in the central regions of the star at high temperatures, reactions of direct fusion of heavy nuclei take place. The energy release in combustion reactions is comparable to the energy release in the β-reaction; however, the powerful neutrino radiation due to the high temperature (K) makes the lifetime of the star at these stages much shorter than the burning time of helium. The probability of detecting such stars is extremely small, and at present there is not a single confident identification of a star in a calm state, releasing energy due to combustion or heavier elements.

Figure: 7.1 Calculation of the evolution of a star with an initial mass of 22 as a function of time from the moment of hydrogen ignition in the core to the onset of collapse. Time (on a logarithmic scale) is counted from the moment the collapse began. The ordinate is the mass in solar units, measured from the center. The stages of thermonuclear combustion of various elements (including layered sources) are noted. The color indicates the intensity of heating (blue) and neutrino cooling (purple). Shaded areas indicate convectively unstable regions of the star. Calculations Heger A., \u200b\u200bWoosley S. (Figure from the review by Langanke K., Martinez-Pinedo G., 2002, nucl-th / 0203071)

Stars: their birth, life and death [Third edition, revised] Shklovsky Iosif Samuilovich

Chapter 12 Star Evolution

Chapter 12 Star Evolution

As already emphasized in Section 6, the overwhelming majority of stars change their main characteristics (luminosity, radius) very slowly. At any given moment, they can be considered as being in a state of equilibrium - a circumstance that we have widely used to clarify the nature of stellar interiors. But the slowness of changes does not mean their absence. It's all about timing evolution, which must be absolutely inevitable for the stars. In its most general form, the problem of the evolution of a star can be formulated as follows. Let's say that there is a star with a given mass and radius. In addition, its initial chemical composition is known, which will be considered constant throughout the entire volume of the star. Then its luminosity follows from the calculation of the model of the star. In the course of evolution, the chemical composition of a star must inevitably change, since because of the thermonuclear reactions supporting its luminosity, the hydrogen content irreversibly decreases with time. In addition, the chemical composition of the star will cease to be uniform. If in its central part the percentage of hydrogen decreases noticeably, then at the periphery it will remain practically unchanged. But this means that as the star evolves, associated with the "burnout" of its nuclear fuel, the model of the star itself, and hence its structure, must change. Changes in luminosity, radius, surface temperature should be expected. As a consequence of such serious changes, the star will gradually change its place in the Hertzsprung - Russell diagram. One should imagine that on this diagram it will describe a certain trajectory or, as they say, a "track".

The problem of stellar evolution is undoubtedly one of the most fundamental problems of astronomy. Essentially, the question is how stars are born, live, “grow old” and die. It is to this problem that this book is devoted. This problem, by its very nature, is an integrated... It is solved by purposeful research by representatives of different branches of astronomy - observers and theorists. After all, studying the stars, it is impossible to immediately say which of them are in a genetic relationship. In general, this problem turned out to be very difficult and for several decades did not lend itself to solution at all. Moreover, until relatively recently, research efforts have often gone in a completely wrong direction. For example, the very presence of the main sequence in the Hertzsprung-Russell diagram has "inspired" many naive researchers to imagine that stars evolve along this diagram from hot blue giants to red dwarfs. But since there is a ratio "mass - luminosity", according to which the mass of stars located along the main sequence should be continuously decreasing, the aforementioned researchers stubbornly believed that the evolution of stars in the indicated direction should be accompanied by a continuous and, moreover, very significant loss of their mass.

All this turned out to be wrong. Gradually, the question of the paths of evolution of stars became clear, although individual details of the problem are still far from being solved. Special merit in understanding the process of stellar evolution belongs to theoretical astrophysicists, specialists in the internal structure of stars, and above all to the American scientist M. Schwarzschild and his school.

The early stage in the evolution of stars, associated with the process of their condensation from the interstellar medium, was considered at the end of the first part of this book. There, in fact, it was not even about the stars, but about protostars... The latter, continuously contracting under the action of gravity, become increasingly compact objects. In this case, the temperature of their bowels increases continuously (see formula (6.2)) until it becomes about several million kelvin. At this temperature, in the central regions of protostars, the first thermonuclear reactions on light nuclei (deuterium, lithium, beryllium, boron), in which the “Coulomb barrier” are relatively low, are “switched on”. When these reactions take place, the contraction of the protostar will slow down. However, the light nuclei will "burn out" rather quickly, since their abundance is small, and the compression of the protostar will continue at almost the same speed (see equation (3.6) in the first part of the book), the protostar will "stabilize", that is, it will stop compressing, only after the temperature in its central part rises so much that proton-proton or carbon-nitrogen reactions "turn on". It will assume an equilibrium configuration under the action of the forces of its own gravity and the difference in gas pressure, which almost exactly compensate each other (see § 6). Strictly speaking, from this moment on, the protostar becomes a star. The young star "sits down" in its place somewhere on the main sequence. Its exact place on the main sequence is determined by the value of the initial mass of the protostar. Massive protostars “land” on the upper part of this sequence, protostars with a relatively small mass (less than the solar mass) “land” on its lower part. Thus, protostars continuously "enter" the main sequence along its entire length, so to speak, "with a wide front."

The "protostellar" stage of stellar evolution is rather fleeting. The most massive stars go through this stage in only a few hundred thousand years. It is therefore not surprising that the number of such stars in the Galaxy is small. Therefore, it is not so easy to observe them, especially when you consider that the places where the process of star formation takes place, as a rule, is submerged in dust clouds that absorb light. But after they “register on their constant area” on the main sequence of the Hertzsprung-Russell diagram, the situation will change dramatically. For a very long time they will be on this part of the diagram, almost without changing their properties. Therefore, most of the stars are observed in the indicated sequence.

The structure of the models of a star, when it has relatively recently "sat down" on the main sequence, is determined by a model calculated on the assumption that its chemical composition is the same throughout the entire volume ("homogeneous model"; see Fig. 11.1, 11.2). As the hydrogen "burns out", the state of the star will change very slowly but steadily, as a result of which the point representing the star will describe a certain "track" on the Hertzsprung-Russell diagram. The nature of the change in the state of a star essentially depends on whether matter is mixing in its interior or not. In the second case, as we saw for some models in the previous section, in the central region of the star, the abundance of hydrogen becomes noticeably lower due to nuclear reactions than in the periphery. Such a star can only be described by an inhomogeneous model. But another path of stellar evolution is also possible: mixing occurs throughout the entire volume of the star, which for this reason always retains a "uniform" chemical composition, although the hydrogen content will continuously decrease over time. It was impossible to say in advance which of these possibilities is realized in nature. Of course, in the convective zones of stars there is always an intense mixing of matter, and within these zones the chemical composition must be constant. But even for those regions of stars where energy transfer by radiation dominates, mixing of matter is also quite possible. After all, one can never exclude systematic rather slow movements of large masses of matter at low speeds, which will lead to mixing. Such movements can arise due to some features of the rotation of the star.

The calculated models of a star, in which both the chemical composition and the measure of inhomogeneity change systematically at a constant mass, form a so-called "evolutionary sequence". By plotting the points corresponding to different models of the evolutionary sequence of a star on the Hertzsprung - Russell diagram, one can obtain its theoretical track on this diagram. It turns out that if the evolution of a star was accompanied by complete mixing of its matter, the tracks would be directed from the main sequence to the left... On the contrary, theoretical evolutionary tracks for inhomogeneous models (i.e., in the absence of complete mixing) always lead the star away right from the main sequence. Which of the two theoretically calculated paths of stellar evolution is correct? As you know, the criterion of truth is practice. In astronomy, practice is the results of observations. Let us look at the Hertzsprung - Russell diagram for star clusters shown in Fig. 1.6, 1.7 and 1.8. We will not find there the stars above and left from the main sequence. But there are a lot of stars on right from it are red giants and subgiants. Therefore, we can consider such stars as leaving the main sequence in the course of their evolution, which is not accompanied by complete mixing of matter in their interior. Explaining the nature of red giants is one of the greatest achievements of the theory of stellar evolution [30]. The very existence of red giants means that the evolution of stars, as a rule, is not accompanied by the mixing of matter in their entire volume. Calculations show that as the star evolves, the size and mass of its convective core continuously decrease [31].

Obviously, the evolutionary sequence of star models by itself still says nothing about pace stellar evolution. The evolutionary timeline can be obtained from the analysis of changes in the chemical composition of different members of the evolutionary sequence of models of the star. Some average hydrogen content in a star can be determined, "weighted" by its volume. We denote this average content through X... Then, obviously, the change with time in the quantity X determines the luminosity of a star, since it is proportional to the amount of thermonuclear energy released in the star in one second. Therefore, one can write:

(12.1)

The amount of energy released during the nuclear transformation of one gram of matter, symbol

means change in value X in one second. We can define the age of a star as the period of time elapsed from the moment when it “sat down” on the main sequence, ie, nuclear hydrogen reactions began in its interior. If the luminosity and the average hydrogen content are known for different members of the evolutionary sequence X, then it is not difficult to find from equation (12.1) the age of a certain model of a star in its evolutionary sequence. Anyone who knows the basics of higher mathematics will understand that from equation (12.1), which is a simple differential equation, the age of a star

defined as the integral

Summing up time intervals

12, we will obviously get the time interval

Passed from the beginning of the evolution of the star. It is this circumstance that is expressed by formula (12.2).

In fig. 12.1 shows the theoretically calculated evolutionary tracks for relatively massive stars. They begin their evolution at the bottom edge of the main sequence. As hydrogen burns out, such stars move along their tracks in a general direction across the main sequence, without going beyond its limits (i.e., staying within its width). This stage of evolution, associated with the presence of stars on the main sequence, is the longest. When the hydrogen content in the core of such a star becomes close to 1%, the rate of evolution will accelerate. To maintain the energy release at the required level with a sharply reduced content of hydrogen "fuel" it is necessary to increase the core temperature as a "compensation". And here, as in many other cases, the star itself regulates its structure (see § 6). An increase in core temperature is achieved by compression stars as a whole. For this reason, the evolutionary tracks turn sharply to the left, that is, the temperature of the star's surface increases. Very soon, however, the contraction of the star stops, as all the hydrogen in the core is burned out. But a new area of \u200b\u200bnuclear reactions “turns on” - a thin shell around an already “dead” (albeit very hot) nucleus. As the star evolves further, this envelope moves further and further away from the center of the star, thereby increasing the mass of the "burnt out" helium core. At the same time, the process of compression of this core and its heating will take place. However, in this case, the outer layers of such a star begin to swell rapidly and very strongly. This means that the surface temperature decreases significantly with a slightly changing flow. Its evolutionary track turns sharply to the right and the star acquires all the features of a red supergiant. Since the star approaches this state rather quickly after the cessation of compression, there are almost no stars filling the gap between the main sequence and the branch of giants and supergiants in the Hertzsprung-Russell diagram. This is clearly seen in such diagrams built for open clusters (see Fig. 1.8). The further fate of red supergiants is still not well understood. We will return to this important issue in the next section. The core can be heated up to very high temperatures, of the order of hundreds of millions of kelvin. At such temperatures, the triple helium reaction "turns on" (see § 8). The energy released during this reaction stops the further compression of the nucleus. After that, the core will expand slightly, and the radius of the star will decrease. The star will become hotter and move to the left in the Hertzsprung-Russell diagram.

The evolution of stars with a lower mass proceeds somewhat differently, for example, M

1, 5M

Note that the evolution of stars, the mass of which is less than the mass of the Sun, is generally inappropriate to consider, since their residence time within the main sequence exceeds the age of the Galaxy. This circumstance makes the problem of the evolution of low-mass stars “uninteresting” or, better to say, “not urgent”. We only note that stars with low mass (less than

0, 3 solar) remain completely "convective" even when they are on the main sequence. They never form a "radiant" nucleus. This tendency is clearly visible in the case of the evolution of protostars (see § 5). If the mass of the latter is relatively large, a radiant core is formed even before the protostar "sits" on the main sequence. And low-mass objects both at the protostellar and stellar stages remain completely convective. In such stars, the temperature at the center is not high enough for the proton-proton cycle to fully work. It is cut off by the formation of the isotope 3 He, and the "normal" 4 He is no longer synthesized. Over 10 billion years (which is close to the age of the oldest stars of this type), about 1% of hydrogen will turn into 3 Not. Consequently, it can be expected that the abundance of 3 He in relation to 1 H will be anomalously high - about 3%. Unfortunately, there is no way to check this prediction of the theory by observation. Stars with such a low mass are red dwarfs, whose surface temperature is completely insufficient to excite helium lines in the optical region. In principle, however, in the far ultraviolet part of the spectrum, resonance absorption lines could be observed by methods of rocket astronomy. However, the extreme weakness of the continuous spectrum precludes even this problematic possibility. It should be noted, however, that a significant, if not most of the red dwarfs are flashing stars of the UV Ceti type (see § 1). The very phenomenon of rapidly repeating flares in such cool dwarf stars is undoubtedly associated with convection, which engulfs their entire volume. Emission lines are observed during flares. Maybe it will be possible to observe the lines 3 Not at such stars? If the mass of the protostar is less than 0 , 08M

The temperature in its interior is so low that no thermonuclear reactions can stop the compression at the stage of the main sequence. Such stars will continuously contract until they become white dwarfs (more precisely, degenerate red dwarfs). Let us return, however, to the evolution of more massive stars.

In fig. 12.2 shows the evolutionary track of a star with a mass equal to 5 M

According to the most detailed calculations carried out using a computer. On this track, the numbers indicate the characteristic stages in the evolution of the star. The explanations for the figure indicate the timing of each stage of evolution. We only point out here that the section of the evolutionary track 1-2 corresponds to the main sequence, section 6-7 corresponds to the stage of the red giant. An interesting decrease in luminosity in the region 5-6, associated with the expenditure of energy for "swelling" of the star. In fig. 12.3 similar theoretically calculated tracks are given for stars of different masses. The numbers marking the different phases of evolution have the same meaning as in Fig. 12.2.

Figure: 12.2:Evolutionary track of a star with a mass of 5 M , (1-2) - combustion of hydrogen in the convective core, 6 , 44 10 7 years old; (2-3) - total compression of the star, 2 , 2 10 6 years old; (3-4) - ignition of hydrogen in a layered source, 1 , 4 10 5 years; (4-5) - combustion of hydrogen in a thick layer, 1 , 2 10 6 years old; (5-6) - expansion of the convective shell, 8 10 5 years; (6-7) - red giant phase, 5 10 5 years; (7-8) - ignition of helium in the core, 6 10 6 years old; (8-9) - disappearance of the convective shell, 10 6 years; (9-10) - combustion of helium in the core, 9 10 6 years old; (10-11) - secondary expansion of the convective shell, 10 6 years; (11-12) - compression of the nucleus as helium burns out; (12-13-14) - layered helium source; (14-?) - neutrino losses, red supergiant.

From a simple consideration of the evolutionary tracks depicted in Fig. 12.3, it follows that more or less massive stars leave the main sequence in a rather "winding" way, forming a branch of giants on the Hertzsprung-Russell diagram. A very rapid increase in the luminosity of stars with lower mass is characteristic as they evolve towards the red giants. The difference in the evolution of such stars compared to more massive ones is that the former form a very dense, degenerate core. Such a core, due to the high pressure of the degenerate gas (see Sec. 10), is capable of "holding" the weight of the layers of the star lying above. It will almost not shrink, and therefore, very hot. Therefore, if the "triple" helium reaction turns on, it will be much later. Except for physical conditions, in the region near the center, the structure of such stars will be similar to the structure of more massive ones. Consequently, their evolution after hydrogen burnout in the central region will also be accompanied by the "swelling" of the outer shell, which will lead their tracks to the region of red giants. However, unlike more massive supergiants, their cores will consist of a very dense degenerate gas (see the diagram in Fig. 11.4).

Perhaps the most outstanding achievement of the theory of stellar evolution developed in this section is its explanation of all the features of the Hertzsprung - Russell diagram for star clusters. A description of these diagrams has already been given in § 1. As already mentioned in the above section, the age of all stars in a given cluster should be considered the same. The initial chemical composition of these stars should also be the same. After all, they all formed from one and the same (albeit rather large) aggregate of the interstellar medium - a gas-dust complex. Different star clusters should differ from each other primarily in age and, in addition, the initial chemical composition of globular clusters should differ sharply from the composition of open clusters.

The lines along which the stars of the clusters are located on the Hertzsprung - Russell diagram do not in any way signify their evolutionary tracks. These lines are the locus of points on the indicated diagram, where stars with different masses have the same age... If we want to compare the theory of stellar evolution with the results of observations, first of all it is necessary to construct theoretically "lines of the same age" for stars with different masses and the same chemical composition. The age of a star at different stages of its evolution can be determined using formula (12.3). In this case, it is necessary to use theoretical tracks of stellar evolution of the type shown in Fig. 12.3. In fig. 12.4 shows the results of calculations for eight stars, the masses of which vary from 5.6 to 2.5 solar masses. On the evolutionary tracks of each of these stars are marked with points of position, which the corresponding stars will take in one hundred, two hundred, four hundred and eight hundred million years of their evolution from the original state on the lower edge of the main sequence. The curves passing through the corresponding points for different stars are “curves of the same age”. In our case, the calculations were carried out for fairly massive stars. The calculated time intervals of their evolution cover at least 75% of their "active life" when they emit thermonuclear energy generated in their depths. For the most massive stars, evolution reaches the stage of secondary compression, which occurs after complete burnout of hydrogen in their central parts.

If we compare the obtained theoretical curve of equal age with the Hertzsprung - Russell diagram for young star clusters (see Fig. 12.5, and also 1.6), then its striking similarity with the main line of this cluster is involuntarily striking. In full accordance with the main tenet of the theory of evolution, according to which more massive stars leave the main sequence faster, the diagram in Fig. 12.5 clearly indicates that the top of this sequence of stars in the cluster bends to the right... The place of the main sequence, where the stars begin to deviate noticeably from it, is the “lower”, the older the cluster. This circumstance alone makes it possible to directly compare the ages of various star clusters. For old clusters, the main sequence ends above somewhere near spectral class A. For young clusters, the entire main sequence is still "intact", up to hot massive stars of spectral class B. For example, this situation is seen in the diagram for the cluster NGC 2264 (Fig. 1.6). Indeed, the line of the same age calculated for this cluster gives a period of its evolution of only 10 million years. Thus, this cluster was born "in the memory" of the ancient human ancestors - the Ramapithecs ... A much older cluster of stars - the Pleiades, whose diagram is shown in Fig. 1.4, has a completely "average" age of about 100 million years. Stars of spectral class B7 are still preserved there. But the Hyades cluster (see Fig. 1.5) is quite old - its age is about one billion years, and therefore the main sequence begins only with class A stars.

The theory of stellar evolution explains another curious feature of the Hertzsprung-Russell diagram for "young" clusters. The point is that the evolutionary times for low-mass dwarf stars are very long. For example, many of them have not yet passed the stage of gravitational contraction in 10 million years (the evolutionary period of the NGC 2264 cluster) and, strictly speaking, are not even stars, but protostars. Such objects, as we know, are located on right from the Hertzsprung - Russell diagram (see Fig. 5.2, where the evolutionary tracks of stars begin at an early stage of gravitational contraction). If, therefore, in a young cluster, dwarf stars have not yet "sat down" on the main sequence, the lower part of the latter will be in such a cluster shifted to the right, which is observed (see Fig. 1.6). Our Sun, as we said above, despite the fact that it has already “exhausted” a noticeable part of its “hydrogen resources”, has not yet left the main sequence band of the Hertzsprung-Russell diagram, although it has evolved for about 5 billion years. Calculations show that the "young", recently "landed" on the main sequence of the Sun emitted 40% less than now, and its radius was only 4% less than the modern one, and the surface temperature was 5200 K (now 5700 K).

The theory of evolution easily explains the features of the Hertzsprung-Russell diagram for globular clusters. First of all, these are very old objects. Their age is only slightly less than the age of the Galaxy. This clearly follows from the almost complete absence of upper main sequence stars in these diagrams. The lower part of the main sequence, as already mentioned in § 1, consists of subdwarfs. It is known from spectroscopic observations that subdwarfs are very poor in heavy elements - there may be tens of times less of them than in "ordinary" dwarfs. Therefore, the initial chemical composition of globular clusters was significantly different from the composition of the substance from which the open clusters were formed: there were too few heavy elements. In fig. 12.6 presents the theoretical evolutionary tracks of stars with a mass of 1.2 solar (this is close to the mass of a star that managed to evolve over 6 billion years), but with different initial chemical compositions. It is clearly seen that after the star "left" from the main sequence, the luminosity for the same phases of evolution at a low metal abundance will be much higher. At the same time, the effective surface temperatures of such stars will be higher.

In fig. 12.7 shows the evolutionary tracks of low-mass stars with a low abundance of heavy elements. On these curves, the points indicate the positions of the stars after six billion years of evolution. The thicker line connecting these points is obviously a line of the same age. If we compare this line with the Hertzsprung - Russell diagram for the globular cluster M 3 (see Fig. 1.8), then the complete coincidence of this line with the line along which the stars of this cluster "leave" the main sequence is immediately striking.

Shown in Fig. 1.8 the diagram also shows a horizontal branch deviating from the sequence of giants to the left. Apparently, it corresponds to stars in the depths of which a "triple" helium reaction is taking place (see Sec. 8). Thus, the theory of stellar evolution explains all the features of the Hertzsprung – Russell diagram for globular clusters to their “ancient ages” and low abundance of heavy elements [32].

Curiously enough, there are several white dwarfs in the Hyades cluster, but not in the Pleiades. Both clusters are relatively close to us, so this interesting difference between the two clusters cannot be explained by different "visibility conditions". But we already know that white dwarfs are formed at the final stage of red giants, whose masses are relatively small. Therefore, for the complete evolution of such a giant, a considerable time is needed - at least a billion years. This time "passed" at the Hyades cluster, but "has not yet come" in the Pleiades. That is why the first cluster already contains a certain number of white dwarfs, while the second does not.

In fig. 12.8 shows a summary schematic diagram of Hertzsprung - Russell for a number of clusters, open and globular. In this diagram, the effect of age differences in different clusters is clearly visible. Thus, there is every reason to assert that the modern theory of stellar structure and the theory of stellar evolution based on it were able to easily explain the main results of astronomical observations. Undoubtedly, this is one of the most outstanding achievements of astronomy of the 20th century.

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Distances to the nearest stars The closest (not counting the Sun) star in the system of which there can be a planet suitable for life is Tau Ceti. It is 11.9 light years from Earth; that is, traveling at the speed of light, it will be possible to reach it