Star evolution. The evolution of stars from the point of view of exact science and the theory of relativity

It is quite natural that stars are not living beings, but they also go through evolutionary stages similar to birth, life and death. Like a person, a star undergoes radical changes throughout its life. But it should be noted that they obviously live longer - millions and even billions of Earth years.

How are stars born? Initially, or rather after the Big Bang, matter in the universe was unevenly distributed. Stars began to form in nebulae, giant clouds of interstellar dust and gases, mostly hydrogen. This matter is affected by gravity, and part of the nebula is compressed. Then round and dense gas and dust clouds are formed - Bok globules. As such a globule continues to thicken, its mass increases due to the attraction of matter from the nebula towards itself. In the inner part of the globule, the gravitational force is strongest, and it begins to heat up and rotate. This is already a protostar. Hydrogen atoms begin to bombard each other and thereby generate a large amount of energy. Eventually the temperature of the central part reaches a temperature of the order of fifteen million degrees Celsius, the core of a new star is formed. The newborn flares up, begins to burn and glow. How long this will continue depends on what was the mass of the born star. What I said at our last meeting. The larger the mass, the shorter the life of the star.
By the way, it depends on the mass whether a protostar can become a star. According to calculations, in order for this contracting celestial body to turn into a star, its mass must be at least 8% of the mass of the Sun. A smaller globule, condensing, will gradually cool down and turn into a transitional object, something in between a star and a planet. Such objects are called brown dwarfs.

The planet Jupiter, for example, is too small to be a star. If Jupiter were more massive, perhaps thermonuclear reactions would begin in its depths, and our solar system would be a system double star. But it's all poetry...

So, the main stage of the life of a star. For most of its existence, the star is in equilibrium state. The force of gravity tends to compress the star, and the energy released as a result of thermonuclear reactions occurring in the star forces the star to expand. These two forces create a stable position of equilibrium - so stable that the star lives like this for millions and billions of years. This phase of a star's life secures its place in the main sequence. -

After shining for millions of years, a large star, that is, a star at least six times heavier than the Sun, begins to burn out. When the core runs out of hydrogen, the star expands and cools, turning into a red supergiant. This supergiant will then contract until it finally explodes in a monstrous and dramatic blazing explosion known as a supernova. It should be noted here that very massive blue supergiants bypass the stage of transformation into a red supergiant and explode much faster in a supernova.
If the remaining supernova core is small, then its catastrophic contraction (gravitational collapse) into a very dense neutron star begins, and if it is large enough, it will contract even more, forming a black hole.

A slightly different demise ordinary star. Such a star lives longer and dies a more peaceful death. The sun, for example, will burn for another five billion years before the hydrogen in its core runs out. Its outer layers will then expand and cool; a red giant is formed. In this form, a star can exist for about 100 million years on the helium formed during its lifetime in its core. But helium also burns out. To top it off, the outer layers will be blown away - they form a planetary nebula, and a dense white dwarf will shrink from the core. Although the white dwarf is hot enough, eventually it will cool down, turning into a dead star, which is called a black dwarf.

Part One ASTRONOMIC ASPECT OF THE PROBLEM

Rice. 12. Globules in a diffusion nebula

When a protostar is compressed, its temperature rises and a significant part of the liberated potential energy radiated into the surrounding space. Since the dimensions of the contracting gas sphere are very large, the radiation from a unit of its surface will be negligible. Since the radiation flux from a unit surface is proportional to the fourth power of temperature (the Stefan-Boltzmann law), the temperature of the surface layers of the star is relatively low, while its luminosity is almost the same as that of an ordinary star with the same mass. Therefore, on the "spectrum - luminosity" diagram, such stars will be located to the right of the main sequence, i.e. they will fall into the region of red giants or red dwarfs, depending on the values ​​of their initial masses. In the future, the protostar continues to shrink. Its dimensions become smaller, and the surface temperature increases, as a result of which the spectrum becomes more and more "early". Thus, moving along the "spectrum - luminosity" diagram, the protostar "sits down" rather quickly on the main sequence. During this period, the temperature of the stellar interior is already sufficient for thermonuclear reactions to begin there. At the same time, the pressure of the gas inside the future star balances the attraction and the gas ball stops shrinking. The protostar becomes a star. It takes relatively little time for protostars to go through this very early stage of their evolution. If, for example, the mass of the protostar is greater than the solar mass, only a few million years are needed; if less, several hundred million years. Since the time of evolution of protostars is relatively short, it is difficult to detect this earliest phase of the development of a star. Nevertheless, stars in this stage, apparently, are observed. We mean very interesting stars type T Taurus, usually immersed in dark nebulae. In 1966, quite unexpectedly, it became possible to observe protostars in the early stages of their evolution. We have already mentioned in the third chapter of this book the discovery by radio astronomy of a number of molecules in the interstellar medium, primarily hydroxyl OH and water vapor H2O. Great was the surprise of radio astronomers when, when surveying the sky at a wavelength of 18 cm, corresponding to the OH radio line, bright, extremely compact (ie, having small angular dimensions) sources were discovered. This was so unexpected that at first they refused even to believe that such bright radio lines could belong to a hydroxyl molecule. It was hypothesized that these lines belonged to some unknown substance, which was immediately given the "appropriate" name "mysterium". However, "mysterium" very soon shared the fate of its optical "brothers" - "nebulium" and "coronia". The fact is that for many decades the bright lines of the nebulae and the solar corona could not be identified with any known spectral lines. Therefore, they were attributed to certain, unknown on earth, hypothetical elements - "nebulium" and "coronia". Let us not condescendingly smile at the ignorance of astronomers at the beginning of our century: after all, there was no theory of the atom then! The development of physics did not leave in periodic system Mendeleev places for exotic "celestials": in 1927 "nebulium" was debunked, the lines of which were identified with complete reliability with the "forbidden" lines of ionized oxygen and nitrogen, and in 1939 -1941. it was convincingly shown that the mysterious "coronium" lines belong to multiply ionized atoms of iron, nickel and calcium. If it took decades to "debunk" "nebulium" and "codonium", then within a few weeks after the discovery it became clear that the lines of "mysterium" belong to ordinary hydroxyl, but only under unusual conditions. Further observations, first of all, revealed that the sources of the "mysterium" have extremely small angular dimensions. This was shown with the help of the then new, very effective method a study called "very long baseline radio interferometry". The essence of the method is reduced to simultaneous observations of sources on two radio telescopes separated from each other by a distance of several thousand km. As it turns out, the angular resolution in this case is determined by the ratio of the wavelength to the distance between the radio telescopes. In our case, this value can be ~3x10 -8 rad or a few thousandths of an arc second! Note that in optical astronomy such an angular resolution is still completely unattainable. Such observations have shown that there are at least three classes of "mysterium" sources. We will be interested in class 1 sources here. All of them are located inside gaseous ionized nebulae, for example, in the famous Orion Nebula. As already mentioned, their sizes are extremely small, many thousands of times smaller sizes nebulae. What is most interesting is that they have a complex spatial structure. Consider, for example, a source located in a nebula called W3.

Rice. 13. Profiles of the four components of the hydroxyl line

On fig. Figure 13 shows the profile of the OH line emitted by this source. As you can see, it consists of a large number of narrow bright lines. Each line corresponds to a certain speed of movement along the line of sight of the cloud emitting this line. The value of this speed is determined by the Doppler effect. The difference in velocities (along the line of sight) between different clouds reaches ~10 km/s. The interferometric observations mentioned above have shown that the clouds emitting each line do not coincide spatially. The picture is as follows: inside a region of approximately 1.5 seconds in size, the arcs move with different speeds about 10 compact clouds. Each cloud emits one specific (by frequency) line. The angular dimensions of the clouds are very small, on the order of a few thousandths of an arc second. Since the distance to the W3 nebula is known (about 2000 pc), the angular dimensions can easily be converted into linear ones. It turns out that the linear dimensions of the region in which the clouds move are of the order of 10 -2 pc, and the dimensions of each cloud are only an order of magnitude larger than the distance from the Earth to the Sun. Questions arise: what are these clouds and why do they radiate so strongly in hydroxyl radio lines? The second question was answered fairly quickly. It turned out that the emission mechanism is quite similar to that observed in laboratory masers and lasers. So, the sources of the "mysterium" are gigantic, natural cosmic masers operating on a wave of the hydroxyl line, the length of which is 18 cm. . As is known, amplification of radiation in lines due to this effect is possible when the medium in which the radiation propagates is "activated" in some way. This means that some "outside" energy source (the so-called "pumping") makes the concentration of atoms or molecules at the initial (upper) level anomalously high. A maser or laser is not possible without a permanent "pump". The question of the nature of the "pumping" mechanism for cosmic masers has not yet been finally resolved. However, rather powerful infrared radiation is most likely to be used as "pumping". Another possible "pumping" mechanism may be some chemical reaction. It is worth interrupting our story about cosmic masers in order to consider what amazing phenomena astronomers encounter in space. One of the greatest technical inventions of our turbulent age, which plays a significant role in the scientific and technological revolution we are now experiencing, is easily implemented in vivo And besides - on a huge scale! The flux of radio emission from some cosmic masers is so great that it could have been detected even at the technical level of radio astronomy 35 years ago, that is, even before the invention of masers and lasers! To do this, it was necessary "only" to know the exact wavelength of the OH radio link and become interested in the problem. By the way, this is not the first case when the most important scientific and technical problems facing mankind are realized in natural conditions. Thermonuclear reactions supporting the radiation of the Sun and stars (see below) stimulated the development and implementation of projects for obtaining nuclear "fuel" on Earth, which should solve all our energy problems in the future. Alas, we are still far from solving this most important task, which nature has solved "easily". A century and a half ago, Fresnel, the founder of the wave theory of light, remarked (on a different occasion, of course): "Nature laughs at our difficulties." As you can see, Fresnel's remark is even more true today. Let us return, however, to cosmic masers. Although the mechanism for "pumping" these masers is still not entirely clear, one can still get a rough idea of ​​\u200b\u200b physical conditions in clouds emitting a line of 18 cm by the maser mechanism. First of all, it turns out that these clouds are quite dense: there are at least 10 8 -10 9 particles in a cubic centimeter, and a significant (and maybe even large) part of them are molecules. The temperature is unlikely to exceed two thousand Kelvin, most likely it is about 1000 Kelvin. These properties differ sharply from those of even the densest clouds of interstellar gas. Considering the still relatively small size of the clouds, we involuntarily come to the conclusion that they rather resemble the extended, rather cold atmospheres of supergiant stars. It is very likely that these clouds are nothing but early stage development of protostars, following immediately after their condensation from the interstellar medium. Other facts speak in favor of this assertion (which the author of this book made back in 1966). In nebulae where cosmic masers are observed, young hot stars are visible (see below). Consequently, the process of star formation has recently ended there and, most likely, continues at the present time. Perhaps the most curious thing is that, as radio astronomical observations show, space masers of this type are, as it were, "immersed" in small, very dense clouds of ionized hydrogen. These clouds contain a lot of cosmic dust, which makes them unobservable in the optical range. Such "cocoons" are ionized by a young, hot star inside them. In the study of star formation processes, infrared astronomy proved to be very useful. Indeed, for infrared rays, interstellar absorption of light is not so significant. We can now imagine the following picture: from a cloud of the interstellar medium, by its condensation, several clots of different masses are formed, evolving into protostars. The rate of evolution is different: for more massive clumps it will be higher (see Table 2 below). Therefore, the most massive bunch will turn into a hot star first, while the rest will linger more or less long at the protostar stage. We observe them as sources of maser radiation in the immediate vicinity of the "newborn" hot star, which ionizes the "cocoon" hydrogen that has not condensed into clumps. Of course, this rough scheme will be refined in the future, and, of course, significant changes will be made to it. But the fact remains: it unexpectedly turned out that for some time (most likely a relatively short time) newborn protostars, figuratively speaking, "scream" about their birth, using the latest methods of quantum radiophysics (i.e. masers) ... After 2 years after the discovery of cosmic hydroxyl masers (line 18 cm) - it was found that the same sources simultaneously emit (also by a maser mechanism) a line of water vapor, the wavelength of which is 1.35 cm. The intensity of the "water" maser is even greater than that of the "hydroxyl ". The clouds emitting the H2O line, although located in the same small volume as the "hydroxyl" clouds, move at different speeds and are much more compact. It cannot be ruled out that other maser lines* will be discovered in the near future. Thus, quite unexpectedly, radio astronomy turned classic problem star formation in the branch of observational astronomy ** . Once on the main sequence and ceasing to shrink, the star radiates for a long time practically without changing its position on the "spectrum - luminosity" diagram. Its radiation is supported by thermonuclear reactions taking place in the central regions. Thus, the main sequence is, as it were, the locus of points on the "spectrum - luminosity" diagram, where a star (depending on its mass) can radiate for a long time and steadily due to thermonuclear reactions. A star's position on the main sequence is determined by its mass. It should be noted that there is one more parameter that determines the position of the equilibrium radiating star on the "spectrum-luminosity" diagram. This parameter is the initial chemical composition of the star. If the relative abundance of heavy elements decreases, the star will "fall" in the diagram below. It is this circumstance that explains the presence of a sequence of subdwarfs. As mentioned above, the relative abundance of heavy elements in these stars is ten times less than in main sequence stars. The residence time of a star on the main sequence is determined by its initial mass. If the mass is large, the radiation of the star has a huge power and it quickly consumes its hydrogen "fuel" reserves. For example, main-sequence stars with a mass several tens of times greater than the solar mass (these are hot blue giants of spectral type O) can radiate steadily while being on this sequence for only a few million years, while stars with a mass close to solar, are on the main sequence 10-15 billion years. Table below. 2, which gives the calculated duration of gravitational contraction and stay on the main sequence for stars of different spectral types. The same table shows the masses, radii, and luminosities of stars in solar units.

table 2

Rice. 14. Evolutionary tracks for stars of different masses on the "luminosity-temperature" diagram

Rice. 15. Hertzsprung-Russell diagram for the star cluster NGC 2254

Rice. 16. Hertzsprung-Russell diagram for the globular cluster M 3. On the vertical axis - relative magnitude

The corresponding diagram clearly shows the entire main sequence, including its upper left part, where hot massive stars are located (color-indicator - 0.2 corresponds to a temperature of 20 thousand K, i.e. class B spectrum). The globular cluster M 3 is an "old" object. It is clearly seen that there are almost no stars in the upper part of the main sequence of the diagram constructed for this cluster. On the other hand, the red giant branch of M 3 is very rich, while NGC 2254 has very few red giants. This is understandable: in the old M 3 cluster, a large number of stars have already “departed” from the main sequence, while in the young cluster NGC 2254 this happened only with a small number of relatively massive, rapidly evolving stars. It is noteworthy that the giant branch for M 3 goes up quite steeply, while for NGC 2254 it is almost horizontal. From the point of view of theory, this can be explained by the significantly lower abundance of heavy elements in M ​​3. Indeed, in the stars of globular clusters (as well as in other stars that concentrate not so much towards the galactic plane as towards the galactic center), the relative abundance of heavy elements is insignificant . On the diagram "color index - luminosity" for M 3 one more almost horizontal branch is visible. There is no similar branch in the diagram constructed for NGC 2254. The theory explains the emergence of this branch as follows. After the temperature of the shrinking dense helium core of a star - a red giant - reaches 100-150 million K, a new nuclear reaction will begin there. This reaction consists in the formation of a carbon nucleus from three helium nuclei. As soon as this reaction begins, the contraction of the nucleus will stop. Subsequently, the surface layers

the stars increase their temperature and the star in the "spectrum - luminosity" diagram will move to the left. It is from such stars that the third horizontal branch of the diagram for M 3 is formed.

Rice. 17. Hertzsprung-Russell summary diagram for 11 star clusters

On fig. Figure 17 schematically shows a summary color-luminosity diagram for 11 clusters, of which two (M 3 and M 92) are globular. It is clearly seen how the main sequences "bend" to the right and upwards in different clusters in full agreement with the theoretical concepts that have already been discussed. From fig. 17, one can immediately determine which clusters are young and which are old. For example, the "double" cluster X and h Perseus is young. It "saved" a significant portion of the main sequence. The M 41 cluster is older, the Hyades cluster is even older, and the M 67 cluster is very old, the color-luminosity diagram for which is very similar to the similar diagram for globular clusters M 3 and M 92. Only the giant branch of globular clusters is higher in agreement with differences in chemical composition, which were mentioned earlier. Thus, the observational data fully confirm and substantiate the conclusions of the theory. It would seem difficult to expect an observational verification of the theory of processes in stellar interiors, which are hidden from us by a huge thickness of stellar matter. And yet the theory here is constantly controlled by the practice of astronomical observations. It should be noted that the compilation of a large number of "color - luminosity" diagrams required a huge amount of work by astronomers-observers and a radical improvement in observation methods. On the other hand, advances in theory internal structure and the evolution of stars would not have been possible without modern computing technology based on the use of high-speed electronic calculating machines. An invaluable service to the theory was also provided by research in the field of nuclear physics, which made it possible to obtain quantitative characteristics of those nuclear reactions that take place in the stellar interior. It can be said without exaggeration that the development of the theory of the structure and evolution of stars is one of the greatest achievements of astronomy in the second half of the 20th century. The development of modern physics opens up the possibility of a direct observational verification of the theory of the internal structure of stars, and in particular the Sun. We are talking about the possibility of detecting a powerful stream of neutrinos, which the Sun should emit if nuclear reactions take place in its depths. It is well known that neutrinos interact extremely weakly with other elementary particles. Thus, for example, neutrinos can pass almost without absorption through the entire thickness of the Sun, while X-rays can pass without absorption only through a few millimeters of the solar interior matter. If we imagine that a powerful beam of neutrinos passes through the Sun with the energy of each particle in

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 actually represent protostars. Astronomers call them T-Taurus stars, after their prototype. By their properties - for example, luminosity - protostars are variable, since their existence has not yet entered a stable phase. Around many of them is a large amount of matter. Powerful wind currents emanate from T-type stars.

Protostars: the beginning of the life cycle

If matter falls on the surface of a protostar, it quickly burns out and turns into heat. As a result, the temperature of protostars is constantly increasing. When it rises so much that nuclear reactions are triggered in the center of the star, the protostar acquires the status of an ordinary one. With the onset of nuclear reactions, the star has a constant source of energy that supports its vital activity for a long time. How long will it be life cycle stars in the universe depends on its original size. However, it is believed that stars with a 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.

Stars of normal size

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

Brightness and luminosity

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

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

Attempts to explain the life cycle of stars

People have long tried to trace the life of the 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 temperature is inversely proportional to the radius of the star) until the increase in density slows down the contraction processes. Then the energy consumption will be higher than its income. At this point, the star will begin to cool rapidly.

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. At the same time, the provisions of his hypothesis were based not only on the theoretical conclusions available in astronomy, but also on the data of the spectral analysis of stars. Lockyer was convinced that chemical elements, which take part in the evolution of celestial bodies, consist of elementary particles- "protoelements". Unlike modern neutrons, protons and electrons, they have not a general, but an individual character. For example, according to Lockyer, hydrogen breaks down 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 and dwarf stars

Stars large sizes are the hottest and brightest. They are usually white or bluish in appearance. Despite the fact that they have giant size, the fuel inside them burns out so quickly that they lose it in just a few million years.

Small stars, in contrast to giant ones, are usually not as bright. They have a red color, live long enough - for billions of years. But among the brightest stars in the sky there are also red and orange ones. An example is the star Aldebaran - the so-called "bull's eye", located in the constellation Taurus; as well as 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 once they expanded very much, and in their diameter they began to exceed the huge red stars (supergiants). The huge area allows these stars to radiate an order of magnitude more energy than the Sun. And this 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 of the size of the Sun. Such stars are called dwarfs. Each celestial body can go through these types of the life cycle of stars - the same star at different segments 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 inside. It turns into helium inside the nuclear core of the star. The sun disposes huge amount fuel, but even it is not infinite - over the past five billion years, half the reserve has been used up.

Lifetime of stars. Life cycle of stars

After the reserves of hydrogen inside the star are exhausted, serious changes come. The remaining hydrogen begins to burn not inside its core, but on the surface. In this case, the lifetime of the star is decreasing more and more. The cycle of stars, at least most of them, in this segment passes into the stage of a red giant. The size of the star becomes larger, and its temperature, on the contrary, becomes smaller. This is how most red giants, as well as supergiants, appear. This process is part of the overall sequence of changes that occur with the stars, which scientists called the evolution of stars. The life cycle of a star includes all its stages: in the end, all stars grow old and die, and the duration of their existence is directly determined by the amount of fuel. Large 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 it live middle star? The life cycle of a star can last from less than 1.5 million years to 1 billion years or more. All this, as was said, depends on its composition and size. Stars like the Sun live between 10 and 16 billion years. Highly bright stars, like Sirius, live for a relatively short time - only a few hundred million years. The life cycle diagram of a star includes the following stages. This is a molecular cloud - the gravitational collapse of the cloud - the birth of a supernova - the evolution of a protostar - the end of the protostellar phase. Then the stages follow: the beginning of the stage of a young star - the middle of life - maturity - the stage of a red giant - a planetary nebula - the stage of a white dwarf. The last two phases are characteristic of small stars.

The nature of planetary nebulae

So, we have briefly considered the life cycle of a star. But what is it? Turning from a huge red giant into a white dwarf, sometimes stars shed their outer layers, and then the core of the star becomes naked. The gas envelope begins to glow under the influence of energy emitted by the star. This stage got its name due to the fact that the luminous gas bubbles in this shell often look like disks around planets. But in fact, they have nothing to do with the 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 the heavenly bodies.

star clusters

Astronomers are very fond of exploring. There is a hypothesis that all luminaries are born precisely in groups, and not one by one. Since the 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 these stars make, they begin at the same time and at the same time. equal conditions. Especially a lot of knowledge can be obtained by studying the dependence of their properties on mass. After all, the age of stars in 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 make beautiful photo, admire them exclusively beautiful view in the planetarium.

The birth of stars and entire galaxies occurs permanently, as well as their death. The disappearance of one star compensates for the appearance of another, therefore it seems to us that the same stars are constantly in the sky.

Stars owe their birth to the process of compression of the interstellar cloud, which is affected by a strong drop in gas pressure. Depending on the mass of the compressed gas, the number of stars born changes: if it is small, then one luminary is born, if it is large, then the formation of a whole cluster is possible.

Stages of the emergence of a star

Here it is necessary to single out two main stages - the fast contraction of the protostar and the slow one. In the first case hallmark is gravity: the matter of a protostar makes practically free fall to its center. At this stage, the temperature of the gas remains unchanged, its duration is about 100 thousand years, and during this time the size of the protostar is reduced very significantly.

And if at the first stage the excess heat was constantly leaving, then the protostar becomes denser. Heat removal is no longer so high, the gas continues to compress and quickly heat up. The slow contraction of the protostar lasts even longer - more than ten million years. Upon reaching an ultra-high temperature (more than a million degrees), thermonuclear reactions take their toll, leading to the termination of compression. After which it forms new star from a protostar.

Life cycle of a star

Stars are like a living organism: they are born, reach their peak of development, and then die. Major changes begin when hydrogen runs out in the central part of the star. It begins to burn out already in the shell, gradually increasing its size, and the star can turn into a red giant or even a supergiant.

All stars have a completely different life cycle, it all depends on the mass. Those that have big weight, live longer and eventually explode. Our sun does not belong to massive stars, therefore, celestial bodies of this type are waiting for the other end: they gradually fade away, turn into a dense structure called a white dwarf.

red giant

Stars that have used up their supply of hydrogen can become enormous. Such luminaries are called red giants. Their distinguishing feature, in addition to size, is an extended atmosphere and very low temperature surfaces. Studies have shown that not all stars go through this stage of development. Only those luminaries with a solid mass become red giants.

Most prominent representatives- Arcturus and Antar, whose visible layers have relatively high temperature, and the rarefied shell has a solid extension. Inside the bodies there is a process of helium ignition, which is characterized by the absence of sharp fluctuations in luminosity.

white dwarf

Stars that are small in size and mass turn into a white dwarf. Their density is extremely high (about a million times higher than the density of water), due to which the substance of the star passes into a state called "degenerate gas". No thermonuclear reactions are observed inside a white dwarf, and only the fact of cooling gives it light. The size of a star in this state is extremely small. For example, many white dwarfs are similar in size to the Earth.

Like any body in nature, the stars also cannot remain unchanged. They are born, develop and finally “die”. The evolution of stars takes billions of years, but there are disputes about the time of their formation. Previously, astronomers believed that the process of their "birth" from stardust requires millions of years, but not so long ago, photographs of a region of the sky from the Great Nebula of Orion were obtained. In a few years there has been a small

In the 1947 photographs, a small group of star-like objects was recorded in this place. By 1954, some of them had already become oblong, and after another five years, these objects broke up into separate ones. So for the first time the process of the birth of stars took place literally in front of astronomers.

Let's take a closer look at how the structure and evolution of stars goes, how they begin and end their endless, by human standards, life.

Traditionally, scientists assume that stars are formed as a result of the condensation of clouds of a gas-dust environment. Under the action of gravitational forces, an opaque gas ball is formed from the formed clouds, dense in structure. Its internal pressure cannot balance the gravitational forces compressing it. Gradually, the ball shrinks so much that the temperature of the stellar interior rises, and the pressure of hot gas inside the ball balances the external forces. After that, 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 a very high temperature in their depths, which contributes to continuous thermonuclear processes (the hydrogen that forms them turns into helium). It is these processes that are the cause of the intense radiation of stars. The time for which they consume the available supply of hydrogen is determined by their mass. The duration of the radiation also depends on this.

When the reserves of hydrogen are depleted, the evolution of stars approaches the stage of formation. This happens as follows. After the cessation of energy release, gravitational forces begin to compress the nucleus. In this case, 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 shrinking helium core and the transformation of helium nuclei into carbon nuclei.

Our Sun is predicted to become a red giant in eight billion years. At the same time, its radius will increase by several tens of times, and the luminosity will increase hundreds of times compared to current indicators.

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

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

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

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