Brief life cycle of a star. Thermonuclear fusion in the interior of stars
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 high that the stars in the center are launched nuclear reactions, 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 the life cycle of a star in the universe will be depends on its initial 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 data spectral analysis stars. Lockyer was convinced that the chemical elements that 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 gigantic dimensions, 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 diameter 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 has a huge amount of 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 the 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. big stars end their lives with a huge, spectacular explosion. More modest ones, on the contrary, die, gradually shrinking to the size of white dwarfs. Then they just fade away.
How long does an average star live? Life cycle stars 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 under equal conditions. Especially a lot of knowledge can be obtained by studying the dependence of their properties on mass. After all, the age of 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.
Part One ASTRONOMIC ASPECT OF THE PROBLEM
4. Evolution of stars Modern astronomy has a large number of arguments in favor of the assertion that stars are formed by the condensation of clouds of gas and dust interstellar medium. The process of formation of stars from this medium continues at the present time. The clarification of this circumstance is one of the greatest achievements of modern astronomy. Until relatively recently, it was believed that all stars were formed almost simultaneously many billions of years ago. The collapse of these metaphysical ideas was facilitated, first of all, by the progress of observational astronomy and the development of the theory of the structure and evolution of stars. As a result, it became clear that many of the observed stars are relatively young objects, and some of them arose when there was already a person on Earth. An important argument in favor of the conclusion that stars are formed from the interstellar gas and dust medium is the location of groups of obviously young stars (the so-called "associations") in the spiral arms of the Galaxy. The fact is that, according to radio astronomical observations, interstellar gas is concentrated mainly in the spiral arms of galaxies. In particular, this is also the case in our Galaxy. Moreover, from detailed "radio images" of some galaxies close to us, it follows that the highest density of interstellar gas is observed at the inner (with respect to the center of the corresponding galaxy) edges of the spiral, which finds a natural explanation, the details of which we cannot dwell on here. But it is precisely in these parts of the spirals that the methods of optical astronomy are used to observe "HII zones", ie, clouds of ionized interstellar gas. In ch. 3 has already been mentioned that the only reason for the ionization of such clouds can be ultraviolet radiation massive hot stars - obviously young objects (see below). Central to the problem of the evolution of stars is the question of the sources of their energy. Indeed, where, for example, does the huge amount of energy necessary to maintain the solar radiation at approximately the observed level for several billion years come from? Every second the Sun emits 4x10 33 ergs, and for 3 billion years it radiated 4x10 50 ergs. There is no doubt that the age of the Sun is about 5 billion years. This follows from at least contemporary assessments age of the Earth by various radioactive methods. It is unlikely that the Sun is "younger" than the Earth. In the last century and at the beginning of this century, various hypotheses were proposed about the nature of the energy sources of the Sun and stars. Some scientists, for example, believed that the source solar energy is the continuous fallout of meteoroids onto its surface, others looked for a source in the continuous compression of the Sun. The potential energy released during such a process could, under certain conditions, be converted into radiation. As we will see below, this source can be quite efficient at an early stage in the evolution of a star, but it cannot provide radiation from the Sun for the required time. successes nuclear physics made it possible to solve the problem of sources of stellar energy as early as the end of the thirties of our century. Such a source is thermonuclear fusion reactions occurring in the interiors of stars at a very high temperature prevailing there (of the order of ten million Kelvin). As a result of these reactions, the rate of which strongly depends on temperature, protons are converted into helium nuclei, and the released energy slowly "leaks" through the interiors of stars and, finally, significantly transformed, is radiated into the world space. It's exclusive powerful source. If we assume that initially the Sun consisted only of hydrogen, which, as a result of thermonuclear reactions, completely turned into helium, then the released amount of energy will be approximately 10 52 erg. Thus, to maintain radiation at the observed level for billions of years, it is enough for the Sun to "use up" no more than 10% of its initial supply of hydrogen. Now we can present a picture of the evolution of some star as follows. For some reason (several of them can be specified), a cloud of the interstellar gas and dust medium began to condense. Pretty soon (of course, on an astronomical scale!) Under the influence of universal gravitational forces, a relatively dense, opaque gas ball is formed from this cloud. Strictly speaking, this ball cannot yet be called a star, since in its central regions the temperature is insufficient for thermonuclear reactions to begin. The pressure of the gas inside the ball is not yet able to balance the forces of attraction of its individual parts, so it will be continuously compressed. Some astronomers used to believe that such "protostars" are observed in individual Nebulae in the form of very dark compact formations, the so-called globules (Fig. 12). Advances in radio astronomy, however, forced us to abandon this rather naive point of view (see below). Usually not one protostar is formed at the same time, but a more or less numerous group of them. In the future, these groups become stellar associations and clusters, well known to astronomers. It is very probable that at this very early stage of the evolution of a star, clumps with a smaller mass form around it, which then gradually turn into planets (see Chap. 9).Rice. 12. Globules in a diffusion nebula
When a protostar contracts, its temperature rises and a significant part of the released potential energy is 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, they discovered bright, extremely compact (i.e., having small angular dimensions) sources. 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 dimensions are extremely small, many thousands of times smaller than the dimensions of the nebula. 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 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 an area of approximately 1.5 seconds, the arcs move at 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, most likely the "pumping" is a sufficiently powerful infrared radiation. Other possible mechanism"pumping" may be some chemical reaction. It is worth interrupting our story about cosmic masers in order to consider with what amazing phenomena astronomers collide in space. One of the greatest technical inventions of our turbulent age, which plays a significant role in the life we now experience. scientific and technological revolution, is easily realized in natural conditions and, moreover, 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 of "pumping" these masers is not yet entirely clear, one can still get a rough idea of the physical conditions in the clouds emitting the 18 cm line by the maser mechanism. First of all, it turns out that these clouds are quite dense: in a cubic centimeter there are at least 10 8 -10 9 particles, and a significant (and maybe a 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. Given the relatively small size 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 more than an early stage in the development of protostars, immediately following 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 have many space 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 the cloud of the interstellar medium, by its condensation, several clots are formed different weight 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 suddenly turned out that for some time (most likely a relatively short time) newborn protostars, figuratively speaking, "scream" about their birth, using the latest methods quantum radiophysics (i.e. masers) ... 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" one. 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 the classical problem of star formation into a 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 giving the computed duration gravitational contraction and staying 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
| years | |||||
|
Spectral class |
Luminosity |
gravitational contraction |
staying on the main sequence | ||
|
G2 (Sun) |
|||||
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 part 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 discussed 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. Development modern physics opens up the possibility of 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 others. 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
Formed by condensation of the interstellar medium. Through observations, it was possible to determine that stars arose in different time and continue to this day.
The main problem in the evolution of stars is the question of the origin of their energy, due to which they glow and radiate a huge amount of energy. Previously, many theories have been put forward that were designed to identify the sources of stellar energy. It was believed that a continuous source of stellar energy is continuous compression. This source is certainly good, but cannot maintain adequate radiation for a long time. In the middle of the 20th century, the answer to this question was found. The radiation source is thermonuclear fusion reactions. As a result of these reactions, hydrogen turns into helium, and the released energy passes through the bowels of the star, transforms and radiates into the world space (it is worth noting that the higher the temperature, the faster these reactions go; that is why hot massive stars leave the main sequence faster).
Now imagine the emergence of a star...
A cloud of the interstellar gas and dust medium began to condense. From this cloud, a fairly dense ball of gas is formed. The pressure inside the ball is not yet able to balance the forces of attraction, so it will shrink (perhaps at this time, clots with a smaller mass form around the star, which eventually turn into planets). When compressed, the temperature rises. Thus, the star gradually settles on the main sequence. Then the pressure of the gas inside the star balances the attraction and the protostar turns into a star.
The early stage of the evolution of a star is very small and the star is immersed in a nebula at this time, so it is very difficult to detect a protostar.
The transformation of hydrogen into helium occurs only in the central regions of the star. In the outer layers, the hydrogen content remains practically unchanged. Since the amount of hydrogen is limited, sooner or later it burns out. The release of energy in the center of the star stops and the core of the star begins to shrink, and the shell to swell. Further, if the star is less than 1.2 solar masses, it sheds the outer layer (the formation of a planetary nebula).
After the shell separates from the star, its inner very hot layers open up, and in the meantime the shell moves further and further away. After several tens of thousands of years, the shell will disintegrate and only a very hot and dense star will remain, gradually cooling down, it will turn into a white dwarf. Gradually cooling down, they turn into invisible black dwarfs. Black dwarfs are very dense and cold stars, about more earth, but having a mass comparable to the mass of the sun. The cooling process of white dwarfs lasts several hundred million years.
If the mass of a star is from 1.2 to 2.5 solar, then such a star will explode. This explosion is called supernova. A bursting star in a few seconds increases its luminosity hundreds of millions of times. Such outbreaks are extremely rare. In our Galaxy, a supernova explosion occurs approximately once every hundred years. After such a flash, a nebula remains, which has a large radio emission, and also scatters very quickly, and the so-called neutron star (more on this later). In addition to the huge radio emission, such a nebula will also be a source of X-ray radiation, but this radiation is absorbed by the earth's atmosphere, so it can only be observed from space.
There are several hypotheses about the cause of stellar explosions (supernovae), but there is no generally accepted theory yet. There is an assumption that this is due to the too rapid decline of the inner layers of the star to the center. The star is rapidly shrinking to catastrophic small size about 10 km, and its density in this state is 10 17 kg / m 3, which is close to the density atomic nucleus. This star consists of neutrons (while the electrons seem to be pressed into protons), which is why it is called "NEUTRON". Its initial temperature is about a billion kelvins, but in the future it will quickly cool down.
This star, due to its small size and rapid cooling, has long been considered impossible to observe. But after some time, pulsars were discovered. These pulsars turned out to be neutron stars. They are named so because of the short-term radiation of radio pulses. Those. the star seems to be blinking. This discovery was made quite by accident and not so long ago, namely in 1967. These periodic pulses are due to the fact that during very fast rotation past our gaze, the cone of the magnetic axis constantly flickers, which forms an angle with the axis of rotation.
A pulsar can only be detected for us under conditions of magnetic axis orientation, and this is approximately 5% of their total number. Some pulsars are not found in radio nebulae, since the nebulae dissipate relatively quickly. After a hundred thousand years, these nebulae cease to be visible, and the age of pulsars is estimated at tens of millions of years.
If the mass of a star exceeds 2.5 solar masses, then at the end of its existence it will, as it were, collapse into itself and be crushed by its own weight. In a matter of seconds, it will turn into a dot. This phenomenon was called "gravitational collapse", and this object was also called a "black hole".
From all of the above, it is clear that final stage The evolution of a star depends on its mass, but it is also necessary to take into account the inevitable loss of this very mass and rotation.
Stars, as you know, get their energy from thermonuclear fusion reactions, and sooner or later every star has a moment when thermonuclear fuel comes to an end. The higher the mass of a star, the faster it burns everything it can and goes to the final stage of its existence. Further events can go according to different scenarios, which one - first of all depends again on the mass.
At the time when the hydrogen in the center of the star “burns out”, a helium core is released in it, which contracts and releases energy. In the future, combustion reactions of helium and subsequent elements may begin in it (see below). The outer layers increase many times under the influence of increased pressure coming from the heated core, the star becomes a red giant.
Depending on the mass of the star, different reactions can take place in it. This determines what composition the star will have by the time the fusion fades.
white dwarfs
For stars with masses up to about 10 MC, the core weighs less than 1.5 MC. After the completion of thermonuclear reactions, the radiation pressure stops, and the nucleus begins to shrink under the influence of gravity. It is compressed until the pressure of the degenerate electron gas, due to the Pauli principle, begins to interfere. The outer layers are shed and dissipate, forming a planetary nebula. The first such nebula was discovered by French astronomer Charles Messier in 1764 and cataloged as M27.
What came out of the core is called a white dwarf. White dwarfs have a density greater than 10 7 g/cm 3 and a surface temperature of about 10 4 K. The luminosity is 2-4 orders of magnitude lower than that of the Sun. Thermonuclear fusion does not take place in it, all the energy emitted by it was accumulated earlier. Thus, white dwarfs slowly cool down and cease to be visible.
A white dwarf still has a chance to be active if it is part of a binary star and draws the mass of a companion onto itself (for example, the companion has become a red giant and filled its entire Roche lobe with its mass). In this case, either hydrogen synthesis can begin in the CNO cycle using the carbon contained in the white dwarf, ending with the shedding of the outer hydrogen layer (“new” star). Or the mass of a white dwarf can grow so much that its carbon-oxygen component will light up, a wave of explosive combustion coming from the center. As a result, heavy elements are formed with the release of a large amount of energy:
12 С + 16 O → 28 Si + 16.76 MeV
28 Si + 28 Si → 56 Ni + 10.92 MeV
The luminosity of the star increases strongly for 2 weeks, then rapidly decreases for another 2 weeks, after which it continues to fall by about 2 times in 50 days. The main energy (about 90%) is emitted in the form of gamma quanta from the nickel isotope decay chain. This phenomenon is called a type 1 supernova.
There are no white dwarfs with a mass of 1.5 or more solar masses. This is explained by the fact that for the existence of a white dwarf, it is necessary to balance the gravitational compression with the pressure of the electron gas, but this happens at masses no more than 1.4 M C , this limitation is called the Chandrasekhar limit. The value can be obtained as a condition of equality of pressure forces to gravitational contraction forces under the assumption that the momenta of electrons are determined by the uncertainty relation for the volume they occupy, and they move at a speed close to the speed of light.
neutron stars
In the case of more massive (> 10 M C) stars, things happen a little differently. The high temperature in the core activates energy-absorbing reactions, such as knocking out protons, neutrons and alpha particles from the nuclei, as well as e-capture of high-energy electrons that compensate for the mass difference two cores. The second reaction creates an excess of neutrons in the nucleus. Both reactions lead to its cooling and general contraction of the star. When the energy of nuclear fusion ends, the contraction turns into an almost free fall of the shell onto the contracting core. This sharply accelerates the rate of fusion in the outer falling layers, which leads to the emission of a huge amount of energy in a few minutes (comparable to the energy that light stars emit in their entire existence).
Due to the high mass, the collapsing nucleus overcomes the pressure of the electron gas and contracts further. In this case, reactions p + e - → n + ν e occur, after which there are almost no electrons that interfere with compression in the nucleus. Compression occurs to sizes of 10 − 30 km, corresponding to the density determined by the pressure of the neutron degenerate gas. The matter falling on the nucleus receives the shock wave reflected from the neutron nucleus and part of the energy released during its compression, which leads to a rapid ejection of the outer shell to the sides. The resulting object is called a neutron star. Most (90%) of the energy released from gravitational contraction is carried away by neutrinos in the first seconds after the collapse. The above process is called a Type II supernova explosion. The energy of the explosion is such that some of them are (rarely) visible to the naked eye even in daytime. The first supernova was recorded by Chinese astronomers in 185 AD. Currently, several hundred outbreaks are recorded per year.
The resulting neutron star has a density ρ ~ 10 14 − 10 15 g/cm 3 . The conservation of angular momentum during the contraction of the star leads to very short revolution periods, usually in the range from 1 to 1000 ms. For ordinary stars, such periods are impossible, because Their gravity will not be able to counteract the centrifugal forces of such rotation. A neutron star has a very large magnetic field, reaching 10 12 -10 13 gauss at the surface, which results in strong electromagnetic radiation. A magnetic axis that does not coincide with the axis of rotation leads to the fact that a neutron star sends periodic (with a rotation period) pulses of radiation in a given direction. Such a star is called a pulsar. This fact helped their experimental discovery and is being used for discovery. It is much more difficult to detect a neutron star by optical methods due to its low luminosity. The period of revolution gradually decreases due to the transition of energy into radiation.
outer layer A neutron star is made up of crystalline matter, mostly iron and its neighboring elements. Most of the rest of the mass is neutrons, pions and hyperons can be in the very center. The density of the star increases towards the center and can reach values much greater than the density of nuclear matter. The behavior of matter at such densities is poorly understood. There are theories about free quarks, including not only the first generation, at such extreme densities of hadronic matter. Superconducting and superfluid states of neutron matter are possible.
There are 2 mechanisms for cooling a neutron star. One of them is the emission of photons, as everywhere else. The second mechanism is neutrino. It prevails as long as the core temperature is above 10 8 K. It usually corresponds to a surface temperature above 10 6 K and lasts 10 5 −10 6 years. There are several ways to emit neutrinos:
Black holes
If the mass of the original star exceeded 30 solar masses, then the core formed in the supernova explosion will be heavier than 3 M C . With such a mass, the pressure of the neutron gas can no longer restrain gravity, and the core does not stop at the stage of a neutron star, but continues to collapse (nevertheless, experimentally discovered neutron stars have masses no more than 2 solar masses, not three). This time, nothing will prevent the collapse, and a black hole is formed. This object has a purely relativistic nature and cannot be explained without GR. Despite the fact that the matter, according to the theory, collapsed into a point - a singularity, a black hole has a non-zero radius, called the Schwarzschild radius:
R W \u003d 2GM / c 2.
The radius denotes the boundary of the gravitational field of a black hole, which is insurmountable even for photons, called the event horizon. For example, the Schwarzschild radius of the Sun is only 3 km. Outside the event horizon, a black hole's gravitational field is the same as that of an ordinary object of its mass. A black hole can only be observed by indirect effects, since it itself does not radiate any noticeable energy.
Despite the fact that nothing can leave the event horizon, a black hole can still create radiation. In the quantum physical vacuum, virtual particle-antiparticle pairs are constantly born and disappear. The strongest gravitational field of a black hole can interact with them before they disappear and absorb the antiparticle. In the event that the total energy of the virtual antiparticle was negative, the black hole loses mass, and the remaining particle becomes real and receives energy sufficient to fly away from the black hole field. This radiation is called Hawking radiation and has a blackbody spectrum. It can be assigned a certain temperature:
The influence of this process on the mass of most black holes is negligible compared to the energy they receive even from the CMB. The exception is relic microscopic black holes, which could have formed in the early stages of the evolution of the Universe. Small sizes speed up the evaporation process and slow down the mass gain process. The last stages of evaporation of such black holes must end in an explosion. No explosions matching the description have ever been recorded.
Matter falling into a black hole heats up and becomes a source of x-rays, which serves as an indirect sign of the presence of a black hole. When matter falls into a black hole big moment momentum, it forms a rotating accretion disk around it, in which particles lose energy and angular momentum before falling into the black hole. In the case of a supermassive black hole, there are two preferred directions along the axis of the disk, in which the pressure of the emitted radiation and electromagnetic effects accelerate the particles that have escaped from the disk. This creates powerful jets of matter in both directions, which can also be registered. According to one theory, this is how the active nuclei of galaxies and quasars are arranged.
A spinning black hole is a more complex object. With its rotation, it “captures” a certain region of space beyond the event horizon (“Lense-Thirring effect”). This area is called the ergosphere, its boundary is called the static limit. The static limit is an ellipsoid coinciding with the event horizon at the two poles of the black hole's rotation.
Rotating black holes have an additional mechanism of energy loss through its transfer to particles that have fallen into the ergosphere. This loss of energy is accompanied by a loss of angular momentum and slows down the rotation.
Bibliography
- S.B. Popov, M.E. Prokhorov "Astrophysics of single neutron stars: radio-quiet neutron stars and magnetars" SAI MSU, 2002
- William J. Kaufman "The Cosmic Frontiers of Relativity" 1977
- Other Internet sources
December 20 10 y.
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 composition of the Great Orion Nebula 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 the stars suggests 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 tend to quickly deplete their supply of nuclear fuel. This is accompanied by a significant loss of mass, in particular, due to the shedding of the outer shells. As a result, only a gradually cooling central part remains, 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.