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 spiral branches Galaxies. 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 it has already been said that the only reason for the ionization of such clouds can be the ultraviolet radiation of 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 great amount energy required to maintain the radiation of the Sun at approximately the observed level for several billion years? 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 at least from modern estimates of the 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 solar radiation for the required time. Advances in 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. These sources are thermal nuclear reactions fusion occurring in the depths 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 world space. This is an exceptionally 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 forces gravity a relatively dense, opaque ball of gas 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 is compressed, 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 future star balances the attraction and the gas ball ceases to shrink. 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 sun, 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 are talking about very interesting T Tauri stars, 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 left no place for exotic "celestials" in Mendeleev's periodic system: in 1927, "nebulium" was debunked, whose lines 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 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 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 more 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 realized in natural conditions and, moreover, on an enormous 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, the founder wave theory Light Fresnel 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 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 cubic centimeter there are at least 10 8 -10 9 particles, 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 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 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 a "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 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 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 an 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 tens of 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 the 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
| 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 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: the old cluster M 3 big number 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 theoretical ideas which 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, the success of the theory of the internal structure and evolution of stars would not have been possible without modern computer science 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 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. It's about 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, a neutrino can fly almost without absorption through the entire thickness of the Sun, while X-rays can pass without absorption only through a few millimeters of the substance of the solar interior. If we imagine that a powerful beam of neutrinos passes through the Sun with the energy of each particle in
The world around us is made up of various chemical elements. How were these elements formed in natural conditions? At present, it is generally accepted that the elements that make up solar system, formed during stellar evolution. Where does the formation of a star begin? Stars condense under the influence of gravitational forces from giant gaseous molecular clouds (the term “molecular” means that the gas consists mainly of matter in molecular form). The mass of matter concentrated in molecular clouds makes up a significant part of the total mass of galaxies. These gaseous clouds of primordial matter consist predominantly of hydrogen nuclei. A small admixture is made up of helium nuclei formed as a result of primary nucleosynthesis in the prestellar epoch.
When the mass of the stellar matter as a result of accretion reaches 0.1 solar masses, the temperature in the center of the star reaches 1 million K, and the life of the protostar begins new stage- thermonuclear fusion reactions. However, these thermonuclear reactions differ significantly from the reactions occurring in stars in a stationary state, such as the Sun. The fact is that the fusion reactions occurring on the Sun:
1 H + 1 H → 2 H + e + + e
require a higher temperature of ~10 million K. The temperature in the center of the protostar is only 1 million K. At this temperature, the deuterium fusion reaction (d 2 H) proceeds efficiently:
2H + 2H → 3He + n + Q,
where Q = 3.26 MeV is the released energy.
Deuterium, as well as 4 He, is formed at the prestellar stage of the evolution of the Universe and its content in the matter of a protostar is 10 -5 of the content of protons. However, even this small amount is sufficient for the appearance of an effective source of energy in the center of the protostar.
The opacity of protostellar matter leads to the fact that convective gas flows begin to appear in the star. Heated gas bubbles rush from the center of the star to the periphery. And the cold matter from the surface descends to the center of the proto-vehicle and supplies an additional amount of deuterium. At the next stage of combustion, deuterium begins to move to the periphery of the protostar, heating its outer shell, which leads to swelling of the protostar. A protostar with a mass equal to that of the Sun has a radius five times that of the Sun.
Compression of stellar matter due to gravitational forces leads to an increase in temperature in the center of the star, which creates conditions for the onset of a nuclear reaction of hydrogen combustion (Fig. 1).
When the temperature in the center of the star rises to 10-15 million K, the kinetic energies of the colliding hydrogen nuclei are sufficient to overcome the Coulomb repulsion and nuclear reactions of hydrogen combustion begin. Nuclear reactions begin in the limited central part of the star. The onset of thermonuclear reactions immediately stop the further compression of the star. The heat released during the hydrogen fusion reaction creates pressure that counteracts gravitational contraction and prevents the star from collapsing. There is a qualitative change in the mechanism of energy release in the star. If before the onset of the nuclear reaction of hydrogen combustion, the heating of the star occurred due to gravitational compression, now another mechanism is opening - energy is released due to nuclear fusion reactions. The star acquires a stable size and luminosity, which, for a star with a mass close to the sun, do not change for billions of years, while the combustion of hydrogen occurs. This is the longest stage in stellar evolution. Thus, the initial stage of thermonuclear fusion reactions consists in the formation of helium nuclei from four hydrogen nuclei. As hydrogen burns in the central part of the star, its reserves there are depleted and helium accumulates. A helium core forms at the center of the star. When the hydrogen in the center of the star has burned out, no energy is released due to the thermonuclear reaction of hydrogen combustion, and gravitational forces come into play again. The helium core begins to shrink. By contracting, the core of the star begins to heat up even more, the temperature in the center of the star continues to rise. The kinetic energy of colliding helium nuclei increases and reaches a value sufficient to overcome the Coulomb repulsion forces.
The next stage of the thermonuclear reaction begins - the combustion of helium. As a result of nuclear reactions of helium combustion, carbon nuclei are formed. Then the combustion reactions of carbon, neon, oxygen begin. As elements with large Z burn, the temperature and pressure in the center of the star increase at an ever-increasing rate, which in turn increases the rate of nuclear reactions (Fig. 2).
If for a massive star (star mass ~ 25 solar masses) the hydrogen burning reaction lasts several million years, then helium burning occurs ten times faster. The combustion process of oxygen lasts about 6 months, and the combustion of silicon occurs in a day. What elements can be formed in stars in a sequential chain of thermonuclear fusion reactions? The answer is obvious. Nuclear fusion reactions of heavier elements can continue as long as the release of energy is possible. At the final stage of thermonuclear reactions during the combustion of silicon, nuclei are formed in the region of iron. This is the final stage of stellar thermonuclear fusion, since the nuclei in the region of iron have the maximum specific binding energy. Nuclear reactions occurring in stars under conditions of thermodynamic equilibrium depend significantly on the mass of the star. This happens because the mass of the star determines the magnitude of the gravitational contraction forces, which ultimately determines the maximum temperature achievable at the center of the star. In table. Table 1 shows the results of a theoretical calculation of possible nuclear fusion reactions for stars of various masses.
Table 1
Theoretical calculation of possible nuclear reactions in stars of different masses
If the initial mass of a star exceeds 10M, the final stage of its evolution is the so-called “supernova explosion”. When a massive star runs out of nuclear power sources, gravitational forces continue to compress the central part of the star. The pressure of the degenerate electron gas is not enough to counteract the compression forces. Compression leads to an increase in temperature. In this case, the temperature rises so much that the splitting of the iron nuclei, which makes up the central part (core) of the star, into neutrons, protons and α-particles begins. At such high temperatures (T ~ 5·10 9 K), the proton + electron pair is effectively converted into a neutron + neutrino pair. Since the cross section for the interaction of low-energy neutrinos (E ν< 10МэВ) с веществом
мало (σ
~ 10 -43 см 2),
то нейтрино быстро покидают центральную часть
звезды, эффективно унося энергию и охлаждая ядро
звезды. Распад ядер железа на более слабо
связанные фрагменты также интенсивно охлаждает
центральную область звезды. Следствием резкого
уменьшения температуры в центральной части
звезды является окончательная потеря
устойчивости в звезде. За несколько секунд ядро
звезды коллапсирует в сильно сжатое состояние
нейтронную звезду или черную дыру. Происходит
взрыв сверхновой с выделением огромной энергии.
В результате образования ударной волны внешняя
оболочка нагревается до температуры ~ 10 9 K и
выбрасывается в окружающее пространство под
действием давления излучения и потока нейтрино.
Невидимая до этого глазом звезда мгновенно
вспыхивает. Энергия, излучаемая сверхновой в
видимом диапазоне, сравнима с излучением целой
галактики.
At the moment of a supernova explosion, the temperature rises sharply and nuclear reactions, the so-called explosive nucleosynthesis, take place in the outer layers of the star. In particular, the resulting intense neutron fluxes lead to the appearance of elements in the range of mass numbers A > 60. A supernova explosion is a rather rare event. In our Galaxy, numbering ~ 10 11 stars, only 3 supernova explosions have been observed over the past 1000 years. However, the frequency of supernova explosions and the amount of matter ejected into interstellar space are quite sufficient to explain the intensity of cosmic rays. After a supernova explosion, the condensed core of a star can form a neutron star or a black hole, depending on the mass of matter left in the central part of the exploding supernova.
Thus, hydrogen is remelted into heavier elements inside the star. Then the formed elements are scattered into the surrounding space as a result of the explosion of supernovae or in less catastrophic processes occurring in red giants. The matter ejected into interstellar space is used again in the process of formation and evolution of stars of the second and subsequent generations. As Population I and Population II stars evolve, heavier and heavier elements are produced.
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 contracts 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 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.
Star- a celestial body in which thermonuclear reactions are going, going or will go. Stars are massive luminous gaseous (plasma) balls. Formed from a gas-dust environment (hydrogen and helium) as a result of gravitational compression. The temperature of matter in the depths of stars is measured in millions of kelvins, and on their surface - in thousands of kelvins. The energy of the vast majority of stars is released as a result of thermonuclear reactions of the conversion of hydrogen into helium, occurring at high temperatures in the inner regions. Stars are often called the main bodies of the universe, since they contain the bulk of the luminous matter in nature. Stars are huge objects, spherical in shape, consisting of helium and hydrogen, as well as other gases. The energy of a star is contained in its core, where every second helium interacts with hydrogen. Like everything organic in our universe, stars arise, develop, change and disappear - this process takes billions of years and is called the process of "Star Evolution".
1. The evolution of stars
Star evolution- the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years, while it radiates light and heat. A star begins its life as a cold rarefied cloud of interstellar gas (a rarefied gaseous medium that fills all the space between stars), shrinking under the influence of its own gravity and gradually taking the shape of a ball. When compressed, the energy of gravity (the universal fundamental interaction between all material bodies) turns into heat, and the temperature of the object increases. When the temperature in the center reaches 15-20 million K, thermonuclear reactions begin and the compression stops. The object becomes a full-fledged star. The first stage of a star's life is similar to that of the sun - it is dominated by the reactions of the hydrogen cycle. It remains in this state for most of its life, being on the main sequence of the Hertzsprung-Russell diagram (Fig. 1) (shows the relationship between the absolute magnitude, luminosity, spectral type and surface temperature of a star, 1910), until the fuel supply runs out at its core. When all the hydrogen in the center of the star turns into helium, a helium core is formed, and the thermonuclear combustion of hydrogen continues on its periphery. During this period, the structure of the star begins to change. Its luminosity increases, the outer layers expand, and the surface temperature decreases - the star becomes a red giant, which form a branch on the Hertzsprung-Russell diagram. The star spends much less time on this branch than on the main sequence. When the accumulated mass of the helium core becomes significant, it cannot support its own weight and begins to shrink; if the star is massive enough, the rising temperature can cause further thermonuclear conversion of helium into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally silicon into iron).
2. Thermonuclear fusion in the interior of stars
By 1939, it was established that the source of stellar energy is thermonuclear fusion occurring in the interiors of stars. Most stars radiate because, in their interiors, four protons combine through a series of intermediate steps into a single alpha particle. This transformation can go in two main ways, called the proton-proton, or p-p-cycle, and the carbon-nitrogen, or CN-cycle. In low-mass stars, the energy release is mainly provided by the first cycle, in heavy stars - by the second. The supply of nuclear fuel in a star is limited and is constantly spent on radiation. The process of thermonuclear fusion, which releases energy and changes the composition of the star's matter, combined with gravity, which tends to compress the star and also releases energy, as well as radiation from the surface, which carries away the released energy, are the main driving forces of stellar evolution. The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in the galaxy actually contains 0.1 to 1 molecule per cm?. A molecular cloud has a density of about a million molecules per cm?. The mass of such a cloud exceeds the mass of the Sun by 100,000-10,000,000 times due to its size: from 50 to 300 light-years across. While the cloud is free to rotate around the center of the home galaxy, nothing happens. However, due to the inhomogeneity of the gravitational field, disturbances may arise in it, leading to local mass concentrations. Such perturbations cause the gravitational collapse of the cloud. One of the scenarios leading to this is the collision of two clouds. Another event causing the collapse could be the passage of a cloud through the dense arm of a spiral galaxy. Also a critical factor could be the explosion of a nearby supernova, the shock wave of which will collide with the molecular cloud at great speed. In addition, a collision of galaxies is possible, capable of causing a burst of star formation, as the gas clouds in each of the galaxies are compressed by the collision. In general, any inhomogeneities in the forces acting on the mass of the cloud can initiate the process of star formation. Due to the inhomogeneities that have arisen, the pressure of the molecular gas can no longer prevent further compression, and the gas begins to gather around the center of the future star under the influence of gravitational attraction. Half of the released gravitational energy goes into heating the cloud, and half into light radiation. In clouds, pressure and density increase towards the center, and the collapse of the central part occurs faster than the periphery. As the contraction progresses, the mean free path of photons decreases, and the cloud becomes less and less transparent to its own radiation. This results in a faster rise in temperature and an even faster rise in pressure. As a result, the pressure gradient balances the gravitational force, a hydrostatic core is formed, with a mass of about 1% of the mass of the cloud. This moment is invisible. The further evolution of the protostar is the accretion of the substance that continues to fall on the "surface" of the core, which, due to this, grows in size. The mass of matter freely moving in the cloud is exhausted, and the star becomes visible in the optical range. This moment is considered the end of the protostellar phase and the beginning of the young star phase. The process of star formation can be described in a single way, but the subsequent stages of the development of a star depend almost entirely on its mass, and only at the very end of stellar evolution can chemical composition play a role.
Thermonuclear fusion in the interior of stars
At this time, for stars with a mass greater than 0.8 solar masses, the core becomes transparent to radiation, and radiative energy transfer in the core will prevail, while the shell at the top remains convective. No one knows for sure what kind of stars of smaller mass arrive on the main sequence, since the time these stars spend in the category of young ones exceeds the age of the Universe. All our ideas about the evolution of these stars are based on numerical calculations.
As the star shrinks, the pressure of the degenerate electron gas begins to increase, and at some radius of the star, this pressure stops the growth of the central temperature, and then begins to lower it. And for stars less than 0.08, this turns out to be fatal: the energy released during nuclear reactions will never be enough to cover the cost of radiation. Such sub-stars are called brown dwarfs, and their fate is constant contraction until the pressure of the degenerate gas stops it, and then gradual cooling with a stop to all nuclear reactions.
Young stars of intermediate mass
Young stars of intermediate mass (from 2 to 8 solar masses) qualitatively evolve in exactly the same way as their smaller sisters, with the exception that they do not have convective zones until the main sequence.
Objects of this type are associated with the so-called. Ae\Be Herbit stars are irregular variables of spectral type B-F5. They also have bipolar jet disks. The exhaust velocity, luminosity, and effective temperature are substantially greater than for τ Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.
Young stars with a mass greater than 8 solar masses
In fact, these are already normal stars. While the mass of the hydrostatic core was accumulating, the star managed to skip all the intermediate stages and heat up the nuclear reactions to such an extent that they compensate for the losses due to radiation. For these stars, the outflow of mass and luminosity is so high that it does not just stop the collapse of the remaining outer areas but pushes them back. Thus, the mass of the formed star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars more than 100-200 solar masses.
mid-life cycle of a star
Among the formed stars there is a huge variety of colors and sizes. They range in spectral class from hot blues to cool reds, and in mass from 0.08 to more than 200 solar masses. The luminosity and color of a star depends on the temperature of its surface, which, in turn, is determined by its mass. Everything, new stars "take their place" on the main sequence according to their chemical composition and mass. We are not talking about the physical movement of the star - only about its position on the indicated diagram, which depends on the parameters of the star. That is, we are talking, in fact, only about changing the parameters of the star.
What happens next depends again on the mass of the star.
Later years and the death of stars
Old stars with low mass
To date, it is not known for certain what happens to light stars after the depletion of the hydrogen supply. Since the universe is 13.7 billion years old, not enough to deplete the supply of hydrogen fuel, modern theories are based on computer simulation of the processes occurring in such stars.
Some stars can only fuse helium in certain active regions, which causes instability and strong solar winds. In this case, the formation of a planetary nebula does not occur, and the star only evaporates, becoming even smaller than a brown dwarf.
But a star with a mass of less than 0.5 solar mass will never be able to synthesize helium even after reactions involving hydrogen cease in the core. Their stellar shell is not massive enough to overcome the pressure produced by the core. Such stars include red dwarfs (such as Proxima Centauri), whose main sequence lifetimes are hundreds of billions of years. After the termination of thermonuclear reactions in their core, they, gradually cooling down, will continue to weakly radiate in the infrared and microwave ranges of the electromagnetic spectrum.
medium sized stars
When a star reaches an average size (from 0.4 to 3.4 solar masses) of the red giant phase, its outer layers continue to expand, the core contracts, and reactions of carbon synthesis from helium begin. The fusion releases a lot of energy, giving the star a temporary reprieve. For a star similar in size to the Sun, this process can take about a billion years.
Changes in the amount of energy emitted cause the star to go through periods of instability, including changes in size, surface temperature, and energy release. The release of energy is shifted towards low-frequency radiation. All this is accompanied by an increasing mass loss due to strong solar winds and intense pulsations. The stars in this phase are called late-type stars, OH-IR stars or Mira-like stars, depending on their exact characteristics. The ejected gas is relatively rich in heavy elements produced in the interior of the star, such as oxygen and carbon. The gas forms an expanding shell and cools as it moves away from the star, allowing the formation of dust particles and molecules. With strong infrared radiation from the central star, ideal conditions are formed in such shells for the activation of masers.
Helium combustion reactions are very sensitive to temperature. Sometimes this leads to great instability. The strongest pulsations arise, which ultimately inform outer layers enough kinetic energy to be ejected and become a planetary nebula. In the center of the nebula, the core of the star remains, which, cooling down, turns into a helium white dwarf, as a rule, having a mass of up to 0.5-0.6 solar and a diameter of the order of the diameter of the Earth.
white dwarfs
The vast majority of stars, including the Sun, end their evolution by shrinking until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a factor of a hundred and the density becomes a million times that of water, the star is called a white dwarf. It is deprived of sources of energy and, gradually cooling down, becomes dark and invisible.
In stars more massive than the Sun, the pressure of degenerate electrons cannot hold back the contraction of the core, and it continues until most of the particles turn into neutrons, packed so densely that the size of the star is measured in kilometers, and the density is 100 million times greater than the density water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.
supermassive stars
After the outer layers of the star, with a mass greater than five solar masses, have scattered to form a red supergiant, the core begins to shrink due to gravitational forces. As the compression increases, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, heavy elements are synthesized, which temporarily restrains the collapse of the nucleus.
Ultimately, as education becomes more and more heavy elements periodic system, iron -56 is synthesized from silicon. Up to this point, the synthesis of elements released a large number of energy, however, it is the iron -56 nucleus that has the maximum mass defect and the formation of heavier nuclei is unfavorable. Therefore, when the iron core of a star reaches a certain value, the pressure in it is no longer able to withstand the colossal force of gravity, and an immediate collapse of the core occurs with the neutronization of its matter.
What happens next is not entirely clear. But whatever it is, in a matter of seconds, it leads to the explosion of a supernova of incredible force.
The accompanying burst of neutrinos provokes a shock wave. Strong neutrino jets and a rotating magnetic field push out most of the material accumulated by the star - the so-called seating elements, including iron and lighter elements. The expanding matter is bombarded by neutrons escaping from the nucleus, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and possibly even California). Thus, supernova explosions explain the presence of elements heavier than iron in the interstellar matter.
The blast wave and jets of neutrinos carry material away from the dying star and into interstellar space. Subsequently, moving through space, this supernova material may collide with other space debris, and possibly participate in the formation of new stars, planets or satellites.
The processes that take place during the formation of a supernova are still being studied, and so far this issue is not clear. It is also questionable what actually remains of the original star. However, two options are being considered:
neutron stars
In some supernovae, the strong gravity in the interior of the supergiant is known to cause electrons to fall into the atomic nucleus, where they fuse with protons to form neutrons. The electromagnetic forces separating nearby nuclei disappear. The core of the star is now a dense ball of atomic nuclei and individual neutrons.
Such stars, known as neutron stars, are extremely small - no more than big city, and have an unimaginably high density. Their orbital period becomes extremely short as the size of the star decreases (due to conservation of angular momentum). Some make 600 revolutions per second. When the axis connecting the north and south magnetic pole of this rapidly rotating star, points to the Earth, it is possible to fix a radiation pulse that repeats at intervals equal to the period of revolution of the star. Such neutron stars are called "pulsars", and became the first discovered neutron stars.
Black holes
Not all supernovae become neutron stars. If the star has a large enough mass, then the collapse of the star will continue and the neutrons themselves will begin to fall inward until its radius becomes less than the Schwarzschild radius. The star then becomes a black hole.
The existence of black holes was predicted by the general theory of relativity. According to general relativity, matter and information cannot leave black hole no way. However, quantum mechanics makes exceptions to this rule possible.
A number of open questions remain. Chief among them: "Are there any black holes at all?" Indeed, in order to say for sure that a given object is a black hole, it is necessary to observe its event horizon. All attempts to do so ended in failure. But there is still hope, since some objects cannot be explained without involving accretion, moreover, accretion onto an object without a solid surface, but the very existence of black holes does not prove this.
Questions are also open: is it possible for a star to collapse directly into a black hole, bypassing a supernova? Are there supernovae that will eventually become black holes? What is the exact effect of the initial mass of a star on the formation of objects at the end of its life cycle?
