Looking into the depths of the universe, astronomers explore the collision of various cosmic forces. The death of a star has lifted the veil of the limits of time and space for us. Modern astronomy has made it possible to see a completely different universe: seething and indomitable. A spectacle accompanied by the death throes of a giant star. Its surface is like a raging sea of fire, covered with bursts of hot gas. Rising waves form a tsunami a thousand meters high. Huge gas plumes rise into the atmosphere, which are larger. In the depths of the star began the process of destruction. This leads to an explosion and the birth of a supernova. In its place, only colored threads and luminous clouds of gases remain.
The amazing thing is that the death of one star gives rise to a whole generation of new stars. Such a change of death and birth determines the entire history of our galaxy - the Milky Way and billions of such galaxies in.
Our view of the cosmos is shaped by rare explosions of stars bright enough to be seen with the naked eye.
In 1054, North American stargazers discovered a supernova while observing the crescent. The same event was observed in China, Korea, the Middle East.
The astronomer Tycho Bragi observed a similar phenomenon in 1572. He wrote about it: "I was so amazed by this spectacle that I was not ashamed to question what my own eyes saw"
The next case, in 1604, was described by Johannes Kepler. Galileo on this made a rationale for a new approach to, taking change as a fundamental component of the cosmos as an idea.
To understand how stars shape the universe, scientists use a whole arsenal of the latest. From giant telescopes high in the mountains to a whole armada of satellites in space. Looking at the stars through telescopes, we see. But this is only a small fraction of what is known as the electromagnetic spectrum.
At one end of the spectrum is short, high-energy X-rays and gamma rays. On the other, long, low energy radio waves, ultrashort waves. A myriad of radio telescopes are used to collect signals emitted by stars in the far reaches of the galaxy. They help scientists to see objects through the thickness of nebulae and gas clusters.
At the other end of the spectrum are ultraviolet x-rays and gamma rays. Shortwave X-rays allow doctors to see through our bodies and see broken bones. Astronomers are looking for it in space, as evidence of the most rapidly occurring processes.
The Crab Nebula is the shell of a supernova that was observed in various places in 1054. Scientists have focused their attention on the deep part of the pulsar. They recorded bursts of radiation that left circular traces in the surrounding gas cloud. Some dying stars have a very strange fate. The universe creates monsters.
Albert Einstein suggested that there are stars with a gravity that does not allow even light to break through. But he dismissed the idea as impossible. What was once beyond comprehension now defines the boundary. Astronomers believe that when a large star bursts in, so much matter enters its core that it can leave the Universe. But gravity has the last word.
By taking advantage of the earth, we can characterize the universe in terms of the criteria we know, including light forms in the electromagnetic spectrum. However, they do not agree with this. How can you identify an object that does not give light?
Astronomers have found the answer in a burst of gamma rays directed towards the center of our galaxy. The radio telescopes concentrated on the sources and detected matter flows in two directions. And that's what they saw.
A black hole that emits streams of gas from the outer layers of a star. They form a rotating disk. It forms magnetic fields, which, rotating, form two high-energy beams, or streams, of matter passing through them.
Astronomers know that black holes are able to concentrate huge amounts of energy in these streams in the blink of an eye. One of them, known as "GROJ 1655-40", rushes through the universe at a speed of 400 thousand kilometers per hour. Four times faster than other stars. It's like a cannon fired from one of the Supernovas.
Black holes, due to the ability to mobilize a huge amount of energy, are of interest to us not only out of curiosity. There is a category of holes that have existed since time immemorial. Ever since the first stars were just born. When those original giants died, they gave birth to black holes.
Gravity fed black holes with cosmic matter and gas. The substance turned for the first time into galaxies, which grew into large ones. Some of them have reached masses billions of times greater than the mass of the Sun.
Emitting energy flows, they warmed up the environment of galaxies. This stopped the jet of gas in the central galaxy, slowing its growth, and causing the growth of peripheral galaxies. But the effect of black holes did not end there.
The galaxy cluster, called Hydra A, is surrounded by hot troughs that emit X-rays. A stream, visible in the radio wave spectrum, breaks out of the central galaxy. The gas at the edges of this stream contains a large amount of iron ions and other metals produced by the supernova explosion. By pushing these metals to the edges of the universe, black holes saturate distant galaxies with the elements needed to form stars and planetary systems like ours.
Gigantic black holes are observed in almost all galaxies in the universe. There is also an increase in the number of powerful energy flows.
We got the role of observers of the thorny life cycle of stars. Being at a colossal distance from them in time and space, we do not understand much.
The launch in 1977 significantly reduced this distance. After surveying the most distant planets of the solar system and their satellites, these vehicles are sent to the outer limits of our system, tens of billions of kilometers from Earth. Moving at a speed of 16 kilometers per second, Voyager 2 will cover a distance of four light years and reach one of our closest stars, Sirius, in 290,000 years.
Watching from our quiet corner in the galaxy, we realized that the stars not only illuminate the Universe, but also saturate it with the matter necessary for life. By watching a star die in an explosion, we gain an understanding of the force that shapes the universe and changes worlds like our own.
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 in these parts of the spirals that the methods of optical astronomy are observed by the methods of optical astronomy "HII zones", i.e. 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 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 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 of solar energy was the continuous fallout of meteoroids onto its surface, others were looking 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. 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. 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 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 likely that at this very early stage in the evolution of a star, clumps of smaller mass form around it, which then gradually turn into planets (see Fig. ch. nine).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 gaseous 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 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, bright, extremely compact (ie, having small angular dimensions) sources were discovered. This was so unexpected that at first they refused even to believe that such bright radio lines could belong to a hydroxyl molecule. It was hypothesized that these lines belonged to some unknown substance, which was immediately given the "appropriate" name "mysterium". However, "mysterium" very soon shared the fate of its optical "brothers" - "nebulium" and "coronia". The fact is that for many decades the bright lines of the nebulae and the solar corona could not be identified with any known spectral lines. Therefore, they were attributed to certain, unknown on earth, hypothetical elements - "nebulium" and "coronia". Let us not condescendingly smile at the ignorance of astronomers at the beginning of our century: after all, there was no theory of the atom then! The development of physics 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 a then still new, very effective research method, 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 greater 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" source of energy (the so-called "pumping") makes the concentration of atoms or molecules at the initial (upper) level abnormally 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 of the wave theory of 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 a cubic centimeter there is 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. 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 contain a lot of cosmic dust, which makes them unobservable in the optical range. Such "cocoons" are ionized by a young, hot star inside them. In the study of star formation processes, infrared astronomy proved to be very useful. Indeed, for infrared rays, interstellar absorption of light is not so significant. We can now imagine the following picture: from a cloud of the interstellar medium, by its condensation, several clots of different masses are formed, evolving into protostars. The rate of evolution is different: for more massive clumps it will be higher (see Table 2 below). Therefore, the most massive bunch will turn into a hot star first, while the rest will linger more or less long at the protostar stage. We observe them as sources of maser radiation in the immediate vicinity of 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 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 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 the globular cluster M 3. On the vertical axis - relative magnitude
The corresponding diagram clearly shows the entire main sequence, including its upper left part, where hot massive stars are located (color-indicator - 0.2 corresponds to a temperature of 20 thousand K, i.e. class B spectrum). The globular cluster M 3 is an "old" object. It is clearly seen that there are almost no stars in the upper part of the main sequence of the diagram constructed for this cluster. On the other hand, the red giant branch of M 3 is very rich, while NGC 2254 has very few red giants. This is understandable: in the old M 3 cluster, a large number of stars have already “departed” from the main sequence, while in the young cluster NGC 2254 this happened only with a small number of relatively massive, rapidly evolving stars. It is noteworthy that the giant branch for M 3 goes up quite steeply, while for NGC 2254 it is almost horizontal. From the point of view of theory, this can be explained by the significantly lower abundance of heavy elements in M 3. Indeed, in the stars of globular clusters (as well as in other stars that concentrate not so much towards the galactic plane as towards the galactic center), the relative abundance of heavy elements is insignificant . On the diagram "color index - luminosity" for M 3 one more almost horizontal branch is visible. There is no similar branch in the diagram constructed for NGC 2254. The theory explains the emergence of this branch as follows. After the temperature of the shrinking dense helium core of a star - a red giant - reaches 100-150 million K, a new nuclear reaction will begin there. This reaction consists in the formation of a carbon nucleus from three helium nuclei. As soon as this reaction begins, the contraction of the nucleus will stop. Subsequently, the surface layers
the stars increase their temperature and the star in the "spectrum - luminosity" diagram will move to the left. It is from such stars that the third horizontal branch of the diagram for M 3 is formed.
Rice. 17. Hertzsprung-Russell summary diagram for 11 star clusters
On fig. Figure 17 schematically shows a summary color-luminosity diagram for 11 clusters, of which two (M 3 and M 92) are globular. It is clearly seen how the main sequences "bend" to the right and upwards in different clusters in full agreement with the theoretical concepts that have already been discussed. From fig. 17, one can immediately determine which clusters are young and which are old. For example, the "double" cluster X and h Perseus is young. It "saved" a significant portion of the main sequence. The M 41 cluster is older, the Hyades cluster is even older, and the M 67 cluster is very old, the color-luminosity diagram for which is very similar to the similar diagram for globular clusters M 3 and M 92. Only the giant branch of globular clusters is higher in agreement with differences in chemical composition, which were 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 computing technology based on the use of high-speed electronic computers. An invaluable service to the theory was also provided by research in the field of nuclear physics, which made it possible to obtain quantitative characteristics of those nuclear reactions that take place in the stellar interior. It can be said without exaggeration that the development of the theory of the structure and evolution of stars is one of the greatest achievements of astronomy in the second half of the 20th century. The development of modern physics opens up the possibility of a direct observational verification of the theory of the internal structure of stars, and in particular the Sun. We are talking about the possibility of detecting a powerful stream of neutrinos, which the Sun should emit if nuclear reactions take place in its depths. It is well known that neutrinos interact extremely weakly with other elementary particles. Thus, for example, 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
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 not only stops the collapse of the remaining outer regions, but pushes them back. Thus, the mass of the formed star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars 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 type 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. All new stars "take their place" on the main sequence according to their chemical composition and mass. We are not talking about the physical movement of the star - only about its position on the indicated diagram, 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, which is not enough to deplete the supply of hydrogen fuel, current theories are based on computer simulations 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. Violent pulsations occur, which eventually impart enough kinetic energy to the outer layers 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 more and more heavy elements of the periodic system are formed, iron -56 is synthesized from silicon. Up to this point, the synthesis of elements released a large amount of energy, but it is the iron-56 nucleus that has the maximum mass defect and the formation of heavier nuclei is 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 can 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 a star is now a dense ball of atomic nuclei and individual neutrons.
Such stars, known as neutron stars, are extremely small - no larger than a major city - and have unimaginably high densities. 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 poles 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 rotation of the star. Such neutron stars were 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 a black hole under any circumstances. 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?
Notes
see also
Links
- Evolution of stars (Physical encyclopedia)
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See what the "Evolution of Stars" is in other dictionaries:
- (from Latin evolutio deployment), in a broad sense, a synonym for development; processes of change (reim. irreversible) occurring in animate and inanimate nature, as well as in social systems. E. can lead to complication, differentiation, increase ... ... Philosophical Encyclopedia
The study of stellar evolution is impossible by observing only one star - many changes in stars proceed too slowly to be noticed even after many centuries. Therefore, scientists study many stars, each of which is at a certain stage in its life cycle. Over the past few decades, modeling of the structure of stars using computer technology has become widespread in astrophysics.
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Thermonuclear fusion in the interior of stars
young stars
The process of star formation can be described in a single way, but the subsequent stages of the evolution of a star depend almost entirely on its mass, and only at the very end of the star's evolution can its chemical composition play a role.
Young low mass stars
Young stars of low mass (up to three solar masses) [ ] , which are on the way to the main sequence , are completely convective, - the convection process covers the entire body of the star. These are still, in fact, protostars, in the centers of which nuclear reactions are just beginning, and all the radiation occurs mainly due to gravitational compression. Until hydrostatic equilibrium is established, the star's luminosity decreases at a constant effective temperature. In the Hertzsprung-Russell diagram, such stars form an almost vertical track, called the Hayashi track. As the contraction slows, the young star approaches the main sequence. Objects of this type are associated with stars of the type T Taurus.
At this time, in stars with a mass greater than 0.8 solar masses, the core becomes transparent to radiation, and radiative energy transfer in the core becomes predominant, since convection is increasingly hampered by the increasing compaction of stellar matter. In the outer layers of the stellar body, convective energy transfer prevails.
It is not known for certain what characteristics the lower-mass stars have at the moment they hit the main sequence, since the time these stars spend in the young category exceeds the age of the Universe [ ] . All ideas about the evolution of these stars are based only on numerical calculations and mathematical modeling.
As the star contracts, the pressure of the degenerate electron gas begins to increase, and when a certain radius of the star is reached, the compression stops, which leads to a halt in the further temperature increase in the core of the star caused by the compression, and then to its decrease. For stars less than 0.0767 solar masses, this does not happen: the energy released during nuclear reactions will never be enough to balance the internal pressure and gravitational contraction. Such "understars" radiate more energy than is produced in the process of thermonuclear reactions, and belong to the so-called brown dwarfs. Their fate is constant contraction until the pressure of the degenerate gas stops it, and then a gradual cooling down with the cessation of all fusion reactions that have begun.
Young stars of intermediate mass
Young stars of intermediate mass (from 2 to 8 solar masses) [ ] evolve qualitatively in exactly the same way as their smaller sisters and brothers, except that they do not have convective zones up to the main sequence.
Objects of this type are associated with the so-called. Ae\Be Herbig stars are irregular variables of spectral type B-F0. They also have discs and bipolar jets. The rate of outflow of matter from the surface, the luminosity, and the effective temperature are significantly higher than for T Tauri, so they effectively heat and disperse the remnants of the protostellar cloud.
Young stars with a mass greater than 8 solar masses
Stars with such masses already have the characteristics of normal stars, because they have passed all the intermediate stages and were able to achieve such a rate of nuclear reactions that compensated for the loss of energy by radiation, while mass was accumulated to achieve hydrostatic equilibrium of the core. For these stars, the outflow of mass and luminosity are so great that they not only stop the gravitational collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, disperse them away. Thus, the mass of the formed star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence of stars with a mass greater than about 300 solar masses in our galaxy.
mid-life cycle of a star
Stars come in a wide variety of colors and sizes. They range in spectral type from hot blues to cool reds, and in mass from 0.0767 to about 300 solar masses, according to recent estimates. The luminosity and color of a star depend on the temperature of its surface, which, in turn, is determined by its mass. All new stars "take their place" on the main sequence according to their chemical composition and mass. This, of course, is not about the physical movement of the star - only about its position on the indicated diagram, which depends on the parameters of the star. In fact, the movement of a star along the diagram corresponds only to a change in the parameters of the star.
The thermonuclear "burning" of matter resumed at a new level causes a monstrous expansion of the star. The star "swells up", becoming very "loose", and its size increases by about 100 times. So the star becomes a red giant, and the helium burning phase lasts about several million years. Almost all red giants are variable stars.
Final stages of stellar evolution
Old stars with low mass
At present, it is not known for certain what happens to light stars after the depletion of the supply of hydrogen in their depths. Because the age of the Universe is 13.7 billion years, which is not enough to deplete the supply of hydrogen fuel in such stars, current theories are based on computer simulations of the processes occurring in such stars.
Some stars can synthesize helium only in some active zones, which causes their instability and strong stellar winds. In this case, the formation of a planetary nebula does not occur, and the star only evaporates, becoming even smaller than a brown dwarf [ ] .
A star with a mass of less than 0.5 solar mass is not able to convert helium even after reactions involving hydrogen cease in its core - the mass of such a star is too small to provide a new phase of gravitational compression to a degree sufficient for "ignition" helium. These stars include red dwarfs, such as Proxima Centauri, whose main sequence lifespan ranges from tens of billions to tens of trillions of years. After the termination of thermonuclear reactions in their nuclei, they, gradually cooling down, will continue to radiate weakly in the infrared and microwave ranges of the electromagnetic spectrum.
medium sized stars
Upon reaching a medium-sized star (from 0.4 to 3.4 solar masses) [ ] phase of the red giant in its core ends with hydrogen, and the reactions of carbon synthesis from helium begin. This process takes place at higher temperatures and therefore the energy flux from the core increases and, as a result, the outer layers of the star begin to expand. The beginning of carbon synthesis marks a new stage in the life of a star and continues for some time. For a star close to the size of the Sun, this process can take about a billion years.
Changes in the amount of radiated energy 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 stellar winds and intense pulsations. Stars in this phase are called "late-type stars" (also "retired stars"), OH-IR stars or Mira-like stars, depending on their exact characteristics. The ejected gas is relatively rich in heavy elements produced in the interior of the star, such as oxygen and carbon. The gas forms an expanding shell and cools as it moves away from the star, allowing the formation of dust particles and molecules. With strong infrared radiation from the source star, ideal conditions are formed in such shells for the activation of cosmic masers.
Helium fusion reactions are very sensitive to temperature. Sometimes this leads to great instability. The strongest pulsations arise, which as a result give the outer layers sufficient acceleration to be thrown off and turn into a planetary nebula. In the center of such a nebula, the bare core of the star remains, in which thermonuclear reactions cease, and, as it cools, it turns into a helium white dwarf, as a rule, having a mass of up to 0.5-0.6 solar masses and a diameter of the order of the diameter of the Earth.
The vast majority of stars, including the Sun, complete their evolution by contracting until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a factor of a hundred and the density becomes a million times higher than the density of water, the star is called a white dwarf. It is deprived of energy sources and, gradually cooling down, becomes an invisible black dwarf.
In stars more massive than the Sun, the pressure of degenerate electrons cannot stop further compression of the nucleus, and the electrons begin to "press" into atomic nuclei, which turns protons into neutrons, between which there is no electrostatic repulsion force. Such neutronization of matter leads to the fact that the size of the star, which now, in fact, is one huge atomic nucleus, is measured in several kilometers, and the density is 100 million times higher than the density of water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.
supermassive stars
After a star with a mass greater than five solar masses enters the stage of a red supergiant, its core begins to contract under the influence of gravitational forces. As the compression increases, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, increasingly heavier elements are synthesized: helium, carbon, oxygen, silicon and iron, which temporarily restrains the collapse of the nucleus.
As a result, as more and more heavy elements of the Periodic Table are formed, iron-56 is synthesized from silicon. At this stage, further exothermic thermonuclear fusion becomes impossible, since the iron-56 nucleus has a maximum mass defect and the formation of heavier nuclei with energy release is impossible. Therefore, when the iron core of a star reaches a certain size, the pressure in it is no longer able to withstand the weight of the overlying layers of the star, and an immediate collapse of the core occurs with the neutronization of its substance.
What happens next is not yet completely clear, but, in any case, the ongoing processes in a matter of seconds lead to a supernova explosion of incredible power.
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 emitted from the stellar core, 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, but this is not the only possible way of their formation, which, for example, is demonstrated by technetium stars.
blast wave and jets of neutrinos carry matter away from a dying star [ ] into interstellar space. Subsequently, as it cools and travels through space, this supernova material may collide with other space junk 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. Also in question is the moment what actually remains of the original star. However, two options are being considered: neutron stars and black holes.
neutron stars
It is known that in some supernovae, strong gravity in the interior of the supergiant causes electrons to be absorbed by the atomic nucleus, where they, merging with protons, form neutrons. This process is called neutronization. The electromagnetic forces separating nearby nuclei disappear. The core of a star is now a dense ball of atomic nuclei and individual neutrons.
Such stars, known as neutron stars, are extremely small - no larger than a major city - and have unimaginably high densities. Their orbital period becomes extremely short as the size of the star decreases (due to the conservation of angular momentum). Some neutron stars make 600 revolutions per second. For some of them, the angle between the radiation vector and the axis of rotation may be such that the Earth falls into the cone formed by this radiation; in this case, it is possible to record a radiation pulse that repeats at intervals equal to the period of revolution of the star. Such neutron stars were called "pulsars", and became the first discovered neutron stars.
Black holes
Not all stars, having passed the phase of a supernova explosion, become neutron stars. If the star has a sufficiently large mass, then the collapse of such a star will continue, and the neutrons themselves will begin to fall inward until its radius becomes less than the Schwarzschild radius. The star then becomes a black hole.
The existence of black holes was predicted by the general theory of relativity. According to this theory,
Life cycle of stars
An ordinary star releases energy by converting hydrogen into helium in a nuclear furnace located in its core. After the star uses up the hydrogen in the center, it begins to burn out in the shell of the star, which increases in size and swells. The size of the star increases, its temperature drops. This process gives rise to red giants and supergiants. The lifespan of each star is determined by its mass. Massive stars end their life cycles with an explosion. Stars like the Sun shrink to become dense white dwarfs. In the process of transforming from a red giant to a white dwarf, a star can shed its outer layers like a light gaseous shell, exposing the core.
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