Evolution of Stars of Different Masses
Astronomers cannot observe the life of one star from beginning to end, because even the shortest-lived stars exist for millions of years - longer than the life of all humanity. Changes in the physical characteristics and chemical composition of stars over time, i.e. Astronomers study stellar evolution by comparing the characteristics of many stars at different stages of evolution.
Physical patterns connecting the observed characteristics of stars are reflected in the color-luminosity diagram - the Hertzsprung - Russell diagram, on which the stars form separate groups - sequences: the main sequence of stars, sequences of supergiants, bright and faint giants, subgiants, subdwarfs and white dwarfs.
For most of its life, any star is on the so-called main sequence color-luminosity diagrams. All other stages of the star's evolution before the formation of a compact remnant take no more than 10% of this time. This is why most of the stars observed in our Galaxy are modest red dwarfs with the mass of the Sun or less. The main sequence contains about 90% of all observed stars.
The lifespan of a star and what it turns into at the end life path, is completely determined by its mass. Stars with masses greater than the Sun live much less than the Sun, and the lifetime of the most massive stars is only millions of years. For the vast majority of stars, the lifetime is about 15 billion years. After a star exhausts its energy sources, it begins to cool and contract. The end product of stellar evolution is compact, massive objects whose density is many times greater than that of ordinary stars.
Stars different weights ultimately come to one of three states: white dwarfs, neutron stars or black holes. If the mass of the star is small, then the gravitational forces are relatively weak and the compression of the star (gravitational collapse) stops. She goes into steady state white dwarf. If the mass exceeds a critical value, compression continues. At very high densities, electrons combine with protons to form neutrons. Soon, almost the entire star consists of only neutrons and has such an enormous density that the huge stellar mass is concentrated in a very small ball with a radius of several kilometers and the compression stops - a neutron star is formed. If the mass of the star is so great that even the formation of a neutron star will not stop the gravitational collapse, then the final stage of the star’s evolution will be a black hole.
Our Sun has been shining for more than 4.5 billion years. At the same time, it constantly consumes hydrogen. It is absolutely clear that no matter how large its reserves are, they will be exhausted someday. And what will happen to the luminary? There is an answer to this question. Life cycle stars can be studied from other similar cosmic formations. After all, there are real patriarchs in space, whose age is 9-10 billion years. And there are very young stars. They are no more than several tens of millions of years old.
Consequently, by observing the state of the various stars with which the Universe is “strewn”, one can understand how they behave over time. Here we can draw an analogy with an alien observer. He flew to Earth and began to study people: children, adults, old people. Thus, in a very short period of time, he understood what changes happen to people throughout life.
The Sun is currently a yellow dwarf - 1
Billions of years will pass, and it will become a red giant - 2
And then it will turn into a white dwarf - 3
Therefore, we can say with all confidence that when the hydrogen reserves in the central part of the Sun are exhausted, the thermonuclear reaction will not stop. The zone where this process will continue will begin to shift towards the surface of our star. But at the same time, gravitational forces will no longer be able to influence the pressure that is generated as a result of the thermonuclear reaction.
Consequently, the star will begin to grow in size and gradually turn into a red giant. This is a space object of a late stage of evolution. But it also happens at an early stage during star formation. Only in the second case does the red giant shrink and turn into main sequence star. That is, one in which the reaction of synthesis of helium from hydrogen takes place. In a word, where the life cycle of a star begins is where it ends.
Our Sun will increase in size so much that it will engulf nearby planets. These are Mercury, Venus and Earth. But don't be afraid. The star will begin to die in a few billion years. During this time, dozens, and maybe hundreds of civilizations will change. A person will pick up a club more than once, and after thousands of years he will sit down at a computer again. This is the usual cyclicity on which the entire Universe is based.
But becoming a red giant doesn't mean the end. The thermonuclear reaction will throw the outer shell into space. And in the center there will remain an energy-deprived helium core. Under the influence of gravitational forces, it will compress and, ultimately, turn into an extremely dense cosmic formation with a large mass. Such remnants of extinct and slowly cooling stars are called white dwarfs.
Our white dwarf will have a radius 100 times smaller than the radius of the Sun, and its luminosity will decrease by 10 thousand times. In this case, the mass will be comparable to the current solar one, and the density will be a million times greater. There are a lot of such white dwarfs in our Galaxy. Their number is 10% of the total number of stars.
It should be noted that white dwarfs are hydrogen and helium. But we will not go into the wilds, but will only note that with strong compression, gravitational collapse can occur. And this is fraught with a colossal explosion. In this case, a supernova explosion is observed. The term "supernova" does not describe the age, but the brightness of the flash. It’s just that the white dwarf was not visible for a long time in the cosmic abyss, and suddenly a bright glow appeared.
Most of the exploding supernova scatters through space at tremendous speed. And the remaining central part is compressed into an even denser formation and is called neutron star. It is the end product of stellar evolution. Its mass is comparable to that of the sun, and its radius reaches only a few tens of kilometers. One cube cm neutron star can weigh millions of tons. There are quite a lot of such formations in space. Their number is about a thousand times less than the ordinary suns with which the Earth's night sky is strewn.
It must be said that the life cycle of a star is directly related to its mass. If it matches the mass of our Sun or is less than it, then a white dwarf appears at the end of its life. However, there are luminaries that are tens and hundreds of times larger than the Sun.
When such giants shrink as they age, they distort space and time so much that instead of a white dwarf a white dwarf appears. black hole. Its gravitational attraction is so strong that even those objects that move at the speed of light cannot overcome it. The dimensions of the hole are characterized by gravitational radius. This is the radius of the sphere bounded by event horizon. It represents a space-time limit. Any cosmic body, having overcome it, disappears forever and never returns back.
There are many theories about black holes. All of them are based on the theory of gravity, since gravity is one of the most important forces in the Universe. And its main quality is versatility. At least, today not a single space object has been discovered that lacks gravitational interaction.
There is an assumption that through black hole you can get into a parallel world. That is, it is a channel to another dimension. Anything is possible, but any statement requires practical evidence. However, no mortal has yet been able to carry out such an experiment.
Thus, the life cycle of a star consists of several stages. In each of them, the luminary appears in a certain capacity, which is radically different from previous and future ones. This is the uniqueness and mystery of outer space. Getting to know him, you involuntarily begin to think that a person also goes through several stages in his development. And the shell in which we exist now is only a transitional stage to some other state. But this conclusion again requires practical confirmation..
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 prevails, while the shell at the top remains convective. No one knows for sure how stars of lower mass arrive on the main sequence, since the time these stars spend in the young category exceeds the age of the Universe. All our ideas about the evolution of these stars are based on numerical calculations.
As the star contracts, the pressure of the degenerate electron gas begins to increase, and at a certain radius of the star, this pressure stops the increase in the central temperature, and then begins to lower it. And for stars smaller than 0.08, this turns out to be fatal: the energy released during nuclear reactions will never be enough to cover the costs of radiation. Such sub-stars are called brown dwarfs, and their fate is constant compression until the pressure of the degenerate gas stops it, and then gradual cooling with the stop of all nuclear reactions.
Young intermediate mass stars
Young stars of intermediate mass (from 2 to 8 times the mass of the Sun) evolve qualitatively in exactly the same way as their smaller sisters, except 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 with irregular variables of spectral type B-F5. They also have bipolar jet disks. The outflow velocity, luminosity and effective temperature are significantly higher 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 jump through all the intermediate stages and heat up nuclear reactions to such an extent that they compensate for radiation losses. For these stars, the outflow of mass and luminosity is so great that it not only stops the collapse of the remaining outer regions, but pushes them back. Thus, the mass of the resulting 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 times the mass of the Sun.
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 blue to cool red, 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, depending on the parameters of the star. That is, we are talking, in fact, only about changing the parameters of the star.
What happens next again depends on the mass of the star.
Later years and death of stars
Old stars with low mass
To date, it is not known for certain what happens to light stars after their hydrogen supply is depleted. Since the universe is 13.7 billion years old, which is not long enough to exhaust its supply of hydrogen fuel, modern theories are based on computer modeling of the processes occurring in such stars.
Some stars can only synthesize helium in some active areas, causing 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 less than 0.5 solar will never be able to synthesize helium even after reactions involving hydrogen cease in the core. Their stellar envelope is not massive enough to overcome the pressure generated by the core. These stars include red dwarfs (such as Proxima Centauri), which have been on the main sequence for hundreds of billions of years. After the cessation of thermonuclear reactions in their core, they, gradually cooling, will continue to weakly emit in the infrared and microwave ranges of the electromagnetic spectrum.
Medium sized stars
When the star reaches average size(from 0.4 to 3.4 solar masses) red giant phase, its outer layers continue to expand, the core contracts, and reactions begin to synthesize carbon from helium. 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 output. Energy output shifts towards low frequency radiation. All this is accompanied by increasing mass loss due to strong solar winds and intense pulsations. 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 star's interior, 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 for the activation of masers are formed in such shells.
Helium combustion reactions are very temperature sensitive. 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, as it cools, turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar and a diameter on the order of the diameter of the Earth.
White dwarfs
The vast majority of stars, including the Sun, end their evolution by contracting until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a hundred times, 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 dark and invisible.
In stars more massive than the Sun, the pressure of degenerate electrons cannot contain the compression of the core, and it continues until most of the particles are converted into neutrons, packed so tightly that the size of the star is measured in kilometers and is 100 million times denser water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.
Supermassive stars
After the outer layers of a star with a mass greater than five solar masses have scattered to form a red supergiant, the core begins to compress due to gravitational forces. As compression increases, 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 heavier and heavier elements of the periodic table are formed, iron-56 is synthesized from silicon. Up until 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 immediate collapse of the core occurs with neutronization of its matter.
What happens next is not entirely clear. But whatever it is, it causes a supernova explosion of incredible power in a matter of seconds.
The accompanying burst of neutrinos provokes a shock wave. Strong jets of neutrinos and a rotating magnetic field push out much of the star's accumulated material - the so-called seed elements, including iron and lighter elements. The exploding matter is bombarded by neutrons emitted from the nucleus, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and perhaps even californium). Thus, supernova explosions explain the presence of elements heavier than iron in interstellar matter.
The blast wave and jets of neutrinos carry material away from the dying star into interstellar space. Subsequently, moving through space, this supernova material may collide with other space debris, and possibly participate in the formation of new stars, planets or satellites.
The processes occurring during the formation of a supernova are still being studied, and so far there is no clarity on this issue. It is also questionable what actually remains of the original star. However, two options are being considered:
Neutron stars
It is known that in some supernovae, strong gravity in the depths of the supergiant causes electrons to fall into the atomic nucleus, where they fuse with protons to form neutrons. The electromagnetic forces separating nearby nuclei disappear. The star's core is now a dense ball of atomic nuclei and individual neutrons.
Such stars, known as neutron stars, are extremely small - no more than the size of a large city - and have an unimaginably high density. Their 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 toward Earth, a pulse of radiation can be detected repeating at intervals equal to the star's orbital period. Such neutron stars were called “pulsars”, and became the first neutron stars to be discovered.
Black holes
Not all supernovae become neutron stars. If the star has a sufficiently large mass, then the collapse of the star will continue and the neutrons themselves will begin to fall inward until its radius becomes less than the Schwarzschild radius. After this, the star 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 conditions. However, quantum mechanics makes exceptions to this rule possible.
There are a number left open questions. Chief among them: “Are there black holes at all?” After all, in order to say for sure that a given object is a black hole, it is necessary to observe its event horizon. All attempts to do this ended in failure. But there is still hope, since some objects cannot be explained without involving accretion, and accretion onto an object without a solid surface, but this does not prove the very existence of black holes.
Questions are also open: is it possible for a star to collapse directly into a black hole, bypassing a supernova? Are there supernovae that will later become black holes? What is the exact influence of a star's initial mass on the formation of objects at the end of its life cycle?
Like any bodies in nature, stars also cannot remain unchanged. They are born, develop and finally “die”. The evolution of stars takes billions of years, but there is debate about the time of their formation. Previously, astronomers believed that the process of their “birth” from stardust took millions of years, but not so long ago photographs of the sky region from the Great Orion Nebula were obtained. Over the course of several years, a small
Photographs from 1947 showed a small group of star-like objects in this location. By 1954, some of them had already become oblong, and five years later these objects broke up into separate ones. Thus, for the first time, the process of star birth took place literally before the eyes of astronomers.
Let's look in detail at the structure and evolution of stars, where their endless, by human standards, life begins and ends.
Traditionally, scientists assume that stars are formed as a result of the condensation of clouds of gas and dust. Under the influence of gravitational forces, an opaque gas ball, dense in structure, is formed from the resulting clouds. His 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 the hot gas inside the ball balances external forces. After this, 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 very high temperatures in their cores, which contributes to continuous thermonuclear processes (the hydrogen that forms them turns into helium). It is these processes that cause intense radiation from stars. The time during which they consume the available supply of hydrogen is determined by their mass. The duration of radiation also depends on this.
When hydrogen reserves are depleted, the evolution of stars approaches the formation stage. This happens as follows. After the release of energy ceases, gravitational forces begin to compress the core. At the same time, the star increases significantly in size. Luminosity also increases as the process continues, but only at thin layer at the core boundary.
This process is accompanied by an increase in the temperature of the contracting helium core and the transformation of helium nuclei into carbon nuclei.
It is predicted that our Sun could become a red giant in eight billion years. Its radius will increase several tens of times, and its luminosity will increase hundreds of times compared to current levels.
The lifespan of a star, as already noted, depends on its mass. Objects with a mass that is less than the Sun “use up” their reserves very economically, so they can shine for tens of billions of years.
The evolution of stars ends with the formation. This happens to those of them whose mass is close to the mass of the Sun, i.e. does not exceed 1.2 of it.
Giant stars tend to quickly deplete their supply of nuclear fuel. This is accompanied by a significant loss of mass, in particular due to the shedding of outer shells. As a result, only a gradually cooling central part remains, in which nuclear reactions have completely stopped. Over time, such stars stop emitting and become invisible.
But sometimes the normal evolution and structure of stars is disrupted. Most often this concerns massive objects that have exhausted all types of thermonuclear fuel. Then they can be converted into neutrons, or And the more scientists learn about these objects, the more new questions arise.
Part one ASTRONOMICAL ASPECT OF THE PROBLEM
4. Evolution of stars Modern astronomy has big amount arguments in favor of the statement that stars are formed by the condensation of clouds of gas and dust in the interstellar medium. The process of star formation from this environment continues to this day. 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 man was already 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 occurs 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 on the inner (relative to the center of the corresponding galaxy) edges of the spiral, which has a natural explanation, the details of which we cannot dwell on here. But it is precisely in these parts of the spirals that “HII zones,” i.e., clouds of ionized interstellar gas, are observed by optical astronomy methods. In ch. 3 it has already been said that the cause of ionization of such clouds can only be ultraviolet radiation from 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 enormous amount of energy needed to maintain the Sun's radiation at approximately the observed level for several billion years come from? Every second the Sun emits 4x10 33 ergs, and over 3 billion years it has emitted 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 using 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 fall of meteoroids onto its surface; others sought the source in the continuous compression of the Sun. The potential energy released during such a process could, under certain conditions, turn into radiation. As we will see below, this source can be quite effective at an early stage of stellar evolution, but it cannot provide radiation from the Sun for the required time. Success nuclear physics made it possible to solve the problem of sources of stellar energy back in the late thirties of our century. Such a source is thermonuclear fusion reactions that occur in the bowels of stars under very prevailing conditions. high temperature(about ten million Kelvin). As a result of these reactions, the speed of which strongly depends on temperature, protons turn into helium nuclei, and the released energy slowly “leaks” through the depths of stars and, in the end, significantly transformed, is emitted into outer space. This is an extremely powerful source. If we assume that initially the Sun consisted only of hydrogen, which as a result of thermonuclear reactions was completely transformed into helium, then the amount of energy released 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 imagine the evolution of a star as follows. For some reasons (several of them can be specified), a cloud of interstellar gas and dust medium began to condense. Quite soon (on an astronomical scale, of course!), under the influence of the forces of universal gravity, a relatively dense opaque gas ball will form from this cloud. Strictly speaking, this ball cannot yet be called a star, since in its central regions the temperature is not sufficient for thermonuclear reactions to begin. The gas pressure inside the ball is not yet able to balance the forces of attraction of its individual parts, so it will continuously compress. Some astronomers previously believed that such “protostars” were observed in individual Nebulae in the form of very dark compact formations, the so-called globules (Fig. 12). The successes of 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. Subsequently, these groups become stellar associations and clusters, well known to astronomers. It is very likely that at this very early stage of the star’s evolution, clumps of lower mass form around it, which then gradually turn into planets (see Chapter 9).Rice. 12. Globules in a diffusion nebula
When a protostar contracts, its temperature rises and a significant part of the released potential energy is radiated into the surrounding space. Since the dimensions of the collapsing gas ball are very large, the radiation per unit of its surface will be insignificant. Since the radiation flux per unit surface is proportional to the fourth power of temperature (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. Subsequently, the protostar continues to contract. 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 will rather quickly “sit down” on the main sequence. During this period, the temperature of the stellar interior is already sufficient for thermonuclear reactions to begin there. In this case, the gas pressure inside the future star balances the attraction and the gas ball stops compressing. A protostar becomes a star. It takes relatively little time for protostars to go through this earliest stage of their evolution. If, for example, the mass of the protostar is greater than that of the Sun, it takes only a few million years; if it is less, it takes several hundred million years. Since the evolutionary time of protostars is relatively short, this earliest phase of star development is difficult to detect. Nevertheless, stars in such a stage are apparently observed. We mean very interesting stars type T Tauri, 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 about the discovery by radio astronomy of a number of molecules in the interstellar medium, primarily hydroxyl OH and water vapor H2O. The surprise of radio astronomers was great when, when surveying the sky at a wavelength of 18 cm, corresponding to the OH radio line, bright, extremely compact (i.e., having small angular dimensions) sources. This was so unexpected that at first they refused to even 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" - "nebulia" and "corona". The fact is that for many decades the bright lines of nebulae and the solar corona could not be identified with any known spectral lines. Therefore, they were attributed to certain hypothetical elements unknown on earth - “nebulium” and “crown”. Let us not smile condescendingly at the ignorance of astronomers at the beginning of our century: after all, there was no atomic theory then! The development of physics has not left in periodic table Mendeleev's place for exotic "celestials": in 1927, "nebulium" was debunked, the lines of which were completely reliably identified 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 “codonia,” then within a few weeks after the discovery it became clear that the “mysterium” lines 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 using the then new, very effective method research, called "radio interferometry at very long baselines." The essence of the method comes down to simultaneous observations of sources on two radio telescopes located at distances of several thousand km from each other. As it turns out, the angular resolution 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 several thousandths of an arcsecond! Note that in optical astronomy such angular resolution is still completely unattainable. Such observations have shown that there are at least three classes of sources of "mysterium". Here we will be interested in 1st class sources. All of them are located inside gaseous ionized nebulae, such as the famous Orion Nebula. As already mentioned, their sizes are extremely small, many thousands of times smaller sizes nebulae. The most interesting thing 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
In 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 magnitude 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 showed that the clouds emitting each line are not spatially aligned. The picture turns out like this: inside an area approximately 1.5 seconds in size, the arcs move with at different speeds about 10 compact clouds. Each cloud emits one specific (frequency) line. The angular dimensions of the clouds are very small, on the order of several thousandths of an arcsecond. Since the distance to the W3 nebula is known (about 2000 pc), the angular dimensions can easily be converted to 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 kind of clouds are these and why do they emit so much in hydroxyl radio lines? The answer to the second question was received quite quickly. It turned out that the radiation mechanism is quite similar to that observed in laboratory masers and lasers. So, the sources of “mysterium” are giant, natural cosmic masers operating at the wave of the hydroxyl line, the length of which is 18 cm. It is in masers (and at optical and infrared frequencies - in lasers) that enormous brightness in the line is achieved, and its spectral width is small . 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 “external” energy source (the so-called “pumping”) makes the concentration of atoms or molecules at the initial (upper) level abnormally high. Without a constantly operating "pumping" a maser or laser is impossible. The question of the nature of the mechanism for “pumping” cosmic masers has not yet been completely resolved. However, most likely the “pumping” is provided by fairly powerful infrared radiation. Another possible pumping mechanism could be certain chemical reactions. It is worth interrupting our story about cosmic masers to think about what amazing phenomena astronomers encounter in space. One of the greatest technical inventions of our turbulent century, which plays a significant role in the scientific and technological revolution we are now experiencing, is easily realized in natural conditions and, moreover, on a huge scale! The flux of radio emission from some cosmic masers is so great that it could have been detected even at the technical level of radio astronomy 35 years ago, i.e. even before the invention of masers and lasers! To do this, you “only” needed to know the exact wavelength of the OH radio link and be interested in the problem. By the way, this is not the first time that the most important scientific and technical problems facing humanity have been realized in natural conditions. Thermonuclear reactions that support the radiation of the Sun and stars (see. below), stimulated the development and implementation of projects for producing nuclear “fuel” on Earth, which in the future should solve all our energy problems. Alas, we are still far from solving this most important problem, which nature 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 we 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, it is still possible to get a rough idea of the physical conditions in the clouds emitting the 18 cm line using the maser mechanism. First of all, it turns out that these clouds are quite dense: per cubic centimeter there are at least 10 8 -10 9 particles, and a significant (and perhaps most) part of them are molecules. The temperature is unlikely to exceed two thousand Kelvin, most likely it is about 1000 Kelvin. These properties are sharply different from the properties of even the densest clouds of interstellar gas. Considering still relatively small sizes 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 also support this statement (which the author of this book expressed back in 1966). In nebulae where cosmic masers are observed, young, hot stars are visible (see below). Consequently, the star formation process there recently ended and, most likely, continues at the present time. Perhaps the most curious thing is that, as radio astronomy observations show, cosmic 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 the young, hot star located inside them. Infrared astronomy has proven to be very useful in studying star formation processes. Indeed, for infrared rays, interstellar absorption of light is not so significant. We can now imagine the following picture: from the cloud of the interstellar medium, through its condensation, several clumps of different masses are formed, evolving into protostars. The rate of evolution is different: for more massive clumps it will be greater (see Table 2 below). Therefore, the most massive clump 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, ionizing the “cocoon” hydrogen that has not condensed into clumps. Of course, this rough scheme will be further refined, and, of course, significant changes will be made to it. But the fact remains: it unexpectedly turned out that for some time (most likely a relatively short time) newborn protostars, figuratively speaking, “scream” about their birth, using using the latest methods quantum radiophysics (i.e. masers)... 2 years after the discovery of cosmic masers on hydroxyl (18 cm line) - it was found that the same sources simultaneously emit (also by a maser mechanism) a line of water vapor, the wavelength of which is 1, 35 cm. The intensity of the “water” maser is even greater than that of the “hydroxyl” one. Clouds emitting the H2O line, although located in the same small volume as “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 having stopped contracting, the star radiates for a long time, practically without changing its position on the spectrum-luminosity diagram. Its radiation is supported by thermonuclear reactions occurring in the central regions. Thus, the main sequence is, as it were, a geometric location of points on the spectrum-luminosity diagram where a star (depending on its mass) can emit for a long time and steadily due to thermonuclear reactions. A star's place 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 emitting 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 time a star stays on the main sequence is determined by its initial mass. If the mass is large, the star’s radiation has enormous power and it quickly uses up its reserves of hydrogen “fuel”. For example, main sequence stars with a mass several tens of times greater than the Sun (these are hot blue giants of spectral class O) can emit steadily while remaining on this sequence for only a few million years, while stars with a mass close to solar, have been on the main sequence for 10-15 billion years. Below is the table. 2, giving the calculated duration of gravitational compression and stay on the main sequence for stars of different spectral classes. The same table shows the values of the masses, radii and luminosities of stars in solar units.
table 2
| years | |||||
|
Spectral class |
Luminosity |
gravitational compression |
stay 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. Along 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 (a color index of 0.2 corresponds to a temperature of 20 thousand K, i.e., a class B spectrum). The globular cluster M3 is an “old” object. It is clearly visible that there are almost no stars in the upper part of the main sequence diagram constructed for this cluster. But the red giant branch of M 3 is very richly represented, while NGC 2254 has very few red giants. This is understandable: the old cluster has M 3 big number stars have already “left” 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 quite steeply upward, while for NGC 2254 it is almost horizontal. From a theoretical point of view, this can be explained by the significantly lower content of heavy elements in M 3. And indeed, in 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 . In the “color index - luminosity” diagram for M 3, another almost horizontal branch is visible. There is no similar branch in the diagram constructed for NGC 2254. The theory explains the appearance of this branch as follows. After the temperature of the contracting dense helium core of the star - a red giant - reaches 100-150 million K, a new nuclear reaction will begin to take place there. This reaction consists of the formation of a carbon nucleus from three helium nuclei. As soon as this reaction begins, the compression of the nucleus will stop. Subsequently, the surface layers
stars increase their temperature and the star on 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. Summary Hertzsprung-Russell diagram for 11 star clusters
In Fig. Figure 17 schematically shows a summary “color-luminosity” diagram for 11 clusters, two of which (M 3 and M 92) are globular. It is clearly visible how the main sequences of different clusters “bend” to the right and upward 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 "preserved" 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 the globular clusters M 3 and M 92. Only the giant branch of the globular clusters is higher in agreement with differences in chemical composition, which were mentioned earlier. Thus, observational data fully confirm and justify the conclusions of the theory. It would seem difficult to expect 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 monitored by the practice of astronomical observations. It should be noted that the compilation of a large number of color-luminosity diagrams required enormous work by observing astronomers and a radical improvement in observation methods. On the other hand, the success of the theory internal structure and the evolution of stars would have been impossible without modern computing technology based on the use of high-speed electronic calculating machines. Research in the field of nuclear physics also provided an invaluable service to the theory, which made it possible to obtain quantitative characteristics of those nuclear reactions that occur in the interior of stars. Without exaggeration, we can say that the development of the theory of the structure and evolution of stars is one of the largest achievements in astronomy of the second half of the 20th century. The development of modern physics opens up the possibility of direct observational testing 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 should be emitted by the Sun if nuclear reactions take place in its depths. It is well known that neutrinos interact extremely weakly with other elementary particles. For example, a neutrino can fly through the entire thickness of the Sun almost without absorption, while X-ray radiation can pass through only a few millimeters of matter in the solar interior without absorption. If we imagine that a powerful beam of neutrinos with the energy of each particle in
