What are the sources of stellar energy? What processes support the "life" of stars? Give an idea about the evolution of ordinary stars and red giants, explain the processes taking place in their interiors. What is the outlook for the evolution of the Sun?

Like all bodies in nature, stars do not remain unchanged, they are born, evolve, and finally "die". To trace the life path of stars and understand how they age, it is necessary to know how they arise. Modern astronomy has a large number of arguments in favor of the assertion that stars are formed by the condensation of clouds of gas-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, some 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.

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 radiates 4*10 33 ergs, and for 3 billion years it radiated 4*10 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.

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 degrees). 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-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 as very dark compact formations, the so-called globules. The success of radio astronomy, however, forced us to abandon this rather naive point of view. 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 of the evolution of a star, clumps of smaller mass form around it, which then gradually turn into planets.

When a protostar contracts, its temperature rises, and a significant part of the released potential energy is radiated into the surrounding space. Since the dimensions of the contracting gaseous sphere are very large, the radiation per unit area of ​​its surface will be negligible. Since the radiation flux from a unit surface is proportional to the fourth power of temperature (the Stefan-Boltzmann law), the temperature of the surface layers of the star is relatively low, while its luminosity is almost the same as that of an ordinary star with the same mass. Therefore, on the "spectrum-luminosity" diagram, such stars will be located to the right of the main sequence, i.e., they will fall into the region of red giants or red dwarfs, depending on the values ​​of their initial masses.

In the future, the protostar continues to shrink. Its dimensions become smaller, and the surface temperature increases, as a result of which the spectrum becomes more and more "early". Thus, moving along the "spectrum - luminosity" diagram, the protostar "sits down" rather quickly on the main sequence. During this period, the temperature of the stellar interior is already sufficient for thermonuclear reactions to begin there. At the same time, the pressure of the gas inside the future star balances the attraction, and the gas ball stops shrinking. The protostar becomes a star.

It takes relatively little time for protostars to go through this very early stage of their evolution. If, for example, the mass of the protostar is greater than the solar mass, only a few million years are needed; if less, several hundred million years. Since the time of evolution of protostars is relatively short, it is difficult to detect this earliest phase of the development of a star. Nevertheless, stars in this stage, apparently, are observed. We are talking about very interesting T Tauri stars, usually immersed in dark nebulae.

Once on the main sequence and ceasing to shrink, the star radiates for a long time practically without changing its position on the "spectrum - luminosity" diagram. Its radiation is supported by thermonuclear reactions taking place in the central regions. Thus, the main sequence is, as it were, the locus of points on the "spectrum - luminosity" diagram, where a star (depending on its mass) can radiate for a long time and steadily due to thermonuclear reactions. A star's position on the main sequence is determined by its mass. It should be noted that there is one more parameter that determines the position of an equilibrium radiating star on the "spectrum-luminosity" diagram. This parameter is the initial chemical composition of the star. If the relative abundance of heavy elements decreases, the star will "fall" in the diagram below. It is this circumstance that explains the presence of a sequence of subdwarfs.

As mentioned above, the relative abundance of heavy elements in these stars is ten times less than in main sequence stars.

The residence time of a star on the main sequence is determined by its initial mass. If the mass is large, the radiation of the star has a huge power, and it quickly consumes its hydrogen "fuel" reserves. For example, main-sequence stars with a mass several tens of times greater than the solar mass (these are hot blue giants of spectral type O) can radiate steadily while being on this sequence for only a few million years, while stars with a mass close to solar, are on the main sequence 10-15 billion years.

The "burning out" of hydrogen (i.e., its transformation into helium in thermonuclear reactions) occurs only in the central regions of the star. This is explained by the fact that the stellar matter is mixed only in the central regions of the star, where nuclear reactions take place, while the outer layers keep the relative content of hydrogen unchanged. Since the amount of hydrogen in the central regions of the star is limited, sooner or later (depending on the mass of the star) almost all of it will "burn out" there.

Calculations show that the mass and radius of its central region, in which nuclear reactions take place, gradually decrease, while the star slowly moves to the right in the "spectrum - luminosity" diagram. This process occurs much faster in relatively massive stars. If we imagine a group of simultaneously formed evolving stars, then over time the main sequence on the "spectrum - luminosity" diagram, constructed for this group, will, as it were, bend to the right.

What will happen to a star when all (or almost all) hydrogen in its core "burns out"? Since the release of energy in the central regions of the star stops, the temperature and pressure there cannot be maintained at the level necessary to counteract the gravitational force that compresses the star. The core of the star will begin to shrink, and its temperature will rise. A very dense hot region is formed, consisting of helium (to which hydrogen has turned) with a small admixture of heavier elements. A gas in this state is called "degenerate". It has a number of interesting properties, which we cannot dwell on here. In this dense hot region, nuclear reactions will not occur, but they will proceed quite intensively on the periphery of the nucleus, in a relatively thin layer. Calculations show that the luminosity of the star and its size will begin to grow. The star, as it were, "swells" and begins to "descend" from the main sequence, moving into the red giant regions. Further, it turns out that giant stars with a lower content of heavy elements will have a higher luminosity for the same size. When a star passes into the stage of a red giant, the rate of its evolution increases significantly.

The next question is what will happen to the star when the helium-carbon reaction in the central regions has exhausted itself, as well as the hydrogen reaction in the thin layer surrounding the hot dense core? What stage of evolution will come after the stage of the red giant? The totality of observational data, as well as a number of theoretical considerations, indicate that at this stage in the evolution of stars, the mass of which is less than 1.2 solar masses, a significant part of their mass, which forms their outer shell, "drops."

A star is a hot ball of gas, heated by nuclear energy and held by gravitational forces. The main information about stars is given by the light they emit and electromagnetic radiation in other regions of the spectrum. The main factors that determine the properties of a star are its mass, chemical composition, and age. Stars must change over time as they radiate energy into space. Information about stellar evolution can be obtained from the Hertzsprung-Russell diagram, which is the dependence of the luminosity of a star on its surface temperature (Fig. 9).

In the Hertzsprung-Russell diagram, the stars are unevenly distributed. About 90% of the stars are concentrated in a narrow band that crosses the diagram diagonally. This lane is called main sequence. Its upper end is located in the region of bright blue stars. The difference in the population of stars located on the main sequence and regions adjacent to the main sequence is several orders of magnitude. The reason is that on the main sequence there are stars in the stage of burning hydrogen, which makes up the bulk of the life of a star. The sun is on the main sequence. Its position is shown in Fig. 9.
The next most populous regions after the main sequence are white dwarfs, red giants, and red supergiants. Red giants and supergiants are mostly stars at the stage of burning helium and heavier nuclei.
The luminosity of a star is the total energy emitted by a star per unit time. The luminosity of a star can be calculated from the energy reaching the Earth if the distance to the star is known.
It is known from thermodynamics that, by measuring the wavelength at the maximum radiation of a black body, one can determine its temperature. A black body with a temperature of 3 K will have a maximum spectral distribution at a frequency of 3·10 11 Hz. A black body with a temperature of 6000 K will emit green light. The temperature 10 6 K corresponds to X-ray radiation. Table 2 shows the wavelength intervals corresponding to the various colors observed in the optical range.

table 2

Color and wavelength

The surface temperature of a star is calculated from the spectral distribution of radiation.
The classification of the spectral type of stars is easy to understand from Table 3.
Each letter characterizes the stars of a certain class. Class O stars are the hottest, class N stars are the coldest. In an O-class star, mainly the spectral lines of ionized helium are visible. The sun belongs to class G, which is characterized by lines of ionized calcium.
Table 4 shows the main characteristics of the Sun. The limits of variation of such characteristics of stars as mass (M), luminosity (L), radius (R) and surface temperature (T) are given in Table 5.

Table 3

Spectral types of stars

Class designation stars	characteristic feature spectral lines	Temperature surface, K
	Ionized helium
	neutral helium
	Ionized calcium
	ionized calcium, neutral metals
	Neutral metals
	neutral metals, absorption bands molecules
	absorption bands cyanide (CN) 2

Rice. 10. Mass-luminosity relation

For main sequence stars with a known mass, the mass-luminosity dependence is shown in Fig. 10 and has the form
L ~ M n , where n = 1.6 for low-mass stars (M < M) and n = 5.4 for high-mass stars (M > M). This means that moving along the main sequence from stars of lower mass to stars of higher mass leads to an increase in luminosity.

Table 4

The main characteristics of the Sun

Luminosity L	3.83 10 33 erg/s (2.4 10 39 MeV/s)
Radiation flux per unit surfaces	6.3 10 7 W / m 2
Average density of matter
Density in the center
Surface temperature
temperature in the center
Chemical composition: hydrogen helium carbon, nitrogen, oxygen, neon, etc.	74% 23% 3%
Age
Acceleration of gravity on a surface	2.7 10 4 cm/s 2
Schwarzschild radius - 2GM / c 2 (c - speed of light)
Rotation period relative to fixed stars
Distance to the center of the Galaxy
Rotation speed around the center galaxies

Table 5

Limits of change in the characteristics of various stars

10-1M< M < 50 M

10-4 L< L < 10 6 L

10-2R< R < 10 3 R

2 10 3 K< T < 10 5 K

The corresponding characteristics of the Sun are taken as the unit of measurement M, R, L, T is the surface temperature.

Thus, more massive stars are also brighter.
In the lower left part of the diagram (Fig. 9) - the second largest group - white dwarfs. In the upper right corner of the diagram, stars with high luminosity but low surface temperatures are grouped - red giants and supergiants. This type of star is less common. The names "giants" and "dwarfs" are associated with the size of the stars. White dwarfs do not obey the mass-luminosity relationship characteristic of main-sequence stars. For the same mass, they have a much lower luminosity than main sequence stars.
A star can be on the main sequence at one point in its evolution and be a giant or white dwarf at another. Most stars are on the main sequence because this is the longest phase of a star's evolution.
One of the essential points in understanding the evolution of the Universe is the idea of ​​the mass distribution of forming stars. By studying the observed mass distribution of stars and taking into account the lifetimes of stars of different masses, one can obtain the mass distribution of stars at the moment of birth. It is established that the probability of the birth of a star of a given mass, very approximately, is inversely proportional to the square of the mass (Salpeter function).

The radiation of stars is maintained mainly due to two types of thermonuclear reactions. In massive stars, these are reactions of the carbon-nitrogen cycle, and in low-mass stars like the Sun, these are proton-proton reactions. In the first, carbon plays the role of a catalyst: it is not consumed itself, but contributes to the transformation of other elements, as a result of which 4 hydrogen nuclei are combined into one helium nucleus.

In principle, a great many other thermonuclear reactions are possible, but calculations show that at temperatures prevailing in the cores of stars, it is the reactions of these two cycles that occur most intensively and give the energy output exactly necessary to maintain the observed stellar radiation.

As you can see, a star is a natural setting for controlled thermonuclear reactions. If the same temperature and pressure of plasma are created in the terrestrial laboratory, then the same nuclear reactions will begin in it. But how to keep this plasma within the laboratory? After all, we do not have a material that would withstand the touch of a substance with a temperature of 10–20 million K without evaporating. And the star does not need this: its powerful gravity successfully resists the gigantic pressure of the plasma.

As long as the proton-proton reaction or carbon-nitrogen cycle proceeds in the star, it is on the main sequence, where it spends the bulk of its life. Later, when a helium core is formed at the star and the temperature in it rises, a “helium flash” occurs, i.e. the reactions of converting helium into heavier elements begin, also leading to the release of energy.

The turbine of a nuclear power plant is a heat engine that determines the overall efficiency of the plant in accordance with the second law of thermodynamics. At modern nuclear power plants, the efficiency is approximately equal. Therefore, to produce 1000 MW of electrical power, the thermal power of the reactor must reach 3000 MW. 2000 MW must be carried away by the water cooling the condenser. This leads to local overheating of natural water bodies and the subsequent emergence of environmental problems.

However, the main problem is to ensure the complete radiation safety of people working at nuclear power plants and to prevent accidental releases of radioactive substances that accumulate in large quantities in the reactor core. Much attention is paid to this problem in the development of nuclear reactors. Nevertheless, after the accidents at some nuclear power plants, in particular at the nuclear power plant in Pennsylvania (USA, 1979) and at the Chernobyl nuclear power plant (1986), the problem of the safety of nuclear energy has become especially acute.

Modern nuclear energy is based on the splitting of atomic nuclei into two lighter ones with the release of energy in proportion to the loss of mass. The source of energy and decay products are radioactive elements. They are associated with the main environmental problems of nuclear energy.

Even more energy is released in the process of nuclear fusion, in which two nuclei merge into one heavier one, but also with a loss of mass and energy release. Hydrogen is the starting element for synthesis, and helium is the final element. Both elements do not have a negative impact on the environment and are practically inexhaustible.

The result of nuclear fusion is the energy of the sun. This process is modeled by man during the explosions of hydrogen bombs. The task is to make nuclear fusion controllable and use its energy purposefully. The main difficulty lies in the fact that nuclear fusion is possible at very high pressures and temperatures of about 100 million °C. There are no materials from which it is possible to manufacture reactors for the implementation of ultra-high-temperature (thermonuclear) reactions. Any material melts and evaporates.

Scientists took the path of searching for the possibility of carrying out reactions in an environment that is not capable of evaporation. There are currently two ways to do this. One of them is based on the retention of hydrogen in a strong magnetic field.

Despite some positive results in the implementation of controlled nuclear fusion, there are opinions that in the short term it is unlikely to be used to solve energy problems. This is due to the unresolved nature of many issues and the need for colossal expenditures for further experimental, and even more industrial, developments.



Diagram "spectrum - luminosity"

Like the Sun, the stars illuminate the Earth, but due to the huge distance to them, the illumination that they create on Earth is many orders of magnitude less than solar. For this reason, technical problems arise when measuring illumination from stars. Astronomers build giant telescopes to pick up the faint rays of stars. The larger the diameter of the telescope lens, the fainter stars can be explored with it. Measurements have shown that, for example, the Polar Star creates illumination on the Earth's surface E = 3.8 10 -9 W / m 2, which is 370 billion times less than the illumination created by the Sun. The distance to the North Star is 200 pc, or about 650 ly. years (r = b 10 18 m). Therefore, the luminosity of the Polar Star L p \u003d 4πr 2 E \u003d 4 3.14 x (6 10 18 m) 2 3.8 10 -9 W / m 2 \u003d 9.1 10 29 W \u003d 4600 L As you can see, despite the small visible the brightness of this star, its luminosity is 4600 times greater than the sun.

The measurements showed that among the stars there are stars hundreds of thousands of times more powerful than the Sun, and stars with luminosities tens of thousands of times smaller than those of the Sun.

Measurements of stellar surface temperatures have shown that the surface temperature of a star determines its visible color and the presence of spectral absorption lines of certain chemical elements in its spectrum. So, Sirius shines in white and its temperature is almost 10,000 K. The Betelgeuse star (α Orion) has a red color and a surface temperature of about 3500 K. The yellow sun has a temperature of 6000 K. By temperature, by color and by the type of spectrum, all the stars were broken into spectral classes, which are denoted by the letters O, B, A, F, G, K, M. The spectral classification of stars is given in the table below.

There is another interesting connection between the spectral class of a star and its luminosity, which is represented as a diagram "spectrum - luminosity (in the luminosities of the Sun)" (it is also called Hertzsprung-Russell diagram in honor of two astronomers - E. Hertzsprung and G. Ressel, who built it). Four groups of stars are clearly distinguished on the diagram.

Main sequence

The parameters of most stars fall on it. Our Sun is one of the main sequence stars. The densities of main sequence stars are comparable to the solar density.

red giants

This group mainly includes red stars with radii ten times greater than the solar one, for example, the star Arcturus (α Bootes), whose radius is 25 times greater than the solar one, and the luminosity is 140 times greater.

supergiants

These are stars with luminosities tens and hundreds of thousands of times greater than the solar one. The radii of these stars are hundreds of times greater than the radius of the Sun. Red supergiants include Betelgeuse (and Orion). With a mass of about 15 times greater than the sun, its radius exceeds the sun by almost 1000 times. The average density of this star is only 2 10 -11 kg / m 3, which is more than 1,000,000 times less than the density of air.

white dwarfs

This is a group of mostly white stars with luminosities hundreds and thousands of times smaller than the sun. They are located at the bottom left of the diagram. These stars have radii almost a hundred times smaller than the sun and are comparable in size to planets. An example of a white dwarf is the star Sirius B, a satellite of Sirius. With a mass almost equal to that of the sun and a size 2.5 times greater than the size of the Earth, this star has a gigantic average density - ρ = 3 10 8 kg/m 3 .

To understand how the observed differences between stars of different groups are explained, let us recall the relationship between luminosity, temperature, and radius of a star, which we used to determine the temperature of the Sun.

Let's compare two stars of spectral type K, one is the main sequence (MS), the other is a red giant (KG). They have the same temperature - T \u003d 4500 K, and the luminosities differ by a thousand times:

i.e., red giants are tens of times larger than main-sequence stars.

Masses of stars it was possible to measure only for the stars that are part of binary systems. And they were determined by the parameters of the orbits of the stars and the period of their revolution around each other using the third generalized Kepler's law. It turned out that the masses of all stars lie within

0.05M ≤ M ≤ 100M

For main sequence stars, there is a relationship between the mass of a star and its luminosity: the greater the mass of a star, the greater its luminosity.

Thus, a star of spectral class B has a mass of about M ≈ 20 M and its luminosity is almost 100,000 times greater than the sun.

Energy source of the sun and stars

According to modern concepts, the source of energy that supports the radiation of the Sun and stars is nuclear energy, which is released during thermonuclear reactions of formation (fusion) of nuclei of helium atoms from the nuclei of hydrogen atoms. During the fusion reaction, the nucleus of the helium atom is formed from four nuclei of hydrogen atoms (four protons), while energy ΔE \u003d 4.8 10 -12 J is released, called binding energy, two elementary particles of neutrinos and two positrons (4Н He + 2е + + 2ν + ΔЕ).

For nuclear reactions to take place, a temperature above several million kelvins is required, at which the protons with the same charges participating in the reaction could receive sufficient energy for mutual approach, overcoming the electric forces of repulsion and merging into one new nucleus. As a result of thermonuclear fusion reactions from hydrogen with a mass of 1 kg, helium is formed with a mass of 0.99 kg, a mass defect Δm = 0.01 kg, and energy is released q = Δmc 2 = 9 10 14 J.

Now we can estimate how long the Sun's hydrogen reserves will last to maintain the observed glow of the Sun, i.e., the lifetime of the Sun. The stock of nuclear energy E \u003d M q \u003d 2 10 30 9 10 14 \u003d 1.8 10 45 J. If we divide this stock of nuclear energy by the luminosity of the Sun L, then we get the lifetime of the Sun:

If we take into account that the Sun consists of at least 70% hydrogen and nuclear reactions occur only in the center, in the solar core, whose mass is about 0.1 M and where the temperature is high enough for thermonuclear reactions to occur, then the lifetime of the Sun and stars, similar to the Sun will be t ≈ 10 10 years

Stars 1 are balls of hot, mostly ionized gas. The ionization of stellar matter is a consequence of its high temperature (from several thousand to several tens of thousands of degrees).

As a result of a study of the chemical composition of the Sun and other stars, it was found that they contain almost all the chemical elements present on Earth and presented in the table of D. I. Mendeleev. It also turned out that in most cases, 70% of the star's mass is hydrogen, 28% - helium and 2% - heavier elements.

You already know that the greater the mass of a star, the stronger the gravitational field it creates. Due to the action of gravitational forces that compress the stellar matter, its temperature, density, pressure increase significantly from the outer layers to the center.

So, for example, the temperature of the outer layers of the Sun is approximately equal to 6 10 3 ° C, and in the center - about 14-15 million ° C, the density of matter in the center of the Sun is approximately equal to 150 g / cm 3 (19 times more than that of iron) , and the pressure from the middle layers to the center increases from 7 10 8 to 3.4 10 11 atm. At such temperatures and pressures, thermonuclear reactions can occur in the core, which are the source of energy for stars.

The radiation power of a star (also called luminosity and denoted by the letter L) is proportional to the fourth power of its mass:

Thermonuclear reactions occurring in the interiors of stars are one of the processes that significantly distinguish stars from planets, since the internal source of planetary heating is radioactive decay. This difference is due to the fact that the mass of any star is obviously greater than the mass of even the largest planet. This can be illustrated by the example of Jupiter. Despite the fact that in many respects it is very similar to a star, its mass turned out to be insufficient for the conditions necessary for the occurrence of thermonuclear reactions to occur in its depths.

As a result of thermonuclear reactions, huge energy is released in the bowels of the Sun, which maintains its glow. Let's consider how this energy goes out to the surface of the Sun.

In the radiant energy transfer zone (Fig. 188), the heat released in the core spreads from the center to the surface of the Sun by radiation, that is, through the absorption and emission of portions of light - quanta - by matter. Since quanta are emitted by atoms in any direction, their path to the surface takes thousands of years.

Rice. 188. Structure of the Sun

In the convection zone, energy is transferred to the surface by rising hot gas flows. Having reached the surface, the gas, radiating energy, cools, condenses and sinks to the base of the zone. In the convective zone, the gas is opaque. Therefore, you can see only those layers that are above it: the photosphere, chromosphere and corona (not indicated in the figure). These three layers belong to the solar atmosphere.

The photosphere ("sphere of light") in the photographs looks like a collection of bright spots - granules (Fig. 189), separated by thin dark lines. The bright spots are streams of hot gas that float to the surface of the convective zone.

Rice. 189. Granules and a spot in the solar photosphere

The chromosphere ("sphere of color") is so named for its reddish-violet color. One of the most interesting phenomena that can be observed in the chromosphere are prominences 2 . The length of the chromosphere reaches 10-15 thousand km.

The outermost part of the Sun's atmosphere is the corona. It extends for millions of kilometers (that is, for a distance of the order of several solar radii), despite the fact that the force of gravity on the Sun is very strong. The large length of the corona is explained by the fact that the movements of atoms and electrons in the corona, heated to a temperature of 1-2 million ° C, occur at great speeds. The solar corona is clearly visible during a solar eclipse (Fig. 190). The shape and brightness of the corona change in accordance with the cycle of solar activity, i.e., with a frequency of 11 years.

Rice. 190. Solar corona (during the total solar eclipse of 1999)

The magnetic field induction on the Sun is only 2 times greater than on the Earth's surface. But from time to time, concentrated magnetic fields arise in a small region of the solar atmosphere, several thousand times stronger than on Earth. They prevent the rise of hot plasma, as a result of which, instead of light granules, a dark area is formed - a sunspot (see Fig. 189). When large groups of spots appear, the power of visible, ultraviolet and X-ray radiation increases sharply, which can adversely affect people's well-being.

The movement of spots across the solar disk is a consequence of its rotation, which occurs with a period equal to 25.4 days relative to the stars.

The final stage of the process of stellar evolution includes several stages. When all the hydrogen in the center of the star turns into helium, the structure of the star begins to noticeably change. Its luminosity increases, the surface temperature decreases, the outer layers expand and the inner layers contract. The star becomes a red giant, i.e., a huge star with high luminosity and very low density. A dense and hot helium core forms in the center. When the temperature in it reaches 100 million ° C, the reaction of converting helium into carbon begins, accompanied by the release of a large amount of energy.

At the next stage, stars like the Sun shed part of their matter, shrink to the size of planets, turning into small, very dense stars - white dwarfs, and slowly cool down.

Questions

1. At a temperature in the core of the order of 14-15 million ° C and pressures from 7 10 8 to 3.4 10 11 atm, the star would have to turn into an expanding gas cloud. But that doesn't happen. What forces do you think oppose the star's expansion?
2. What is the source of energy emitted by a star?
3. What physical process is the source of the internal heating of the planet?
4. What causes sunspots to form?
5. What are the layers of the solar atmosphere?
6. Tell us about the main stages of the evolution of the Sun.

2 Prominences are huge, up to hundreds of thousands of kilometers long, plasma formations in the solar corona, which have a higher density and lower temperature than the coronal plasma surrounding them.