The lifetime of stars consists of several stages, passing through which for millions and billions of years the luminaries are steadily striving for the inevitable finale, turning into bright flashes or gloomy black holes.
The lifetime of a star of any type is an incredibly long and complex process, accompanied by phenomena on a cosmic scale. Its versatility is simply impossible to fully trace and study, even using the entire arsenal of modern science. But on the basis of that unique knowledge accumulated and processed over the entire period of the existence of terrestrial astronomy, whole layers of valuable information become available to us. This makes it possible to connect the sequence of episodes from the life cycle of the luminaries into relatively coherent theories and model their development. What are these stages?
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Episode I. Protostars
The life path of stars, like all objects of the macrocosm and microcosm, begins from birth. This event originates in the formation of an incredibly huge cloud, inside which the first molecules appear, therefore the formation is called molecular. Sometimes another term is used that directly reveals the essence of the process - the cradle of stars.
Only when in such a cloud, due to insurmountable circumstances, an extremely rapid compression of its constituent particles with mass, i.e., gravitational collapse, occurs, the future star begins to form. The reason for this is a surge of gravitational energy, part of which compresses the gas molecules and heats up the parent cloud. Then the transparency of the formation gradually begins to disappear, which contributes to even greater heating and an increase in pressure in its center. The final episode in the protostellar phase is the accretion of matter falling onto the core, during which the nascent luminary grows and becomes visible after the pressure of the emitted light literally sweeps away all the dust to the outskirts.
Find protostars in the Orion Nebula!
This huge panorama of the Orion Nebula is derived from imagery. This nebula is one of the largest and closest cradles of stars to us. Try to find protostars in this nebula, since the resolution of this panorama allows you to do this.
Episode II. young stars
Fomalhaut, image from the DSS catalog. There is still a protoplanetary disk around this star.
The next stage or cycle of a star's life is the period of its cosmic childhood, which, in turn, is divided into three stages: the young luminaries of the small (<3), промежуточной (от 2 до 8) и массой больше восьми солнечных единиц. На первом отрезке образования подвержены конвекции, которая затрагивает абсолютно все области молодых звезд. На промежуточном этапе такое явление не наблюдается. В конце своей молодости объекты уже во всей полноте наделены качествами, присущими взрослой звезде. Однако любопытно то, что на данной стадии они обладают колоссально сильной светимостью, которая замедляет или полностью прекращает процесс коллапса в еще не сформировавшихся солнцах.
Episode III. The heyday of the life path of a star
Sun shot in H line alpha. Our star is in its prime.
In the middle of their lives, cosmic bodies can have a wide variety of colors, masses and dimensions. The color palette varies from bluish hues to red, and their mass can be much less than the sun, or exceed it by more than three hundred times. The main sequence of the life cycle of stars lasts about ten billion years. After that, hydrogen ends in the core of the cosmic body. This moment is considered to be the transition of the life of the object to the next stage. Due to the depletion of hydrogen resources in the core, thermonuclear reactions stop. However, during the period of the newly begun compression of the star, a collapse begins, which leads to the occurrence of thermonuclear reactions already with the participation of helium. This process stimulates the expansion of the star, which is simply incredible in scale. And now it is considered a red giant.
Episode IV The end of the existence of stars and their death
Old luminaries, like their young counterparts, are divided into several types: low-mass, medium-sized, supermassive stars, and. As for objects with a small mass, it is still impossible to say exactly what processes take place with them in the last stages of existence. All such phenomena are hypothetically described using computer simulations, and not based on careful observations of them. After the final burnout of carbon and oxygen, the atmospheric shell of the star increases and its gas component rapidly loses. At the end of their evolutionary path, the luminaries are repeatedly compressed, while their density, on the contrary, increases significantly. Such a star is considered to be a white dwarf. Then, in its life phase, the period of a red supergiant follows. The last in the life cycle of a star is its transformation, as a result of a very strong compression, into a neutron star. However, not all such cosmic bodies become such. Some, most often the largest in terms of parameters (more than 20-30 solar masses), pass into the category of black holes as a result of collapse.
Interesting facts from the life cycles of stars
One of the most peculiar and remarkable information from the stellar life of the cosmos is that the vast majority of the luminaries in ours are at the stage of red dwarfs. Such objects have a mass much less than that of the Sun.
It is also quite interesting that the magnetic attraction of neutron stars is billions of times higher than the similar radiation of the earthly body.
Effect of mass on a star
Another no less entertaining fact is the duration of the existence of the largest known types of stars. Due to the fact that their mass is capable of hundreds of times greater than the solar mass, their release of energy is also many times greater, sometimes even millions of times. Consequently, their life span is much shorter. In some cases, their existence fits into just a few million years, against the billions of years of the life of stars with a small mass.
An interesting fact is also the opposite of black holes to white dwarfs. It is noteworthy that the former arise from the most gigantic stars in terms of mass, and the latter, on the contrary, from the smallest.
In the Universe there is a huge number of unique phenomena that can be talked about endlessly, because the cosmos is extremely poorly studied and explored. All human knowledge about stars and their life cycles, which modern science has, is mainly obtained from observations and theoretical calculations. Such little-studied phenomena and objects give rise to constant work for thousands of researchers and scientists: astronomers, physicists, mathematicians, chemists. Thanks to their continuous work, this knowledge is constantly accumulated, supplemented and changed, thus becoming more accurate, reliable and comprehensive.
EVOLUTION OF STARS- change over time physical. parameters and observed characteristics of stars as a result. nuclear reactions, energy and mass loss. For stars in close binary systems of beings, the exchange of matter between companions plays a role. For the evolution of such stars, see Art. close binary stars.
Main observable characteristics of a star are its luminosity L(with a known distance) and temperature G, the surface of the star, determined by the distribution of energy in the spectrum. Approximately T s is equal to effective temperature T e. E. h. represented as a line (track) on the plane lg L, lg T e (i.e., on Hertzsprung - Ressell diagram, GRD).
Introduction
Stars are born from dense interstellar clouds, in which thermal and hydrodynamic clouds develop. instability (see star formation). The consequence of these instabilities is hydrodynamic. collapse of part of the cloud, ending with the formation of a gravitationally bound object - a protostar. The collapse is uneven. Rapid compression of the center, part leads to the formation of a hydrostatically equilibrium core with a mass (for the total mass of the collapsing cloud, the mass of the Sun), and then follows a long stage accretions on it the rest of the cloud (shell). The time of formation of a protostar from the beginning of the collapse is 10 -10 6 years. Protostars shine due to the release of gravitas. compression energy. A certain contribution to the luminosity is also made with the participation 
, small numbers of which were formed at very early stages of the evolution of the Universe (see Fig. Nucleosynthesis).As mass increases and compression temp-pa center. regions of the core of the protostar is growing. When it reaches values of ~ 10 7 K (which is possible for stars with a mass exceeding hydrogen combustion begins (fusion reactions of hydrogen conversion into helium). Energy losses for radiation are compensated by the energy released during hydrogen combustion. The star enters the main sequence (MS) GRD For more information about the initial stage of the E.Z., see Art. protostars.
The formation of stars is accompanied by the outflow of the material of the shell, so that the mass of the star on the MS is less than the beginning. the mass of the collapsing cloud. Observations show that at the protostar stage, the rate of mass loss in stars is c (T Tauri stars). During the time of arrival at the GP (from 6 * 10 6 years for 
up to 2 10 7 years for 
the mass of the star will decrease by The luminosity of stars increases rapidly with increasing mass (see Fig. Weight - luminosity dependence). At the stars with
the luminosity at the accretion stage turns out to be so high that it causes a powerful outflow of matter, and the mass of the star being born M turns out to be significantly less. masses M0 collapsing cloud: for
A star that radiates by releasing nuclear energy slowly evolves as its chemistry changes. composition. Naib. The star spends time at the stage when its center is at its center. area is burning hydrogen. This stage is called GP on GRD. The majority of observed stars are located near the MS. The long duration of this stage is connected, firstly, with the fact that hydrogen is the most high-calorie nuclear fuel. When one helium nucleus (alpha particle) is formed from 4 hydrogen nuclei, only 12 C is released from 3 alpha particles, i.e., the energy release per unit mass is 10 times less. Secondly, stars on the MS emit much less radiation than at subsequent stages of evolution, and as a result, it turns out that the lifetime on the MS is two to three orders of magnitude longer than the time of the entire subsequent evolution. Accordingly, the number of stars on the MS significantly exceeds the number of brighter stars.
After the burning of hydrogen in the center of the star and the formation of a helium core, the release of nuclear energy in it stops and the core begins to contract intensively. Hydrogen continues to burn in a thin shell surrounding the helium core (the so-called layer source). At the same time, the shell expands, the luminosity of the star increases, the surface temperature decreases, and the star becomes a red giant (in the case of less massive stars) or a supergiant (red or yellow) in the case of more massive stars (see Fig. Red giants and supergiants). The process of subsequent evolution is determined mainly by the mass of the star M.
In stars, nuclear combustion ends after the formation of a carbon (12 C) with an admixture of oxygen stellar core with a mass of approx. 1. After resetting the entire shell surrounding this core, it turns into a "dead" star - white dwarf.
massive stars 
undergo evolution. the path of combustion up to the formation of a stellar core from the most stable (max. binding energy per nucleon) element 56 Fe. In such a nucleus, the release of nuclear energy is impossible, the increase in pressure does not compensate for the increase in gravitational forces during growth and the slow quasi-static. compression is replaced by a rapid collapse - there is a loss of hydrodynamic. stability and explosion supernova. With rapid compression to a density r, close to the density of matter in the atomic nucleus, a huge number of gravitations are released. energy - times more than during the entire time of nuclear evolution, lasting tens of millions of years. The vast majority of this energy is carried away by neutrinos. After the explosion and ejection of the shell, a residue is formed in the form neutron star- the second type of "dead" stars.
In stars of intermediate mass, a degenerate carbon-oxygen core is formed, the mass of which is so large that it can no longer exist in the form of a white dwarf, but continues to contract until an increase in temperature and density leads to rapid (explosive) combustion carbon (carbon flash) and the complete expansion of the entire star. This expansion is also observed as a supernova explosion, in place of which no remnant remains.
Finally, for the most massive stars, the collapse may not stop at the stage of a neutron star, but continue further, forming a relativistic object - black hole. Watching. manifestations of the process of formation of a black hole are not yet known. It is possible that the increase in luminosity here is so insignificant that such a collapse is difficult to detect (a "silent" collapse). However, even in this case, the collapse should be accompanied by a powerful burst of neutrino radiation, almost like during the formation of a neutron star, and, in addition, the star that existed before the collapse began will disappear (extinguish).
Throughout almost the entire star evolution stable with respect to decomp. types of disturbances. Naib. Two types of perturbations are important: hydrodynamic and thermal. Hydrodynamic the perturbations are associated with random perturbations in the density and size of the star. Stability with respect to such perturbations is ensured by the fact that during compression (expansion) of the pressure force P rise (fall) faster than gravity. This leads to the fact that during random compression or expansion, a force arises that returns the star to its equilibrium state. The change in pressure during fast processes occurs almost adiabatically, so the stability is determined by the adiabatic exponent, which must be greater than 4/3 ( S- ud. entropy; see Art. gravitational collapse). Since the pressure of matter in a star is determined by the mixture of ideal gas with radiation, and, as a rule, stars are hydrodynamically stable. An example of an unstable star is a pre-supernova with an iron core, in which the increase in pressure during compression is insufficient. This means that part of the energy is spent on the photodisintegration of iron with the formation of neutrons, protons and alpha particles, and g decreases significantly and can approach unity.
Stability with respect to thermal perturbations is provided by the negative heat capacity of the star. Negative heat capacity can be explained on the basis of the virial theorem. As applied to stars, which are described by a state equation with an adiabatic exponent of 5/3, this theorem says that in equilibrium the thermal energy of a star is half abs. the magnitude of its gravity. energy (negative), i.e. the total energy of the star is negative and equal to half of the gravitational one.
Any random release of energy increases the total energy of the star, i.e., reduces its abs. size. Therefore, in the new equilibrium position, the star must expand in order to decrease in abs. the magnitude of the value of gravity. energy. In accordance with this, the value of the thermal energy of the star (and hence the temperature) in the new state will decrease, since it is half abs. gravity values. energy. Thus, the release of energy leads to a decrease in temperature, which is called. negative heat capacity. When denied. heat capacity, the random release of heat will reduce the temperature, and therefore reduce the release of heat in nuclear reactions, the rate of which drops rapidly with decreasing temperature. On the contrary, the random loss of energy will be compensated by compression and an increase in the rate of heat release.
On some critical stages, the heat capacity of the star becomes positive. Then thermal instability develops and a thermal flash occurs. Naib, the mechanism of development of thermal instability is obvious in the presence of a degenerate core, where pressure and int. the energy of matter is practically independent of temperature. In this case, heat release leads to an increase in temperature, which does not affect the increase in pressure and therefore is not accompanied by expansion. Since the rate of nuclear reactions increases rapidly with increasing temperature, a self-accelerating release of nuclear energy and a thermal flash (nuclear explosion) occur.
The processes that determine E. z., proceed with different characteristic times, of which we note the hydrodynamic thermal and nuclear hydrodynamic. time characterizes the rate of change in the parameters of a star when matter moves at speeds comparable to the speed of sound u sound. In order of magnitude where R is the characteristic size of a star. For an equilibrium star 
Hydrodynamic time of the order of free fall time: 
Thermal time determines the rate of cooling or heating of a star. When cooling in the absence of nuclear combustion, since the energy reserve is of the order of gravitational. star energy; in this case t th often called Kelvin-Helmholtz time. In the case of rapid nuclear combustion in the absence of Hydrodynamic. movements, when the heating time, where is the rate of energy release, and C v- heat capacity at post, volume.
Nuclear time determines the rate of change of chemical. composition (element concentrations) during nuclear combustion. Usually use the concentration (content) by weight X i- the fraction of the mass of a unit volume attributable to a given element i. Nuclear time very sharply (exponentially) depends on temperature. In normal stars, where hydrostatic is maintained. equilibrium, this time, as a rule, is much longer than other characteristic times. For fast nuclear combustion, t n is related to thermal time:
where q- calorie content of nuclear fuel (energy released during the combustion of a unit mass of fuel
Throughout almost the entire E. z. - from the stage of a young contracting star to the later stages - time is minimal. from all characteristic times. Only in pre-supernovae, where nuclear equilibrium (equilibrium with respect to strong interaction reactions) takes place, time is the shortest. Usually, a star maintains an approximate equilibrium of relatively fast processes (eg, hydrostatic equilibrium), and the time of evolution is determined by one of the slow processes.
At the gravitational stage compression, the inequality 
The star is in hydrostatic. equilibrium, evolution is determined by the loss of energy (with a characteristic time, the main nuclear reactions practically do not occur.
On the MS, this inequality persists, but the evolution is determined by nuclear reactions and hydrostatic takes place. and thermal equilibrium.
After the formation of the helium core, the compression of the central regions and the expansion of the shell, the rate of nuclear reactions in the center of the star increases so much that t n becomes order deviations from thermal equilibrium occur in the massive shell around the helium core. Hydrodynamic time remains minimal, and hydrostatic. the equilibrium of the star is not disturbed.
With a flash in the carbon-oxygen core, leading to the complete expansion of the star, both and turn out to be much less than t h, which leads to a violation of the hydrostatic. balance and explosion.
In the nuclei of massive presupernovae, where nuclear equilibrium takes place, the value of the E.z. determined by the rate of energy loss, as in young contracting stars. It ends with the loss of hydro-dynamic. stability and rapid collapse. Hydrodynamic instability is not associated with change with change in the structure of the equilibrium state of the star. The development of thermal instability is associated with a rapid decrease and ends with an explosion when these times become shorter
So, if we exclude several critical moments, stars in their mass are globally stable with respect to mechanical. and thermal disturbances. A variety of properties of the matter of stars, in particular the presence of zones of variable. , thin combustion layers, extended shells, leads to the development of local instabilities, which do not lead to the destruction of the star, since they are usually stabilized by nonlinear effects when finite amplitudes of perturbations are reached. Existence of certain types variable stars associated with the development of such local instabilities.
Main the factor that determines the temperature distribution in a star is the rate of energy loss (luminosity), which depends on opacity stellar interiors. Speed E. z. without energy sources is determined by the reserves of heat and gravity. energy and cooling rate, and the "switching on" of nuclear reactions is equivalent to an increase in thermal energy reserves and a decrease in the rate of evolution. Faktich. The luminosity of a star is determined by its structure and does not depend on the rate of nuclear reactions. Consider, for example, the transition from the stage of gravitational compression to the GP stage of the star with If the star radiated only due to the stock of gravitational. energy, then the characteristic time of its life (the time of E. z.) was bylet. As energy is emitted and compressed, the temperature in the center of the star increases and nuclear heat release increases until it balances the radiation losses (luminosity). Starting from this moment, the gravitational the compression stops and the star "freezes" on the MS until the hydrogen burns out and a helium core is formed. For such a star, due to the burning of hydrogen, the lifetime increases by almost three orders of magnitude, reaching ~ 10 10 years. Similarly, the burning of the next nuclear fuel "freezes" the star in some other state. Point (on HRD). in which the "freezing" of the star occurs, determines the dependence of the rate of nuclear reactions of a given fuel on temperature. The larger the fuel core, the greater the temperature required to provide a given heat release rate (due to an increase in altitude the Coulomb barrier of the nucleus fuel). However, with an increase in temperature and density, the luminosity of the star, which is a state function, also increases. Therefore, as the evolution and formation of more and more heavy elements in the center. luminosity grows almost monotonically in the nucleus.
At high temperatures, neutrino losses play an increasingly important role in the cooling of the star. In the later stages, neutrino losses are several orders of magnitude higher than losses due to photon emission and, accordingly, accelerate the E.Z.
Star evolution equations
Usually (to simplify calculations) the star is considered non-rotating and spherically symmetrical. In the process of evolution of the mass of the star is in a hydrostatic state. equilibrium determined by the equation
where is the mass contained inside the radius r,
Density, pressure, determined by the state level
Here the first term is the gas pressure, the second is the radiation, is the gas constant, a is the constant of the radiation density. For stars with a mass on the GP, corrections to the equation of state associated with the non-ideal nature of matter play the role. The temperature distribution is determined by the energy level
(E-internal energy per unit mass, - the rate of energy loss per unit mass of matter due to neutrino radiation), heat transfer equations
In the zone radiant balance(k - opacity),
in convective zone and 
in the convective core with post. entropy S. Convective energy flow Fc in the shell is calculated according to the approximate mixing path theory (see Fig. convective instability).
Equilibrium equations are solved for boundary conditions at the center ( r= 0, L= 0 at t = 0) and at the level photosphere, where optical thickness
at m = M. The latter condition becomes more complicated for stars at the stage of red supergiants and giants, when the star has an extended shell of low density and high luminosity.
In the process of nuclear combustion, a slow change in chem. composition of the star and, as a result, changes in all its parameters. Main ur-niami describing the evolution of chemical. composition are:
Here: t p , ma , and m 12C - masses of proton, a-particle and carbon 
and-contents (by mass) of hydrogen, helium and-rate of energy release and energy-tich. output for the corresponding chains of nuclear reactions (see below). When calculating the late stages of the evolution of massive stars, the combustion of heavier elements is taken into account. Stars with less mass have a center, a temp swarm
T s less than ~ 1.5-10 7 K main. reactions are the source of nuclear energy hydrogen cycle(pp-cycle). At large masses and the center, the temperature of the stars, hydrogen burns predominately. in carbon-nitrogen cycle(CNO cycle). cp. the amount of energy released during the fusion of one 4 He nucleus (minus the energy carried away by neutrinos): in the pp-cycle 26.2 MeV, and in the CNO-cycle MeV. Corresponding power release rates:
(T9- temp-pa in billion K, r in g / cm 3). The appearance of a convective core in MS sleep stars is associated with the transition from the pp- to the CNO-cycle, which has a sharper dependence of the burning rate on temperature. Helium burning takes place in the so-called. For-reactions - fusion reactions of three He nuclei: 
The a-reaction is accompanied by the reaction 
which corresponds to
The release of heat during the formation of one nucleus 12 C and 16 O, respectively, is equal to
Building a star model (see also Star Modeling)in the moment
requires knowledge of its state at the previous time step of the numerical model tn-1 to find the rate of release of gravity. energy
and definitions of chem. composition
where are the right parts of equations (7), 
Along with the explicit time step scheme above, an implicit one is used when F i , P/ r 2
calculated at the moment t n or represent a linear combination of values taken at the moments Solution of the system of ordinary differentials. Equations (1) - (6) are complicated by the presence of singular points in the center of the star and, therefore, the integration is carried out towards the center and from the surface with a stitching into the c-l. intermediate point [M. Schwarzschild method]. From the conditions of the stitching, the center is found, the values of r with, T with and also L and T e. Dr. the solution is to split the star into N spherical layers and replacement of differentials. differential equations [Henyi's method (L. Nepueu)]. The latter method is better adapted to the use of computers. To build a hydrostatic models also use a method based on the solution of hydrodynamic. non-stationary equations with viscosity.
Nuclear evolution of stars
Calculations E. h. are presented as tracks on the HDD. As already noted b. hours of the lifetime of a star are spent on the MS.
The lifetime of such a star on the MS (point BUT in fig. 1) ok. 10 10 years, and its structure is similar to the structure sun. During this stage, in the center, regions of the star, hydrogen "burns out" into helium. When the mass of the helium core reaches ~ 10% of the mass of the star, a departure from the MS becomes noticeable (point AT). Slight increase in luminosity in the area AB is associated with a decrease in opacity due to a decrease in the number of electrons during the synthesis of helium from hydrogen. After the burning of hydrogen in the center of the star and the formation of a helium core, the removal of energy from it can only be compensated by the energy released during compression. This leads to compression and heating of the shell that retained hydrogen, which ignites in a thin layer surrounding the helium core (layer source).
The energy released during the compression of the helium core and in the hydrogen layer source comes out. Partially, it is absorbed by the hydrogen shell, the edges gradually swell, reducing the eff. temp-py at post, luminosity (section BC).
As the shell expands and the mass of the helium core increases, two factors begin to play a decisive role in the behavior of the star: convection developing in the shell and degeneracy arising in the core. The expansion of the shell and the drop in temperature in it contribute to the expansion of the external. convective zone, which the star had on the MS. The development of convection leads to an improvement in heat removal, which, thanks to the negative. heat capacity of a star, causes its compression, an increase in temperature, heat release and luminosity. An increase in luminosity contributes to an increase in the radiant temperature gradient, which further enhances convection. T. o. a positive feedback occurs and convection captures, therefore, a part of the mass of the star, approaching the layer source. The luminosity grows, and the star moves on the GRD from the point With to the point D(region of red giants).
As the star moves towards the point D there is an accelerated combustion of hydrogen, the mass isothermal. of the helium core increases, which, under the condition of equilibrium, leads to an increase in its density. Since the temperature of the nucleus is close to the temperature of the hydrogen layer source and increases slightly, an increase in density leads to degeneration of the nucleus. The pressure in it practically ceases to depend on the temperature. Under these conditions, a small increase in the temperature of the core associated with the ignition of helium has almost no effect on pressure, the star acquires positive heat capacity, which leads to a sharp increase in the rate of helium combustion ( helium flash). Indeed, while the energy release during helium combustion is small, the star is located on the HRD near the point D and the growth of temperature and density leads to an increase in energy release, which in turn increases the temperature. A positive feedback occurs, leading to a thermal helium flash in the core. The development of the outbreak continues until the increase in temperature removes the degeneracy in the core, the star acquires a "normal" negative. heat capacity and further combustion of helium will continue quietly in a non-degenerate nucleus. A feature of the helium flash is that it is hidden in the depths of the star and external. its manifestations are almost absent. After the formation of a non-degenerate core, the star descends from the point D and turns left towards the line EF(horizontal branch of giants), where it is until the helium in the core turns into carbon. The newly formed carbon core becomes degenerate, the ignition of helium in a layer source and the formation of a two-layer helium-hydrogen burning layer lead to the development of convection in the shell, and the same development pattern is repeated again, with the star returning almost along the same line to the point D.
Unlike hydrogen layered sources, where combustion proceeds quietly, helium layered sources are unstable with respect to the development of a thermal flash. The nature of this flare, as well as the flare in the helium core, is related to the positive. heat capacity leading to positive feedback. However, in the layer, the heat capacity is due not to degeneracy (helium is not degenerate here), but to the geometry of the combustion region (thin layer) and the rapid increase in the rate of energy release with increasing temperature during helium combustion. The mechanism of layered combustion instability is not as obvious as in the case of a flare in a degenerate core, and requires detailed calculations to substantiate it.
T. o., in a neighborhood of a point D there are quiet stars with helium cores and flaring stars with carbon ones. Flares contribute to the outflow of matter, so as the carbon core grows, the total mass of the star decreases. After several hundreds of flares (an approximate figure, since no one has been able to consistently calculate so many flares), as a result of the rapid outflow of matter and the growth of the nucleus, the mass over the helium-hydrogen eff. temp-ry and. hence the movement of the star to the left. After the exhaustion of fuel in layer sources (point G), the luminosity is maintained only due to the heat capacity of the core, which quickly cools down, the star moves down the GRD and turns into a white dwarf (point h). At this stage, the star is up to complete cooling. Observations indicate that the outflow of matter near the point D occurs unevenly, which means that a fraction of the mass is dumped immediately before the start of the star’s movement to the left, forming planetary nebula.
Stars with. For stars with a lifetime on the MS, it exceeds the cosmological. time (2*10 10 years), and all of them are either on the MS or moving towards it. In stars, the combustion of hydrogen is accompanied by an increase in density in the center of the star and the approach of the core to a degenerate state. At 
the helium core formed after hydrogen burnout becomes degenerate, and the shell strongly swells, leading to an increase in luminosity and a decrease in surface temperature (Fig. 2). The star becomes a red giant. The degenerate core is unstable with respect to a helium flash. A helium flash in the core leads to its expansion and removal of degeneracy; in this case, no more than 1% of helium burns.
Rice. 2. Evolutionary tracks of stars [with initial chemical composition Xz(abundance of elements heavier than helium) - = 0.03] from the main sequence to the helium flash (for M= 0.8 and 1.5) or before the ignition of carbon in the center (for the Numbers indicate the mass of the star, the dots correspond to the main sequence and the moments of ignition of helium and carbon in the core.
Stars of low mass with a nondegenerate helium core and a hydrogen shell after a helium flash are located on the HRD near the horizontal giant branch (SHG, Fig. 3). On this branch, the stars are helium cores with a mass surrounded by hydrogen shells decomp. masses. After helium burns out in the core, its rapid compression begins until the helium layer source ignites. The star on the GRD moves up and to the right to a line called asymptotic. giant branch (ABG). On this line, the star consists of a degenerate carbon-oxygen core and two layer sources (helium and hydrogen) located very close to each other. A hydrogen shell is located above them, the mass of which can reach. An amazing property of stars on the AGB is that their position on the HRD depends only on the mass of the carbon core and practically does not depend on the mass of the hydrogen shell. Luminosity L stars on AWG is determined by f-loy
where M co is the mass of the carbon-oxygen core. With growth MCO the star moves on the GRD up the AGB. This movement is not calm.
Rice. 3. Coarse evolutionary tracks of stars with initial masses M= 1.5, 25 Bold lines correspond to the main stages of combustion in the core (corresponding reactions are indicated next). For M<2
. 3, a helium flash occurs in the core (HFN), then the quiet combustion of 4 He begins in the core. After 4 He burns out in the core, the star passes to the early asymptotic giant branch (RAN). When the core, in which 4 He has burned out, reaches mass, thermal flashes (TV) begin in the helium layer source. At the AGB stage, mass loss occurs, which ends with a rapid ejection of the rest of the hydrogen envelope in the form of a planetary nebula (PN). The CO nucleus mass turns into a white dwarf. The evolution of more massive stars with 
at the AVG stage and beyond, the process is similar. The circle with rays marks the beginning of the glow of the planetary nebula, when T, the star reaches 3 · 10 4 K and gas ionization in the PT begins.
Rice. 4. Evolutionary track of a star transforming into a white dwarf, starting from RAVG; initial composition:
. The dots give the position of the star before the next thermal burst, its number is indicated. OM is the envelope of luminosity minima during outbursts. Star tracks are shown in the region of flare minima nos. 7, 9, and 10. The shaded areas are on the MS and in the region of helium burning in the core (HTC), where approximate evolutionary tracks of stars with are given. The dashed line on the left corresponds to a star of constant radius 
The small thickness of layer sources leads to thermal flashes (TS). The number of flashes while moving along the AGB increases with the growth of the mass of the hydrogen shell and can exceed several. thousand. The time between bursts also depends mainly on the mass of the nucleus and is determined by the expression
in years), and the luminosity of the star at the outburst maximum
A characteristic property of AGB stars is their intense mass loss. It is believed that stars lose their entire hydrogen shell and turn into a white dwarf with a mass. The mechanism of mass loss is not entirely clear, but it is believed (chapter based on observational data) that part of the mass is lost in the form of a quiet outflow, and the rest (several tenths of it is quickly thrown off in the form of a spherical shell, observed as a planetary nebula. The evolutionary track of the core of a planetary nebula c, turning into a white dwarf, is shown in Fig. 4 (schematically such tracks are shown in Fig. 3. Times on dashed marks t i and the corresponding masses of hydrogen shells M oh, equal
Stars with mass. In such stars, the mass of the core reaches. When the nucleus is compressed, carbon is ignited in it. The combustion of carbon in the degenerate core of a star c is unstable, the reaction leads to an explosion and complete expansion of the star. It is possible that such explosions cause the observed bursts of supernovae of the first type. In the cores of stars from the beginning. masses exceeding (up to the carbon nucleus is not degenerate. Degeneracy occurs at the stage of formation of the nucleus from For
The degenerate kernel shrinks as a result Neutronizing substances 24 Mg, compression turns into gravity. collapse. In this case, the nucleus is heated due to nonequilibrium neutronization. in stars with a mass 
thermal instability develops in the degenerate core, which, as in a helium flash, leads to the removal of degeneracy and the transition to a quiet burning regime up to the appearance of 56 Fe at the center of the star. The fate of such a star is similar to the fate of more massive stars.
The evolution of massive stars. Burning in the center, the regions of these stars takes place in the absence of degeneracy up to the formation of an iron core. Estimated evolutions. tracks of massive stars after the formation of the helium core are sensitive to physical. assumptions, calculation method and are very diverse. This manifests itself in different the shape of the loops on the HRD (similar to the loops for in Fig. 2), as well as in the values of eff. the temperature of the star at the helium burning stage. The difference in physical assumptions consists in choosing a criterion of convective instability, which takes into account [P. Ledoux's criterion] or does not take into account [K. Schwarzschild's criterion] the stabilizing role of the chemical gradient. composition. Related to this is the behavior of the so-called. a semi-convective zone, which appears above the convective core in sleep stars of the hydrogen burning stage and has a very small excess of the temperature gradient over the adiabatic one. In models that take into account the gradient of chem. composition, the semi-convection zone is separated from the convective core by a radiant layer, which prevents mixing. If, on the other hand, the Schwarzschild criterion is used, then partial mixing occurs and the evolution conditions change significantly. Helium burning occurs in the region of blue supergiants, while in the case of the Ledoux criterion, helium burns out in the region of red supergiants with
With increasing mass, the value of where is critical increases. luminosity
At L = Lc the force of light pressure on electrons balances the force of gravity. attraction of atomic nuclei. In the process of the star's motion on the HRD to the right into the region of red supergiants after the formation of a helium core in the shell, where zones of incomplete ionization of helium and hydrogen appear, the opacity increases sharply and L/L c becomes greater than one. At this stage, a sharp increase in the rate of mass loss of the star is possible, so that the entire hydrogen envelope can be lost. Observations show the existence of very bright helium Wolf-Rayet (WR, see below) stars. wolf - Raye stars at), to-rykh there is a powerful outflow of matter with a mass flux At the stage of formation of WR stars, the mass flux could be much greater.
The calculation of the evolution of massive stars requires a self-consistent accounting for mass loss, so that the quantity M was obtained in the calculations unambiguously as L, R, T e,. T. to. mass loss time M/M much more hydrodynamic. time of a star, a star at the stage of expiration can be represented as a static. core and stationary outflowing shell, the mass of the swarm inside the critical. the radius of the stream is much less than the mass of the star; on the critical radius r to speed v to equal (see Stellar wind).The flow rate drops rapidly as you move to dense int. layers of the star, and the shell smoothly turns into a static one. core. Only preliminary calculations of evolution with self-consistent allowance for mass loss have been made, although there are many evolutions. calculations with phenomenological taking into account the loss of mass, the type of dependencies
(L, R, M in units 
Rice. 5. Evolutionary tracks of stars with masses 15 and 25 bb" and BC- areas of helium combustion in the core; CD- combustion in a double (H - He) layered source; DE- combustion of carbon. The calculations were brought to the point of loss of stability (indicated by a cross in a circle), dashed tracks correspond to not quite confident calculations.
The calculation of the evolution of two stars with post masses (M = 15 and up to the formation of an iron core in the presupernova state is shown in Fig. 5. After the combustion of carbon, the evolution of the core proceeds very quickly, due to an increase in the neutrino loss rate, so that the state of the shell almost does not change and the star moves slowly along the GRD until the onset of collapse.Observations of supernova 1987A in the Large Magellanic Cloud showed that the pre-supernova here was a blue, and not a red supergiant, as shown in Fig. 5. This may be due to the fact that either a reset means , parts of the hydrogen shell, or the star evolved on a track along loops entering the blue region.If carbon ignited at the moment when the star was in the blue region, its apparent position on the HRD remained almost unchanged until the loss of stability and a supernova explosion. calculation shows that the appearance of loops is stochastic in nature, so we can only talk about the probabilities of the location of a star in the region of blue, yellow or red supergiants in a pre-supernova state.
Stars that have turned into red and yellow giants and supergiants, after the formation of a helium core, become determined. area unstable relative to the buildup of mechanical. and are observed as variable stars with regular brightness fluctuations ( cepheids and RR Lyrae-type stars). Main The reason for the excitation of oscillations in these stars is the anomalous behavior of opacity in the zone of incomplete helium ionization, the thickness of which increases with increasing temperature (see Fig. Pulsations of the stars). Outside the MS, there are other types of variable stars with regular, semi-regular, and irregular variability. The reason for the variability of regular variables that are at the stages of E. h. before and after the MS, is the presence of powerful convective shells, leading to the generation of shock waves during stellar flares, similar to solar flares, but many orders of magnitude more powerful.
Presupernovae and supernovae
Supernovae of the second type (with hydrogen lines in the spectra and remnants in the form pulsars) are the product of the evolution of massive stars, the cores of these stars lose stability and collapse after an increase in the center, temperature, so much that the dissociation of the 56 Fe and adiabatic nuclei begins. the index becomes less than 4/3. The value of g, averaged over a star, determines its hydrodynamic. sustainability. The instability occurs when
In the expression, the term on the right is related to the effects of the general theory of relativity and is equal to zero in the Newtonian theory, in which it separates stable states from unstable ones. According to the calculation results presented in Fig. 5. stellar cores at a point shortly after the loss of stability are characterized by the following parameters:
Here M, is the mass of the nucleus; T s and r c - central temperature and density, -neutrino luminosity, -photon luminosity, -radius of the photosphere; the numbers in brackets indicate the order of magnitude. Stars with a mass of approx. 8, a degenerate carbon-oxygen core with a mass of 1.39 is formed, which, before a thermal flash, is characterized by a trace, with the parameters: ( r i, is the radius of the nucleus). Thermal bursts of stellar nuclei, leading to the complete expansion of the star and the release of energy ~ 10 51 erg, are associated with observed bursts of type I supernovae, in the spectra of which hydrogen is not observed, and pulsars were not found in the explosion remnants. Supernova explosions of the type intermediate between types I and II (hydrogen lines are almost invisible, but neutron stars can form), are apparently associated with a loss of stability in the cores of stars of intermediate mass 
or with the entry of these stars into binary systems.
hydrodynamic calculations. the collapse of the cores of massive stars showed that the overwhelming majority of the released gravitational energyerg) is carried away by a neutrino. The inner parts of the star turn out to be opaque for the neutrinos born there, and a neutrino photosphere is formed inside the star. The neutrino heating of the falling shell, the burning out of the remaining nuclear fuel in it during the collapse, as well as the rebound of the falling shell from the surface of the formed neutron star are not sufficient to eject matter from the kinetic energy. energy erg (characteristic of supernovae). Main the reasons for this are that the neutrino flux slows down the fall of the shell, and the shock wave formed during the rebound of the shell is additionally weakened due to the expenditure of most of its energy for dissociation in the shell of atomic nuclei of the iron peak (i.e., nuclei with mass numbers close to 56). Rapid energy losses due to the emission of neutrinos from the region of the neutrino photosphere lead to an increase in the temperature gradient and the development of convection. This can significantly increase the energy of each emitted neutrino and, accordingly, the cross section of its interaction with matter, which contributes to the explosion.
The energy of a supernova explosion can be drawn from the rotational energy of the forming neutron star, which reaches 10 53 erg. The most important role in the transformation of the energy of rotation into the energy of the explosion is played by the magnetic field. field. Therefore, such an explosion is called. magnetorotational. In a differentially rotating shell around a neutron star, the azimuthal magnetic field increases linearly in time. fields due to the winding of lines of force. When the magn. the pressure will increase sufficiently, it is formed, the edges increase when propagating in a medium with a decreasing density and due to the work of the magnetic. piston. Calculations show that ~3-5% of the rotational energy can be converted into kinetic energy. ejection energy. This is sufficient to explain the observed supernovae. In contrast to the explosion mechanisms of spherically symmetrical stars, where energy is released in a fraction of a second, in a magneto-rotational explosion, energy release can be delayed for several. hours; in this case, the rotation period of the resulting neutron star can exceed 10 milliseconds (the rotation rate will be<~ 1/10 предельной, совместимой
с устойчивостью нейтронной звезды).
The last stages of stellar evolution
A star, which has no sources of energy, shines due to cooling, and the equilibrium in it is maintained by the pressure of degenerate electrons or neutrons. Fun-dam. a fact is the presence of a mass limit in cold stars, associated with the fact that with increasing density, relativistic degeneracy of electrons occurs, and then neutrons. Therefore, sufficiently massive stars lose their stability and go into a state of relativistic collapse with the formation of a black hole. At densities g/cm 3 the substance consists of electrons and nuclei. electron is narrower at g / cm 3 (m z is the number of nucleons per electron), so you can use the equation of state of the relativistic degenerate electron gas
For the barotropic equation of state P = P(p) the equilibrium of a star is determined by equations (1) and (2). In the case of a polytropy, from (1) and (2) the equilibrium equation follows:
star mass
From equation (9) it follows that the primal mass of a star does not depend on r s. For the equation of state (8) mass
Rice. 6. Dependence of mass on central density for equilibrium cold stars. The upper dashed line corresponds to the equation of state for "pure" neutrons, the lower one, with hyperons taken into account.
The mass of stars, for which the pressure is determined by degenerate electrons, cannot exceed ( Chandrasekara limit). Stars, in which the pressure of degenerate electrons predominates, called. white dwarfs due to their small size and hot surface. On the graph for cold stars (Fig. 6), white dwarfs are located to the left of the first maximum. For iron composition = 28/13; with taking into account neutronization and Coulomb corrections to the equation of state max, the mass of an iron white dwarf is approximately when the center, the density is ~1.4x At a higher density, m z increases due to neutronization and the equilibrium mass falls. In this case, the equilibrium models are unstable, and stability is restored when the main. nonrelativistic degenerate neutrons begin to contribute to the pressure (the minimum is shown in Fig. 6, where at such high densities the nuclear interaction plays an important role, therefore, in stable neutron stars (between the minimum and the second maximum), the neutron gas is not ideal. Relativistic neutron degeneracy and general relativity effects lead to to loss of stability.As a result, the limiting mass of a neutron star (for realistic equations of state)
Stars from the beginning mass lose matter in the process of evolution on the AGB and turn into white dwarfs. More massive stars that do not have time to lose mass and lose stability either fly apart as a result of the explosive combustion of carbon, or turn into neutron stars decomp. types. If the excess mass is not released during the collapse, then a relativistic collapse of the nucleus occurs and the formation of a black hole occurs. The precursors of black holes are naib, massive stars from the beginning. by the masses
Lit.: Frank-Kamenetsky D. A., Physical processes inside stars, M., 1959; Schwarzschild M., Structure and evolution of stars, trans. from English, M., 1961; The internal structure of stars, ed. L. Allera. D. M. McLaughlin, trans. from English, M., 1970; Masevich A. G., Tutukov A. V., Evolution of stars; theory and observations, M., 1988; Bisnovaty-Kogan GS, Physical questions of the theory of stellar evolution. M.. 1989. G. S. Bisnovaty-Kogan.
Let us briefly consider the main stages in the evolution of stars.
Changes in the physical characteristics, internal structure, and chemical composition of a star over time.
Fragmentation of matter. .
It is assumed that stars are formed during the gravitational compression of fragments of a gas and dust cloud. So, the so-called globules can be the places of star formation.
A globule is a dense opaque molecular dust (gas and dust) interstellar cloud, which is observed against the background of luminous clouds of gas and dust in the form of a dark round formation. It consists mainly of molecular hydrogen (H 2) and helium ( He ) with an admixture of molecules of other gases and solid interstellar dust particles. Temperature of the gas in the globule (mainly the temperature of molecular hydrogen) T≈ 10 h 50K, average density n~ 10 5 particles / cm 3, which is several orders of magnitude greater than in the densest ordinary gas and dust clouds, diameter D~ 0.1 h one . Mass of globules M≤ 10 2 × M ⊙ . Some globules contain young types T Taurus.
The cloud is compressed by its own gravity due to gravitational instability, which can occur either spontaneously or as a result of the interaction of the cloud with a shock wave from a supersonic stellar wind stream from another nearby source of star formation. Other reasons for the emergence of gravitational instability are also possible.
Theoretical studies show that under the conditions that exist in ordinary molecular clouds (T≈ 10 ÷ 30K and n ~ 10 2 particles / cm 3), the initial one can occur in cloud volumes with mass M≥ 10 3 × M ⊙ . In such a contracting cloud, further decay into less massive fragments is possible, each of which will also be compressed under the influence of its own gravity. Observations show that in the Galaxy, in the process of star formation, not one, but a group of stars with different masses, for example, an open star cluster, is born.
With compression in the central regions of the cloud, the density increases, as a result of which there comes a moment when the substance of this part of the cloud becomes opaque to its own radiation. In the bowels of the cloud, a stable dense condensation occurs, which astronomers call oh.
Fragmentation of matter - the decay of a molecular dust cloud into smaller parts, the further of which leads to the appearance.
is an astronomical object that is in the stage , from which after some time (for the solar mass this time T ~ 10 8 years) normal is formed.
With a further fall of matter from the gaseous shell onto the nucleus (accretion), the mass of the latter, and consequently, the temperature and increase so much that the gas and radiant pressure are compared with the forces . Kernel compression stops. The formed one is surrounded by a gas-dust shell that is opaque for optical radiation, passing only infrared and longer-wave radiation to the outside. Such an object (-cocoon) is observed as a powerful source of radio and infrared radiation.
With a further increase in the mass and temperature of the core, light pressure stops accretion, and the remnants of the shell disperse into outer space. A young one appears, the physical characteristics of which depend on its mass and initial chemical composition.
The main source of energy for a star being born is, apparently, the energy released during gravitational contraction. This assumption follows from the virial theorem: in a stationary system, the sum of the potential energy E p all members of the system and twice the kinetic energy 2 E to of these terms is zero:
E p + 2 E c = 0. (39)
The theorem is valid for systems of particles moving in a limited region of space under the action of forces whose magnitude is inversely proportional to the square of the distance between the particles. It follows that the thermal (kinetic) energy is equal to half of the gravitational (potential) energy. When a star is compressed, the total energy of the star decreases, while the gravitational energy decreases: half of the change in gravitational energy leaves the star through radiation, and the thermal energy of the star increases due to the second half.
Young low mass stars(up to three solar masses), which are on the way to the main sequence, are completely convective; the process of convection covers all areas of the star. These are still, in fact, protostars, in the center of which nuclear reactions are just beginning, and all the radiation occurs mainly due to. It has not yet been established whether stars decrease at a constant effective temperature. In the Hertzsprung-Russell diagram, such stars form an almost vertical track, called the Hayashi track. As the compression slows down, the young one approaches the main sequence.
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 contraction stops, which stops the further growth of the central temperature caused by the contraction, 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 . Such "understars" radiate more energy than is formed during nuclear reactions, and belong to the so-called; their fate is a constant contraction until the pressure of the degenerate gas stops it, and then a gradual cooling with the cessation of all nuclear reactions that have begun.
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.
Stars with mass greater than 8 solar massesalready have the characteristics of normal stars, because they have gone through all the intermediate stages and were able to achieve such a rate of nuclear reactions that they compensate for the loss of energy by radiation while the mass of the nucleus accumulates. In these stars, the outflow of mass is so great that it not only stops the collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, thaws them away. Thus, the mass of the formed star is noticeably less than the mass of the protostellar cloud.
Main sequence
The temperature of the star rises until in the central regions it reaches values sufficient to turn on thermonuclear reactions, which then become the main source of energy for the star. For massive stars ( M > 1 ÷ 2 × M ⊙ ) is the "combustion" of hydrogen in the carbon cycle; for stars with a mass equal to or less than the mass of the Sun, energy is released in a proton-proton reaction. passes into the equilibrium stage and takes its place on the main sequence of the Hertzsprung-Russell diagram: in a star of large mass, the temperature in the core is very high ( T ≥ 3 × 107 K ), energy production is very intense, - on the main sequence it occupies a place above the Sun in the region of early ( O … A , (F )); in a star of small mass, the temperature in the core is relatively low ( T ≤ 1.5 × 107K ), energy production is not so intense, - on the main sequence it takes place near or below the Sun in the region of late (( F ), G , K , M ).
On the main sequence, it spends up to 90% of the time allotted by nature for its existence. The time a star spends in the main sequence stage also depends on the mass. Yes, with mass M ≈ 10 ÷ 20 × M ⊙ O or B is in the main sequence stage for about 10 7 years, while the red dwarf K 5 with mass M ≈ 0.5 × M ⊙ is in the main sequence stage for about 10 11 years, that is, a time comparable to the age of the Galaxy. Massive hot stars quickly pass into the next stages of evolution, cold dwarfs are in the main sequence stage all the time of the existence of the Galaxy. It can be assumed that red dwarfs are the main type of population of the Galaxy.
Red giant (supergiant).
The rapid burning of hydrogen in the central regions of massive stars leads to the appearance of a helium core in them. With a fraction of the mass of hydrogen of a few percent in the nucleus, the carbon reaction of the conversion of hydrogen into helium almost completely stops. The core is compressed, which leads to an increase in its temperature. As a result of the heating caused by the gravitational contraction of the helium core, hydrogen "lights up" and energy release begins in a thin layer located between the core and the extended shell of the star. The shell expands, the radius of the star increases, the effective temperature decreases and grows. "leaves" the main sequence and passes into the next stage of evolution - into the stage of a red giant or, if the mass of the star M > 10 × M⊙ , into the red supergiant stage.
With an increase in temperature and density, helium begins to “burn” in the core. At T ~ 2 × 10 8 K and r ~ 10 3 ¸ 10 4 g / cm 3 begins a thermonuclear reaction, which is called triple a -process: out of three a -particles (helium nuclei 4 He ) one stable carbon nucleus 12 C is formed. With the mass of the star's core M< 1,4 × M ⊙ тройной a - the process leads to the explosive nature of the energy release - a helium flash, which for a particular star can be repeated many times.
In the central regions of massive stars that are in the giant or supergiant stage, an increase in temperature leads to the successive formation of carbon, carbon-oxygen, and oxygen cores. After carbon burnout, reactions occur, as a result of which heavier chemical elements are formed, possibly also iron nuclei. Further evolution of a massive star can lead to shell ejection, a flare of a star like a Nova or, with the subsequent formation of objects that are the final stage in the evolution of stars: a white dwarf, a neutron star, or a black hole.
The final stage of evolution is the stage of evolution of all normal stars after these have exhausted their thermonuclear fuel; cessation of thermonuclear reactions as a source of energy for the star; the transition of a star, depending on its mass, to the stage of a white dwarf, or a black hole.
White dwarfs are the last stage in the evolution of all normal stars with mass M< 3 ÷ 5 × M ⊙ after exhaustion of thermonuclear fuel by these mi. Having passed the stage of a red giant (or subgiant), such a shell sheds and exposes the core, which, cooling down, becomes a white dwarf. Small radius (R b.c ~ 10 -2 × R ⊙ ) and white or blue-white (T b.c ~ 10 4 K) determined the name of this class of astronomical objects. The mass of a white dwarf is always less than 1.4×M⊙ - it is proved that white dwarfs with large masses cannot exist. With a mass comparable to the mass of the Sun, and sizes comparable to those of the large planets of the solar system, white dwarfs have a huge average density: ρ b.c ~ 10 6 g / cm 3, that is, a weight of 1 cm 3 of white dwarf matter weighs a ton! Acceleration of free fall on the surface g b.c ~ 10 8 cm / s 2 (compare with acceleration on the surface of the Earth - g c ≈980 cm/s 2). With such a gravitational load on the inner regions of the star, the equilibrium state of the white dwarf is maintained by the pressure of the degenerate gas (mainly the degenerate electron gas, since the contribution of the ionic component is small). Recall that a gas is called degenerate if there is no Maxwellian velocity distribution of particles. In such a gas, at certain values of temperature and density, the number of particles (electrons) having any speed in the range from v = 0 to v = v max will be the same. v max is determined by the density and temperature of the gas. With a white dwarf mass M b.c > 1.4 × M ⊙ the maximum speed of electrons in a gas is comparable to the speed of light, the degenerate gas becomes relativistic and its pressure is no longer able to resist gravitational compression. The radius of the dwarf tends to zero - "collapses" into a point.
The thin, hot atmospheres of white dwarfs are either composed of hydrogen, with virtually no other elements found in the atmosphere; or from helium, while there is hundreds of thousands of times less hydrogen in the atmosphere than in the atmospheres of normal stars. According to the type of spectrum, white dwarfs belong to spectral classes O, B, A, F. To “distinguish” white dwarfs from normal stars, the letter D is placed before the designation (DOVII, DBVII, etc. D is the first letter in the English word Degenerate - degenerate). The radiation source of a white dwarf is the supply of thermal energy that the white dwarf received while being the core of the parent star. Many white dwarfs inherited from their parent a strong magnetic field, the strength of which H ~ 10 8 E. It is believed that the number of white dwarfs is about 10% of the total number of stars in the Galaxy.
On fig. 15 shows a photograph of Sirius - the brightest star in the sky (α Canis Major; m v = -1 m ,46; class A1V). The disk visible in the picture is the result of photographic irradiation and diffraction of light on the telescope lens, that is, the disk of the star itself is not resolved in the photograph. The rays coming from the photographic disk of Sirius are traces of the distortion of the wave front of the light flux on the elements of the telescope's optics. Sirius is located at a distance of 2.64 from the Sun, the light from Sirius takes 8.6 years to reach the Earth - thus, it is one of the stars closest to the Sun. Sirius is 2.2 times more massive than the Sun; his M v = +1 m ,43, that is, our neighbor radiates 23 times more energy than the Sun.
Figure 15.The uniqueness of the photograph lies in the fact that, together with the image of Sirius, it was possible to obtain an image of his satellite - the satellite “glows” with a bright dot to the left of Sirius. Sirius - telescopically: Sirius itself is denoted by the letter A, and its satellite by the letter B. The apparent magnitude of Sirius B m v \u003d +8 m,43, that is, it is almost 10,000 times weaker than Sirius A. The mass of Sirius B is almost exactly equal to the mass of the Sun, the radius is about 0.01 of the radius of the Sun, the surface temperature is about 12000K, but Sirius B radiates 400 times less than the Sun . Sirius B is a typical white dwarf. Moreover, this is the first white dwarf discovered, by the way, by Alven Clark in 1862 during visual observation through a telescope.
Sirius A and Sirius B revolve around in common with a period of 50 years; the distance between components A and B is only 20 AU.
According to the apt remark of V.M. Lipunov, “they “ripen” inside massive stars (with a mass of more than 10×M⊙ )”. The nuclei of stars evolving into a neutron star have 1.4× M ⊙ ≤ M ≤ 3 × M ⊙ ; after the sources of thermonuclear reactions run out and the parent ejects a significant part of the matter with a flash, these nuclei will become independent objects of the stellar world, possessing very specific characteristics. The compression of the core of the parent star stops at a density comparable to the nuclear one (ρ n. h ~ 10 14 h 10 15 g/cm3). With such a mass and density, the radius of the born only 10 consists of three layers. The outer layer (or outer crust) is formed by a crystal lattice of iron atomic nuclei ( Fe ) with a possible small admixture of atomic nuclei of other metals; the thickness of the outer crust is only about 600 m with a radius of 10 km. Beneath the outer crust is another inner hard crust, composed of iron atoms ( Fe ), but these atoms are overenriched with neutrons. The thickness of this bark≈ 2 km. The inner crust borders on the liquid neutron core, the physical processes in which are determined by the remarkable properties of the neutron liquid - superfluidity and, in the presence of free electrons and protons in it, superconductivity. It is possible that in the very center the matter may contain mesons and hyperons.
They rotate rapidly around an axis - from one to hundreds of revolutions per second. Such rotation in the presence of a magnetic field ( H ~ 10 13 h 10 15 Oe) often leads to the observed effect of pulsation of the star's radiation in different ranges of electromagnetic waves. We saw one of these pulsars inside the Crab Nebula.
Total number the rotation speed is already insufficient for the ejection of particles, so this cannot be a radio pulsar. However, it is still large, and the surrounding neutron star captured by the magnetic field cannot fall, that is, the accretion of matter does not occur.
Accretor (X-ray pulsar). The rotation speed is reduced to such an extent that now nothing prevents the matter from falling onto such a neutron star. The plasma, falling, moves along the lines of the magnetic field and hits a solid surface in the region of the poles, heating up to tens of millions of degrees. A substance heated to such high temperatures glows in the X-ray range. The area in which the falling matter stops with the surface of the star is very small - only about 100 meters. This hot spot, due to the rotation of the star, periodically disappears from view, which the observer perceives as pulsations. Such objects are called X-ray pulsars.
Georotator. The rotation speed of such neutron stars is low and does not prevent accretion. But the dimensions of the magnetosphere are such that the plasma is stopped by the magnetic field before it is captured by gravity.
If it is a component of a close binary system, then there is a “transfer” of matter from a normal star (the second component) to a neutron one. The mass may exceed the critical one (M > 3×M⊙ ), then the gravitational stability of the star is violated, nothing can resist gravitational contraction, and “leaves” under its gravitational radius
r g = 2 × G × M/c 2 , (40)
turning into a black hole. In the above formula for r g: M is the mass of the star, c is the speed of light, G is the gravitational constant.
A black hole is an object whose gravitational field is so large that neither a particle, nor a photon, nor any material body can reach the second cosmic velocity and escape into outer space.
A black hole is a singular object in the sense that the nature of the flow of physical processes inside it is still inaccessible to a theoretical description. The existence of black holes follows from theoretical considerations, in reality they can be located in the central regions of globular clusters, quasars, giant galaxies, including the center of our galaxy.
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 according to 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. Traveling at 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 precisely in these parts of the spirals that the methods of optical astronomy are used to observe "HII zones", ie, clouds of ionized interstellar gas. In ch. 3 it has already been said that the only reason for the ionization of such clouds can be the ultraviolet radiation of massive hot stars - obviously young objects (see below). Central to the problem of the evolution of stars is the question of the sources of their energy. Indeed, where, for example, does 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 radiation from the Sun 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 contracts, its temperature rises and a significant part of the released potential energy is radiated into the surrounding space. Since the dimensions of the contracting gas sphere are very large, the radiation from a unit of its surface will be negligible. Since the radiation flux from a unit surface is proportional to the fourth power of temperature (the Stefan-Boltzmann law), the temperature of the surface layers of the star is relatively low, while its luminosity is almost the same as that of an ordinary star with the same mass. Therefore, on the "spectrum - luminosity" diagram, such stars will be located to the right of the main sequence, i.e. they will fall into the region of red giants or red dwarfs, depending on the values of their initial masses. In the future, the protostar continues to shrink. Its dimensions become smaller, and the surface temperature increases, as a result of which the spectrum becomes more and more "early". Thus, moving along the "spectrum - luminosity" diagram, the protostar "sits down" rather quickly on the main sequence. During this period, the temperature of the stellar interior is already sufficient for thermonuclear reactions to begin there. At the same time, the pressure of the gas inside the future star balances the attraction and the gas ball stops shrinking. The protostar becomes a star. It takes relatively little time for protostars to go through this very early stage of their evolution. If, for example, the mass of the protostar is greater than the solar mass, only a few million years are needed; if less, several hundred million years. Since the time of evolution of protostars is relatively short, it is difficult to detect this earliest phase of the development of a star. Nevertheless, stars in this stage, apparently, are observed. We 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, Fresnel, the founder of the wave theory of light, remarked (on a different occasion, of course): "Nature laughs at our difficulties." As you can see, Fresnel's remark is even more true today. Let us return, however, to cosmic masers. Although the mechanism 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 unexpectedly 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 part of the main sequence. The M 41 cluster is older, the Hyades cluster is even older, and the M 67 cluster is very old, the "color - luminosity" diagram for which is very similar to the similar diagram for globular clusters M 3 and M 92. Only the giant branch of globular clusters is higher in agreement with differences in chemical composition, which were discussed earlier. Thus, the observational data fully confirm and substantiate the conclusions of the theory. It would seem difficult to expect an observational verification of the theory of processes in stellar interiors, which are hidden from us by a huge thickness of stellar matter. And yet the theory here is constantly controlled by the practice of astronomical observations. It should be noted that the compilation of a large number of "color - luminosity" diagrams required a huge amount of work by astronomers-observers and a radical improvement in observation methods. On the other hand, the success of the theory of the internal structure and evolution of stars would not have been possible without modern 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
