Evolution of stars of different masses. How stars die

Hello dear readers! I would like to talk about the beautiful night sky. Why about night? You ask. Because the stars are clearly visible on it, these beautiful luminous little dots on the black-blue background of our sky. But in fact they are not small, but simply huge, and because of the great distance they seem so tiny.

Have any of you imagined how stars are born, how they live their lives, what is it like for them in general? I suggest you read this article now and imagine the evolution of stars along the way. I have prepared a couple of videos for a visual example 😉

The sky is dotted with many stars, among which are scattered huge clouds of dust and gases, mainly hydrogen. Stars are born precisely in such nebulae, or interstellar regions.

A star lives so long (up to tens of billions of years) that astronomers are unable to trace the life of even one of them from beginning to end. But they have the opportunity to observe different stages of star development.

Scientists combined the data obtained and were able to trace the stages of life of typical stars: the moment of birth of a star in an interstellar cloud, its youth, average age, old age and sometimes very spectacular death.

The birth of a star.

The formation of a star begins with the compaction of matter inside a nebula. Gradually, the resulting compaction decreases in size, shrinking under the influence of gravity. During this compression, or collapse, energy is released that heats up the dust and gas and causes them to glow.

There is a so-called protostar. The temperature and density of matter in its center, or core, is maximum. When the temperature reaches about 10,000,000°C, thermal processes begin to occur in the gas. nuclear reactions.

The nuclei of hydrogen atoms begin to combine and turn into the nuclei of helium atoms. This fusion releases a huge amount of energy. This energy, through the process of convection, is transferred to the surface layer, and then, in the form of light and heat, is emitted into space. This is how a protostar turns into a real star.

The radiation that comes from the core heats the gaseous environment, creating pressure that is directed outward, and thus preventing the gravitational collapse of the star.

The result is that it finds equilibrium, that is, it has constant dimensions, a constant surface temperature and a constant amount of energy released.

Astronomers call a star at this stage of development main sequence star, thus indicating the place it occupies on the Hertzsprung-Russell diagram. This diagram expresses the relationship between a star's temperature and luminosity.

Protostars, which have a small mass, never warm up to the temperatures required to initiate a thermonuclear reaction. These stars, as a result of compression, turn into dim red dwarfs , or even dimmer brown dwarfs . The first brown dwarf star was discovered only in 1987.

Giants and dwarfs.

The diameter of the Sun is approximately 1,400,000 km, its surface temperature is about 6,000°C, and it emits yellowish light. It has been part of the main sequence of stars for 5 billion years.

The hydrogen “fuel” on such a star will be exhausted in approximately 10 billion years, and mainly helium will remain in its core. When there is no longer anything left to “burn”, the intensity of radiation directed from the core is no longer sufficient to balance the gravitational collapse of the core.

But the energy that is released in this case is enough to heat up the surrounding matter. In this shell, the synthesis of hydrogen nuclei begins and more energy is released.

The star begins to glow brighter, but now with a reddish light, and at the same time it also expands, increasing in size tens of times. Now such a star called a red giant.

The red giant's core contracts, and the temperature rises to 100,000,000°C or more. Here the fusion reaction of helium nuclei occurs, turning it into carbon. Thanks to the energy that is released, the star still glows for about 100 million years.

After the helium runs out and the reactions die out, the entire star gradually, under the influence of gravity, shrinks to almost the size of . The energy released in this case is enough for the star to (now a white dwarf) continued to glow brightly for some time.

The degree of compression of matter in a white dwarf is very high and, therefore, it has a very high density - the weight of one tablespoon can reach a thousand tons. This is how the evolution of stars the size of our Sun takes place.

Video showing the evolution of our Sun into a white dwarf

A star with five times the mass of the Sun has a much shorter life cycle and evolves somewhat differently. Such a star is much brighter, and its surface temperature is 25,000 ° C or more; the period of stay in the main sequence of stars is only about 100 million years.

When such a star enters the stage red giant , the temperature in its core exceeds 600,000,000°C. It undergoes fusion reactions of carbon nuclei, which are converted into heavier elements, including iron.

The star, under the influence of the released energy, expands to sizes that are hundreds of times larger than its original size. A star at this stage called a supergiant .

The energy production process in the core suddenly stops, and it shrinks within a matter of seconds. With all this, a huge amount of energy is released and a catastrophic shock wave.

This energy passes through the entire star and throws a significant part of it with the force of an explosion into space, causing a phenomenon known as supernova explosion .

To better visualize everything that has been written, let’s look at the diagram of the evolutionary cycle of stars

In February 1987, a similar flare was observed in a neighboring galaxy, the Large Magellanic Cloud. This supernova briefly glowed brighter than a trillion Suns.

The core of the supergiant compresses and forms a celestial body with a diameter of only 10-20 km, and its density is so high that a teaspoon of its substance can weigh 100 million tons!!! Such a celestial body consists of neutrons andcalled a neutron star .

A neutron star that has just formed is different high speed rotation and very strong magnetism.

This creates a powerful electromagnetic field that emits radio waves and other types of radiation. They spread from magnetic poles stars in the form of rays.

These rays, due to the rotation of the star around its axis, seem to scan outer space. When they rush past our radio telescopes, we perceive them as short flashes, or pulses. That's why such stars are called pulsars.

Pulsars were discovered thanks to the radio waves they emit. It has now become known that many of them emit light and X-ray pulses.

The first light pulsar was discovered in the Crab Nebula. Its pulses are repeated 30 times per second.

The pulses of other pulsars are repeated much more often: PIR (pulsating radio source) 1937+21 flashes 642 times per second. It’s even hard to imagine this!

Stars that have the greatest mass, tens of times the mass of the Sun, also flare up like supernovae. But due to their enormous mass, their collapse is much more catastrophic.

The destructive compression does not stop even at the stage of formation of a neutron star, creating a region in which ordinary matter ceases to exist.

There is only one gravity left, which is so strong that nothing, not even light, can escape its influence. This area is called black hole.Yeah, evolution big stars scary and very dangerous.

In this video we will talk about how a supernova turns into a pulsar and into a black hole.

I don’t know about you, dear readers, but personally, I really love and am interested in space and everything connected with it, it’s so mysterious and beautiful, it’s breathtaking! The evolution of stars has told us a lot about the future of our and all.

Studying stellar evolution is impossible by observing just one star - many changes in stars occur too slowly to be noticed even after many centuries. Therefore, scientists study many stars, each of which is at a certain stage life cycle. Over the past few decades, modeling of the structure of stars using computer technology has become widespread in astrophysics.

Encyclopedic YouTube

1 / 5

✪ Stars and stellar evolution (narrated by astrophysicist Sergei Popov)

✪ Stars and stellar evolution (narrated by Sergey Popov and Ilgonis Vilks)

✪ Evolution of stars. Evolution of a blue giant in 3 minutes

✪ Surdin V.G. Stellar Evolution Part 1

✪ S. A. Lamzin - “Stellar Evolution”

Subtitles

Thermonuclear fusion in the interior of stars

Young stars

The process of star formation can be described in a unified way, but the subsequent stages of a star's evolution depend almost entirely on its mass, and only at the very end of the star's evolution can its chemical composition play a role.

Young low mass stars

Young low-mass stars (up to three solar masses) [ ], which are approaching the main sequence, are completely convective - the convection process covers the entire body of the star. These are essentially protostars, in the centers of which nuclear reactions are just beginning, and all radiation occurs mainly due to gravitational compression. Until hydrostatic equilibrium is established, the star's luminosity decreases at a constant effective temperature. On the Hertzsprung-Russell diagram, such stars form an almost vertical track called the Hayashi track. As the compression slows, the young star approaches the main sequence. Objects of this type are associated with T Tauri stars.

At this time, for stars with a mass greater than 0.8 solar masses, the core becomes transparent to radiation, and radiative energy transfer in the core becomes predominant, since convection is increasingly hampered by the increasing compaction of stellar matter. In the outer layers of the star’s body, convective energy transfer prevails.

It is not known for certain what characteristics do stars of lower mass have at the moment they enter the main sequence, since the time these stars spent in the young category exceeds the age of the Universe [ ] . All ideas about the evolution of these stars are based only on numerical calculations and mathematical modeling.

As the star contracts, the pressure of the degenerate electron gas begins to increase and when a certain radius of the star is reached, the compression stops, which leads to a stop in the further increase in temperature in the core of the star caused by the compression, and then to its decrease. For stars smaller than 0.0767 solar masses, this does not happen: the energy released during nuclear reactions is never enough to balance the internal pressure and gravitational compression. Such “understars” emit more energy than is produced during thermonuclear reactions, and are classified as so-called brown dwarfs. Their fate is constant compression until the pressure of the degenerate gas stops it, and then gradual cooling with the cessation of all thermonuclear reactions that have begun.

Young intermediate mass stars

Young stars of intermediate mass (from 2 to 8 solar masses) [ ] evolve qualitatively in exactly the same way as their smaller sisters and brothers, with the exception that they do not have convective zones up to the main sequence.

Objects of this type are associated with the so-called. Ae\Be Herbig stars with irregular variables of spectral class B-F0. They also exhibit disks and bipolar jets. The rate of outflow of matter from the surface, luminosity and effective temperature are significantly higher than for T Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.

Young stars with a mass greater than 8 solar masses

Stars with such masses already have the characteristics of normal stars, since they went through all the intermediate stages and were able to achieve such a rate of nuclear reactions that compensated for the energy lost to radiation while mass accumulated to achieve hydrostatic equilibrium of the core. For these stars, the outflow of mass and luminosity are so great that they not only stop the gravitational collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, disperse them away. Thus, the mass of the resulting star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars with a mass greater than about 300 solar masses.

Mid-life cycle of a star

Stars come in a wide variety of colors and sizes. By spectral class they range from hot blue to cold red, by mass - from 0.0767 to about 300 Solar masses according to the latest estimates. The luminosity and color of a star depend on its surface temperature, which in turn is determined by its mass. All new stars “take their place” on the main sequence according to their chemical composition and mass. Naturally, we are not talking about the physical movement of the star - only about its position on the indicated diagram, depending on the parameters of the star. In fact, the movement of a star along the diagram corresponds only to a change in the parameters of the star.

The thermonuclear “burning” of matter, resumed at a new level, causes a monstrous expansion of the star. The star "swells", becoming very "loose", and its size increases approximately 100 times. So the star becomes a red giant, and the helium burning phase lasts about several million years. Almost all red giants are variable stars.

Final stages of stellar evolution

Old stars with low mass

At present, it is not known for certain what happens to light stars after the supply of hydrogen in their cores is depleted. Since the age of the Universe is 13.7 billion years, which is not enough for the hydrogen fuel supply in such stars to be depleted, modern theories are based on computer modeling of the processes occurring in such stars.

Some stars can only synthesize helium in certain active zones, causing instability and strong stellar winds. In this case, the formation of a planetary nebula does not occur, and the star only evaporates, becoming even smaller than a brown dwarf [ ] .

A star with a mass less than 0.5 solar is not able to convert helium even after reactions involving hydrogen stop in its core - the mass of such a star is too small to provide new phase gravitational compression to a degree sufficient to “ignite” helium. Such stars include red dwarfs, such as Proxima Centauri, whose residence time on the main sequence ranges from tens of billions to tens of trillions of years. After the cessation of thermonuclear reactions in their cores, they, gradually cooling, will continue to weakly emit in the infrared and microwave ranges of the electromagnetic spectrum.

Medium sized stars

Upon reaching star average size(from 0.4 to 3.4 solar masses) [ ] of the red giant phase, hydrogen runs out in its core, and reactions of synthesis of carbon from helium begin. This the process is underway with more high temperatures and therefore the flow of energy from the core increases and, as a result, the outer layers of the star begin to expand. The beginning of carbon synthesis marks a new stage in the life of a star and continues for some time. For a star similar in size to the Sun, this process can take about a billion years.

Changes in the amount of energy emitted cause the star to go through periods of instability, including changes in size, surface temperature and energy release. Energy output shifts towards low frequency radiation. All this is accompanied by increasing mass loss due to strong stellar winds and intense pulsations. Stars in this phase are called “late-type stars” (also “retired stars”), OH -IR stars or Mira-like stars, depending on their exact characteristics. The ejected gas is relatively rich in heavy elements produced in the interior of the star, such as oxygen and carbon. The gas forms an expanding shell and cools as it moves away from the star, allowing the formation of dust particles and molecules. With strong infrared radiation source stars are formed in such shells ideal conditions to activate cosmic masers.

Thermonuclear combustion reactions of helium are very sensitive to temperature. Sometimes this leads to great instability. Strong pulsations arise, which as a result impart sufficient acceleration to the outer layers to be thrown off and turn into a planetary nebula. In the center of such a nebula, the bare core of the star remains, in which thermonuclear reactions stop, and as it cools, it turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar masses and a diameter on the order of the diameter of the Earth.

The vast majority of stars, including the Sun, complete their evolution by contracting until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a hundred times, and the density becomes a million times higher than the density of water, the star is called a white dwarf. It is deprived of energy sources and, gradually cooling, becomes an invisible black dwarf.

In stars more massive than the Sun, the pressure of degenerate electrons cannot stop further compression of the core, and electrons begin to be “pressed” into atomic nuclei, which turns protons into neutrons, between which there are no electrostatic repulsion forces. This neutronization of matter leads to the fact that the size of the star, which is now, in fact, one huge atomic nucleus, is measured in several kilometers, and its density is 100 million times higher than the density of water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.

Supermassive stars

After a star with a mass greater than five solar masses enters the red supergiant stage, its core begins to shrink under the influence of gravity. As the compression proceeds, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, increasingly heavier elements are synthesized: helium, carbon, oxygen, silicon and iron, which temporarily restrains the collapse of the core.

As a result, as increasingly heavier elements of the Periodic Table are formed, iron-56 is synthesized from silicon. At this stage, further exothermic thermonuclear fusion becomes impossible, since the iron-56 nucleus has a maximum mass defect and the formation of heavier nuclei with the release of energy is impossible. Therefore, when the iron core of a star reaches a certain size, the pressure in it is no longer able to withstand the weight of the overlying layers of the star, and immediate collapse of the core occurs with neutronization of its matter.

What happens next is not yet completely clear, but, in any case, the processes taking place in a matter of seconds lead to a supernova explosion of incredible power.

Strong neutrino jets and a rotating magnetic field push out much of the star's accumulated material. [ ] - so-called seating elements, including iron and lighter elements. The exploding matter is bombarded by neutrons escaping from the stellar core, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and perhaps even californium). Thus, supernova explosions explain the presence of elements heavier than iron in interstellar matter, but this is not the only possible way of their formation, which, for example, is demonstrated by technetium stars.

blast wave And neutrino jets carry matter away from dying star [ ] into interstellar space. Subsequently, as it cools and moves through space, this supernova material can collide with other cosmic “salvage” and, possibly, participate in the formation of new stars, planets or satellites.

The processes occurring during the formation of a supernova are still being studied, and so far there is no clarity on this issue. Also questionable is what actually remains of the original star. However, two options are being considered: neutron stars and black holes.

Neutron stars

It is known that in some supernovae, strong gravity in the depths of the supergiant forces electrons to be absorbed by the atomic nucleus, where they merge with protons to form neutrons. This process is called neutronization. The electromagnetic forces separating nearby nuclei disappear. The star's core is now a dense ball of atomic nuclei and individual neutrons.

Such stars, known as neutron stars, are extremely small - no more than the size of a large city - and have an unimaginably high density. Their orbital period becomes extremely short as the size of the star decreases (due to the conservation of angular momentum). Some neutron stars rotate 600 times per second. For some of them, the angle between the radiation vector and the axis of rotation may be such that the Earth falls into the cone formed by this radiation; in this case, it is possible to detect a radiation pulse repeating at intervals equal to the star’s orbital period. Such neutron stars were called “pulsars”, and became the first to be discovered. neutron stars.

Black holes

Not all stars, after going through the supernova explosion phase, become neutron stars. If the star has a sufficiently large mass, then the collapse of such a star will continue, and the neutrons themselves will begin to fall inward until its radius becomes less than the Schwarzschild radius. After this, the star becomes a black hole.

The existence of black holes was predicted by the general theory of relativity. According to this theory,

The evolution of stars is a change in physicality. characteristics, internal structures and chemistry composition of stars over time. The most important tasks of the theory of E.Z. - explanation of the formation of stars, changes in their observable characteristics, study of the genetic connection of various groups of stars, analysis of their final states.

Since in the part of the Universe known to us, approx. 98-99% of the mass of the observed matter is contained in stars or has passed the stage of stars, explanation by E.Z. yavl. one of the most important problems in astrophysics.

A star in a stationary state is a gas ball, which is in a hydrostatic state. and thermal equilibrium (i.e., the action of gravitational forces is balanced by internal pressure, and energy losses due to radiation are compensated by the energy released in the bowels of the star, see). The “birth” of a star is the formation of a hydrostatically equilibrium object, the radiation of which is supported by its own. energy sources. The “death” of a star is an irreversible imbalance leading to the destruction of the star or its catastrophe. compression.

Isolation of gravitational energy can play a decisive role only when the temperature of the star’s interior is insufficient for nuclear energy release to compensate for energy losses, and the star as a whole or part of it must contract to maintain equilibrium. The release of thermal energy becomes important only after nuclear energy reserves have been exhausted. T.o., E.z. can be represented as a consistent change in the energy sources of stars.

Characteristic time E.z. too large for all evolution to be traced directly. Therefore the main E.Z. research method yavl. construction of sequences of star models describing changes in internal structures and chemistry composition of stars over time. Evolution. the sequences are then compared with the results of observations, for example, with (G.-R.d.), summing up the observations large number stars at different stages of evolution. A particularly important role is played by comparison with G.-R.d. for star clusters, since all stars in a cluster have the same initial chemical. composition and formed almost simultaneously. According to G.-R.d. clusters of different ages, it was possible to establish the direction of the E.Z. Evolution in detail. sequences are calculated by numerically solving a system of differential equations describing the distribution of mass, density, temperature and luminosity over a star, to which are added the laws of energy release and opacity of stellar matter and equations describing changes in chemical properties. star composition over time.

The course of a star's evolution depends mainly on its mass and initial chemistry. composition. The rotation of the star and its magnetic field can play a certain, but not fundamental, role. field, however, the role of these factors in E.Z. has not yet been sufficiently researched. Chem. The composition of a star depends on the time at which it was formed and on its position in the Galaxy at the time of formation. Stars of the first generation were formed from matter, the composition of which was determined by cosmology. conditions. Apparently, it contained approximately 70% by mass hydrogen, 30% helium and an insignificant admixture of deuterium and lithium. During the evolution of first-generation stars, heavy elements (following helium) were formed, which were ejected into interstellar space as a result of the outflow of matter from stars or during stellar explosions. Stars of subsequent generations were formed from matter containing up to 3-4% (by mass) of heavy elements.

The most direct indication that star formation in the Galaxy is still ongoing is the phenomenon. existence of massive bright stars range. classes O and B, the lifetime of which cannot exceed ~ 10 7 years. The rate of star formation in modern times. era is estimated at 5 per year.

2. Star formation, stage of gravitational compression

According to the most common point of view, stars are formed as a result of gravitational forces. condensation of matter in the interstellar medium. The necessary division of the interstellar medium into two phases - dense cold clouds and a rarefied medium with a higher temperature - can occur under the influence of Rayleigh-Taylor thermal instability in the interstellar magnetic field. field. Gas-dust complexes with mass , characteristic size (10-100) pc and particle concentration n~10 2 cm -3 . are actually observed due to their emission of radio waves. Compression (collapse) of such clouds requires certain conditions: gravity. particles of the cloud must exceed the sum of the energy of thermal motion of the particles, the rotational energy of the cloud as a whole and the magnetic field. cloud energy (Jeans criterion). If only the energy of thermal motion is taken into account, then, accurate to a factor of the order of unity, the Jeans criterion is written in the form: align="absmiddle" width="205" height="20">, where is the mass of the cloud, T- gas temperature in K, n- number of particles per 1 cm3. With typical modern interstellar clouds temperature K can only collapse clouds with a mass not less than . The Jeans criterion indicates that for the formation of stars of the actually observed mass spectrum, the concentration of particles in collapsing clouds must reach (10 3 -10 6) cm -3, i.e. 10-1000 times higher than observed in typical clouds. However, such concentrations of particles can be achieved in the depths of clouds that have already begun to collapse. It follows from this that it happens through a sequential process, carried out in several steps. stages, fragmentation of massive clouds. This picture naturally explains the birth of stars in groups - clusters. At the same time, questions related to the thermal balance in the cloud, the velocity field in it, and the mechanism determining the mass spectrum of fragments still remain unclear.

Collapsed stellar mass objects are called protostars. Collapse of a spherically symmetric non-rotating protostar without a magnetic field. fields includes several. stages. At the initial moment of time, the cloud is homogeneous and isothermal. It is transparent to its own. radiation, so the collapse comes with volumetric energy losses, Ch. arr. due to the thermal radiation of the dust, the cut transmits its kinetic. energy of a gas particle. In a homogeneous cloud there is no pressure gradient and compression begins in free fall with a characteristic time , where G- , - cloud density. With the beginning of compression, a rarefaction wave appears, moving towards the center at the speed of sound, and since collapse occurs faster where the density is higher, the protostar is divided into a compact core and an extended shell, into which the matter is distributed according to the law. When the concentration of particles in the core reaches ~ 10 11 cm -3 it becomes opaque to the IR radiation of dust grains. The energy released in the core slowly seeps to the surface due to radiative thermal conduction. The temperature begins to increase almost adiabatically, this leads to an increase in pressure, and the core becomes hydrostatic. balance. The shell continues to fall onto the core, and it appears at its periphery. The kernel parameters at this time weakly depend on total mass protostars: K. As the mass of the core increases due to accretion, its temperature changes almost adiabatically until it reaches 2000 K, when the dissociation of H 2 molecules begins. As a result of energy consumption for dissociation, and not an increase in kinetic. particle energy, the adiabatic index value becomes less than 4/3, pressure changes are not able to compensate for gravitational forces and the core collapses again (see). A new core with parameters is formed, surrounded by a shock front, onto which the remnants of the first core accrete. A similar rearrangement of the nucleus occurs with hydrogen.

Further growth of the core at the expense of the shell matter continues until all the matter falls onto the star or is scattered under the influence of or, if the core is sufficiently massive (see). Protostars with a characteristic time of shell matter t a >t kn, therefore their luminosity is determined by the energy release of the collapsing nuclei.

A star, consisting of a core and an envelope, is observed as an IR source due to the processing of radiation in the envelope (the dust of the envelope, absorbing photons of UV radiation from the core, emits in the IR range). When the shell becomes optically thin, the protostar begins to be observed as an ordinary object of stellar nature. The most massive stars retain their shells until thermonuclear burning of hydrogen begins at the center of the star. Radiation pressure limits the mass of stars to probably . Even if more massive stars are formed, they turn out to be pulsationally unstable and may lose their power. part of the mass at the stage of hydrogen combustion in the core. The duration of the stage of collapse and scattering of the protostellar shell is of the same order as the free fall time for the parent cloud, i.e. 10 5 -10 6 years. Illuminated by the core, clumps of dark matter from the remnants of the shell, accelerated by the stellar wind, are identified with Herbig-Haro objects (stellar clumps with an emission spectrum). Low-mass stars, when they become visible, are in the G.-R.D. region occupied by T Tauri stars (dwarf), more massive ones are in the region where Herbig emission stars are located (irregular early spectral classes with emission lines in spectra).

Evolution. tracks of protostar cores with constant mass at the hydrostatic stage. compressions are shown in Fig. 1. For stars of low mass, at the moment when hydrostatic is established. equilibrium, the conditions in the nuclei are such that energy is transferred to them. Calculations show that the surface temperature of a fully convective star is almost constant. The radius of the star is continuously decreasing, because she continues to shrink. With a constant surface temperature and a decreasing radius, the luminosity of the star should also fall on the G.-R.D. This stage of evolution corresponds to the vertical sections of the tracks.

As the compression continues, the temperature in the interior of the star increases, the matter becomes more transparent, and stars with align="absmiddle" width="90" height="17"> have radiant cores, but the shells remain convective. Less massive stars remain completely convective. Their luminosity is controlled by a thin radiant layer in the photosphere. The more massive the star and the higher its effective temperature, the larger its radiative core (in stars with align="absmiddle" width="74" height="17"> the radiative core appears immediately). In the end, almost the entire star (with the exception of the surface convective zone for stars with a mass) goes into a state of radiative equilibrium, in which all the energy released in the core is transferred by radiation.

3. Evolution based on nuclear reactions

At a temperature in the nuclei of ~ 10 6 K, the first nuclear reactions begin - deuterium, lithium, boron burn out. The primary quantity of these elements is so small that their burnout practically does not withstand compression. The compression stops when the temperature at the center of the star reaches ~ 10 6 K and hydrogen ignites, because The energy released during thermonuclear combustion of hydrogen is sufficient to compensate for radiation losses (see). Homogeneous stars, in the cores of which hydrogen burns, form on the G.-R.D. initial main sequence (IMS). Massive stars reach the NGP faster than low-mass stars, because their rate of energy loss per unit mass, and therefore the rate of evolution, is higher than that of low-mass stars. Since entering the NGP E.z. occurs on the basis of nuclear combustion, the main stages of which are summarized in table. Nuclear combustion can occur before the formation of iron group elements, which have the highest binding energy among all nuclei. Evolution. tracks of stars on G.-R.D. are shown in Fig. 2. The evolution of the central values ​​of temperature and density of stars is shown in Fig. 3. At K main. source of energy yavl. reaction of the hydrogen cycle, at large T- reactions of the carbon-nitrogen (CNO) cycle (see). Side effect CNO cycle phenomenon establishing equilibrium concentrations of nuclides 14 N, 12 C, 13 C - 95%, 4% and 1% by weight, respectively. The predominance of nitrogen in the layers where hydrogen combustion occurred is confirmed by the results of observations, in which these layers appear on the surface as a result of the loss of external. layers. In stars in the center of which the CNO cycle is realized ( align="absmiddle" width="74" height="17">), a convective core appears. The reason for this is very strong addiction energy release depending on temperature: . The flow of radiant energy ~ T 4(see), therefore, it cannot transfer all the energy released, and convection must occur, which is more efficient than radiative transfer. In the most massive stars, more than 50% of the stellar mass is covered by convection. The importance of the convective core for evolution is determined by the fact that nuclear fuel is uniformly depleted in a region much larger than the region of effective combustion, while in stars without a convective core it initially burns out only in a small vicinity of the center, where the temperature is quite high. The hydrogen burnout time ranges from ~ 10 10 years for to years for . The time of all subsequent stages of nuclear combustion does not exceed 10% of the time of hydrogen combustion, therefore stars at the stage of hydrogen combustion form on the G.-R.D. densely populated region - (GP). In stars with a temperature in the center that never reaches the values ​​necessary for the combustion of hydrogen, they shrink indefinitely, turning into “black” dwarfs. Burnout of hydrogen leads to an increase in avg. molecular weight substances of the core, and therefore to maintain hydrostatic. equilibrium, the pressure in the center must increase, which entails an increase in the temperature in the center and the temperature gradient across the star, and consequently, the luminosity. An increase in luminosity also results from a decrease in the opacity of matter with increasing temperature. The core contracts to maintain nuclear energy release conditions with a decrease in hydrogen content, and the shell expands due to the need to transfer the increased energy flow from the core. On G.-R.d. the star moves to the right of the NGP. A decrease in opacity leads to the death of convective cores in all but the most massive stars. The rate of evolution of massive stars is the highest, and they are the first to leave the MS. The lifetime on the MS is for stars with ca. 10 million years, from ca. 70 million years, and from ca. 10 billion years.

When the hydrogen content in the core decreases to 1%, the expansion of the shells of stars with align="absmiddle" width="66" height="17"> is replaced by a general contraction of the star necessary to maintain energy release. Compression of the shell causes heating of hydrogen in the layer adjacent to the helium core to the temperature of its thermonuclear combustion, and a layer source of energy release arises. In stars with mass , in which it depends less on temperature and the region of energy release is not so strongly concentrated towards the center, there is no stage of general compression.

E.z. after hydrogen burns out depends on their mass. The most important factor, influencing the course of evolution of stars with mass , yavl. degeneracy of electron gas at high densities. Due to the high density, the number of quantum states with low energy is limited due to the Pauli principle and electrons fill quantum levels with high energy, significantly exceeding the energy of their thermal motion. Key Feature degenerate gas is that its pressure p depends only on the density: for non-relativistic degeneracy and for relativistic degeneracy. The gas pressure of electrons is much greater than the pressure of ions. This follows what is fundamental for E.Z. conclusion: since the gravitational force acting on a unit volume of a relativistically degenerate gas depends on density in the same way as the pressure gradient, there must be a limiting mass (see), such that at align="absmiddle" width="66" height ="15"> electron pressure cannot counteract gravity and compression begins. Limit weight align="absmiddle" width="139" height="17">. The boundary of the region in which the electron gas is degenerate is shown in Fig. 3. In low-mass stars, degeneracy plays a noticeable role already in the process of formation of helium nuclei.

The second factor determining E.z. at later stages, these are neutrino energy losses. In the depths of the stars T~10 8 K main. a role in the birth is played by: photoneutrino process, decay of plasma oscillation quanta (plasmons) into neutrino-antineutrino pairs (), annihilation of electron-positron pairs () and (see). The most important feature of neutrinos is that the star’s matter is almost transparent to them and neutrinos freely carry energy away from the star.

The helium core, in which conditions for helium combustion have not yet arisen, is compressed. The temperature in the layered source adjacent to the core increases, and the rate of hydrogen combustion increases. The need to transfer an increased energy flow leads to expansion of the shell, for which part of the energy is wasted. Since the luminosity of the star does not change, the temperature of its surface drops, and on the G.-R.D. the star moves to the region occupied by red giants. The star's restructuring time is two orders of magnitude less than the time it takes for hydrogen to burn out in the core, so there are few stars between the MS strip and the region of red supergiants. With a decrease in the temperature of the shell, its transparency increases, as a result of which an external appearance appears. convective zone and the luminosity of the star increases.

The removal of energy from the core through the thermal conductivity of degenerate electrons and neutrino losses in stars delays the moment of helium combustion. The temperature begins to increase noticeably only when the core becomes almost isothermal. The combustion of 4 He determines the E.Z. from the moment when the energy release exceeds the energy loss through thermal conductivity and neutrino radiation. The same condition applies to the combustion of all subsequent types nuclear fuel.

A remarkable feature of stellar cores made of degenerate gas, cooled by neutrinos, is “convergence” - the convergence of tracks, which characterize the relationship between density and temperature Tc in the center of the star (Fig. 3). The rate of energy release during compression of the core is determined by the rate of addition of matter to it through a layer source, and depends only on the mass of the core for a given type of fuel. A balance of inflow and outflow of energy must be maintained in the core, therefore the same distribution of temperature and density is established in the cores of stars. By the time 4 He ignites, the mass of the nucleus depends on the content of heavy elements. In nuclei of degenerate gas, the combustion of 4 He has the character of a thermal explosion, because the energy released during combustion goes to increase the energy of the thermal motion of electrons, but the pressure remains almost unchanged with increasing temperature until the thermal energy of the electrons is equal to the energy of the degenerate gas of electrons. Then the degeneracy is removed and the core rapidly expands - a helium flash occurs. Helium flares are likely accompanied by the loss of stellar matter. In , where massive stars have long finished evolution and red giants have masses, stars at the helium burning stage are on the horizontal branch of the G.-R.D.

In the helium cores of stars with align="absmiddle" width="90" height="17"> the gas is not degenerate, 4 He ignites quietly, but the cores also expand due to increasing Tc. In the most massive stars, the combustion of 4 He occurs even when they are active. blue supergiants. Expansion of the core leads to a decrease T in the region of the hydrogen layer source, and the luminosity of the star after the helium burst decreases. For supporting thermal equilibrium the shell contracts, and the star leaves the region of red supergiants. When the 4 He in the core is depleted, compression of the core and expansion of the shell begin again, the star again becomes a red supergiant. A layered combustion source of 4 He is formed, which dominates the energy release. External appears again. convective zone. As helium and hydrogen burn out, the thickness of the layer sources decreases. A thin layer of helium combustion turns out to be thermally unstable, because with a very strong sensitivity of energy release to temperature (), the thermal conductivity of the substance is insufficient to extinguish thermal disturbances in the combustion layer. During thermal outbreaks, convection occurs in the layer. If it penetrates into layers rich in hydrogen, then as a result of a slow process ( s-process, see) elements are synthesized with atomic masses from 22 Ne to 209 B.

Radiation pressure on dust and molecules formed in the cold, extended shells of red supergiants leads to continuous loss of matter at a rate of up to a year. Continuous mass loss can be supplemented by losses caused by instability of layer combustion or pulsations, which can lead to the release of one or more. shells. When the amount of substance above the carbon-oxygen core becomes less than a certain limit, the shell is forced to compress in order to maintain the temperature in the combustion layers until the compression is capable of maintaining combustion; star on G.-R.D. moves almost horizontally to the left. At this stage, the instability of the combustion layers can also lead to expansion of the shell and loss of matter. While the star is hot enough, it is observed as a core with one or more. shells. When layer sources shift toward the surface of the star so much that the temperature in them becomes lower than that required for nuclear combustion, the star cools, turning into a white dwarf with , radiating due to the consumption of thermal energy of the ionic component of its matter. The characteristic cooling time of white dwarfs is ~ 10 9 years. The lower limit on the masses of single stars turning into white dwarfs is unclear, it is estimated at 3-6. In c stars, the electron gas degenerates at the stage of growth of carbon-oxygen (C,O-) stellar cores. As in the helium cores of stars, due to neutrino energy losses, a “convergence” of conditions occurs in the center and at the moment of combustion of carbon in the C,O core. The combustion of 12 C under such conditions most likely has the nature of an explosion and leads to the complete destruction of the star. Complete destruction may not occur if . Such a density is achievable when the core growth rate is determined by the accretion of satellite matter in a close binary system.

INTRODUCTION

CHAPTER 1. Evolution of stars

CHAPTER 2.Thermonuclear fusion in the interior of stars and the birth of stars

CHAPTER 3. Mid-life cycle of a star

CHAPTER 4. Later years and death of stars

CONCLUSION

Literature

INTRODUCTION

Modern scientific sources indicate that the universe consists of 98% stars, which “in turn” are the main element of the galaxy. Information sources give different definitions this concept, here are some of them:

A star is a celestial body in which thermonuclear reactions have occurred, have occurred, or will occur. Stars are massive luminous balls of gas (plasma). Formed from a gas-dust environment (hydrogen and helium) as a result of gravitational compression. The temperature of matter in the interior of stars is measured in millions of kelvins, and on their surface - in thousands of kelvins. The energy of the vast majority of stars is released as a result of thermonuclear reactions converting hydrogen into helium, occurring at high temperatures in internal areas. Stars are often called the main bodies of the Universe, since they contain the bulk of luminous matter in nature.

Stars are huge, spherical objects made of helium and hydrogen, as well as other gases. The energy of a star is contained in its core, where helium interacts with hydrogen every second.

Like everything organic in our universe, stars arise, develop, change and disappear - this process takes billions of years and is called the process of “Star Evolution”.

CHAPTER 1. Evolution of stars

Evolution of stars- the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years while it emits light and heat.

A star begins its life as a cold, rarefied cloud of interstellar gas (a rarefied gaseous medium that fills all the space between stars), compressing under its own gravity and gradually taking the shape of a ball. When compressed, gravitational energy (the universal fundamental interaction between all material bodies) turns into heat, and the temperature of the object increases. When the temperature in the center reaches 15-20 million K, thermonuclear reactions begin and compression stops. The object becomes a full-fledged star. The first stage of a star's life is similar to the solar one - it is dominated by reactions of the hydrogen cycle. It remains in this state for most of its life, being on the main sequence of the Hertzsprung-Russell diagram (Fig. 1) (showing the relationship between absolute magnitude, luminosity, spectral class and surface temperature of the star, 1910), until the fuel reserves in its core. When all the hydrogen in the center of the star is converted into helium, a helium core is formed, and thermonuclear burning of hydrogen continues at its periphery. During this period, the structure of the star begins to change. Its luminosity increases, its outer layers expand, and its surface temperature decreases - the star becomes a red giant, which form a branch on the Hertzsprung-Russell diagram. The star spends significantly less time on this branch than on the main sequence. When the accumulated mass of the helium core becomes significant, it cannot withstand own weight and begins to shrink; if the star is massive enough, the increasing temperature can cause further thermonuclear transformation of helium into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally silicon into iron).

Rice. 1. Hertzsprung-Russell diagram

Evolution of a class G star using the example of the Sun

CHAPTER 2. Thermonuclear fusion in the interior of stars

By 1939, it was established that the source of stellar energy was thermonuclear fusion occurring in the depths of stars. Most stars radiate because in their core four protons combine through a series of intermediate steps into a single alpha particle. This transformation can occur in two main ways, called the proton-proton, or p-p, cycle, and the carbon-nitrogen, or CN, cycle. In low-mass stars, energy release is mainly provided by the first cycle, in heavy stars - by the second. The supply of nuclear fuel in a star is limited and is constantly spent on radiation. The process of thermonuclear fusion, which releases energy and changes the composition of the star's matter, in combination with gravity, which tends to compress the star and also releases energy, as well as radiation from the surface, which carries away the released energy, are the main driving forces of stellar evolution.

The Birth of Stars

The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in a galaxy actually contains between 0.1 and 1 molecule per cm³. The molecular cloud has a density of about a million molecules per cm³. The mass of such a cloud exceeds the mass of the Sun by 100,000-10,000,000 times due to its size: from 50 to 300 light years in diameter.

While the cloud rotates freely around the center of its home galaxy, nothing happens. However, due to the heterogeneity gravitational field disturbances may arise in it, leading to local concentrations of mass. Such disturbances cause gravitational collapse of the cloud. One of the scenarios leading to this is the collision of two clouds. Another event causing collapse could be the passage of a cloud through a dense arm spiral galaxy. Also a critical factor could be the explosion of a nearby supernova, the shock wave of which will collide with the molecular cloud at enormous speed. It is also possible that galaxies collide, which could cause a burst of star formation as the gas clouds in each galaxy are compressed by the collision. In general, any inhomogeneities in the forces acting on the mass of the cloud can initiate the process of star formation.

Due to the inhomogeneities that have arisen, the pressure of the molecular gas can no longer prevent further compression, and the gas begins to gather around the center of the future star under the influence of gravitational attraction forces. Half of the released gravitational energy goes to heating the cloud, and half goes to light radiation. In clouds, pressure and density increase towards the center, and the collapse of the central part occurs faster than the periphery. As it contracts, the mean free path of photons decreases, and the cloud becomes less and less transparent to its own radiation. This leads to more rapid growth temperature and an even faster increase in pressure. As a result, the pressure gradient balances gravitational force, a hydrostatic core is formed, weighing about 1% of the mass of the cloud. This moment is invisible. The further evolution of the protostar is the accretion of matter that continues to fall onto the “surface” of the core, which due to this grows in size. The mass of freely moving matter in the cloud is exhausted and the star becomes visible in the optical range. This moment is considered the end of the protostellar phase and the beginning of the young star phase.

The lifespan of stars consists of several stages, passing through which for millions and billions of years the luminaries steadily strive towards the inevitable finale, turning into bright flares 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 modern science. But based on the unique knowledge accumulated and processed over the entire period of the existence of terrestrial astronomy, whole layers of the most valuable information become available to us. This makes it possible to link the sequence of episodes from the life cycle of 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 with birth. This event originates in the formation of an incredibly huge cloud, within 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 that have mass occurs, i.e., gravitational collapse, does a future star begin to form. The reason for this is a surge of gravitational energy, part of which compresses gas molecules and heats up the mother 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 star 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 comes from images. 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 catalogue. 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: young stars of minor (<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 a star's life

The sun photographed in the H alpha line. Our star is in his prime.

In the middle of their lives, cosmic luminaries can have a wide variety of colors, masses and dimensions. The color palette varies from bluish shades to red, and their mass can be significantly less than the sun's mass, or more than three hundred times greater. The main sequence of the life cycle of stars lasts about ten billion years. After which the core of the cosmic body runs out of hydrogen. This moment is considered to be the transition of the object’s life to the next stage. Due to the depletion of hydrogen resources in the core, thermonuclear reactions stop. However, during the period of renewed compression of the star, collapse begins, which leads to the occurrence of thermonuclear reactions with the participation of helium. This process stimulates a simply incredible expansion of the star. And now it is considered a red giant.

Episode IV. The end of the existence of stars and their death

Old stars, like their young counterparts, are divided into several types: low-mass, medium-sized, supermassive stars, and. As for objects with low mass, it is still impossible to say exactly what processes occur 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 star’s atmospheric envelope increases and its gas component rapidly loses. At the end of their evolutionary path, the stars are compressed many times, and their density, on the contrary, increases significantly. Such a star is considered to be a white dwarf. Its life phase is then followed by a red supergiant period. The last thing in the life cycle of a star is its transformation, as a result of very strong compression, into a neutron star. However, not all such cosmic bodies become like this. Some, most often the largest in parameters (more than 20-30 solar masses), become black holes as a result of collapse.

Interesting facts about the life cycles of stars

One of the most peculiar and remarkable information from the stellar life of space 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 earth’s star.

Effect of mass on a star

Another equally interesting fact is the duration of existence of the largest known types of stars. Due to the fact that their mass can be hundreds of times greater than that of the sun, their energy release is also many times greater, sometimes even millions of times. Consequently, their life span is much shorter. In some cases, their existence lasts only a few million years, compared to the billions of years of life of low-mass stars.

An interesting fact is also the contrast between black holes and 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.

There are a huge number of unique phenomena in the Universe that we can talk about endlessly, because space is extremely poorly studied and explored. All human knowledge about stars and their life cycles that modern science possesses is mainly derived from observations and theoretical calculations. Such little-studied phenomena and objects provide the basis for constant work for thousands of researchers and scientists: astronomers, physicists, mathematicians, and chemists. Thanks to their continuous work, this knowledge is constantly accumulated, supplemented and changed, thus becoming more accurate, reliable and comprehensive.