Evolution of stars of different masses. The evolution of stars from the point of view of exact science and the theory of relativity

25 nearest stars mV - visual magnitude; r is the distance to the star, pc; L is the luminosity (radiation power) of the star, expressed in units of the luminosity of the Sun (3.86–1026

Types of Stars Compared to other stars in the Universe, the Sun is a dwarf star and belongs to the category of normal stars, in the depths of which hydrogen is converted into helium. One way or another, but the types of stars roughly describe the life cycle of one separately

"LIFE WORLD" (Lebenswelt) is one of the central concepts of Husserl's late phenomenology, formulated by him as a result of overcoming the narrow horizon of a strictly phenomenological method by addressing the problems of world connections of consciousness. Such inclusion of the "global"

Life cycle of a virus Each virus enters a cell in its own unique way. Having penetrated, he must first of all take off his outer clothing in order to expose, at least partially, his nucleic acid and start copying it. The work of the virus is well organized.

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 incredibly long and difficult process accompanied by phenomena of a cosmic scale. Its versatility is simply impossible to fully trace and study, even using the entire arsenal 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 large 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 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.

Stellar evolution in astronomy is the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years, while it radiates light and heat. during such colossal periods of time, the changes are very significant.

The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in the galaxy actually contains 0.1 to 1 molecule per cm3. A molecular cloud, on the other hand, 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 across.

The evolution of a star begins in a giant molecular cloud, also called a stellar cradle.

As long as the cloud circulates freely around the center of the native galaxy, nothing happens. However, due to the inhomogeneity of the gravitational field, disturbances can arise in it, leading to local mass concentrations. Such perturbations cause the gravitational collapse of the cloud. One of the scenarios leading to this is the collision of two clouds. Another collapse-causing event could be the passage of a cloud through the dense arm of a spiral galaxy. Also a critical factor may be the explosion of a nearby supernova, the shock wave of which will collide with the molecular cloud at great speed. In addition, a collision of galaxies is possible, capable of causing a burst of star formation, as the gas clouds in each of the galaxies are compressed by the collision. In general, any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.

any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.

In the course of this process, the inhomogeneities of the molecular cloud will be compressed under the influence of their own gravity and gradually take the shape of a ball. When compressed, the gravitational energy is converted into heat, and the temperature of the object increases.

When the temperature in the center reaches 15–20 million K, thermonuclear reactions begin and the compression stops. The object becomes a full-fledged star.

The subsequent stages of a star's evolution depend almost entirely on its mass, and only at the very end of a star's evolution can its chemical composition play its role.

The first stage of a star's life is similar to that of the sun - it is dominated by the 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, until the fuel reserves in its core run out. When all the hydrogen in the center of the star turns into helium, a helium core is formed, and the thermonuclear combustion of hydrogen continues on the periphery of the core.

Small and cold red dwarfs slowly burn their hydrogen reserves and remain on the main sequence for tens of billions of years, while massive supergiants leave the main sequence after only a few tens of millions (and some only a few million) years after formation.

At present, it is not known for certain what happens to light stars after the depletion of the supply of hydrogen in their interiors. Since the universe is 13.8 billion years old, which is not enough to deplete the supply of hydrogen fuel in such stars, modern theories are based on computer simulation of the processes occurring in such stars.

According to theoretical concepts, some of the light stars, losing their substance (stellar wind), will gradually evaporate, becoming smaller and smaller. Others, red dwarfs, will slowly cool down over billions of years, continuing to radiate weakly in the infrared and microwave ranges of the electromagnetic spectrum.

Medium-sized stars like the Sun stay on the main sequence for an average of 10 billion years.

It is believed that the Sun is still on it, as it is in the middle of its life cycle. As soon as the star depletes the supply of hydrogen in the core, it leaves the main sequence.

As soon as the star depletes the supply of hydrogen in the core, it leaves the main sequence.

Without the pressure generated by the fusion reactions to balance the internal gravity, the star begins to contract again, as it did earlier in the process of its formation.

The temperature and pressure rise again, but, unlike in the protostar stage, to a much higher level.

The collapse continues until, at a temperature of approximately 100 million K, thermonuclear reactions involving helium begin, during which helium is converted into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally silicon to iron).

The collapse continues until, at a temperature of approximately 100 million K, thermonuclear reactions involving helium begin.

The thermonuclear "burning" of matter resumed at a new level causes a monstrous expansion of the star. The star "swells up", becoming very "loose", and its size increases by about 100 times.

The star becomes a red giant, and the helium burning phase continues for about several million years.

What happens next also depends on the mass of the star.

By the stars medium size the reaction of thermonuclear combustion of helium can lead to an explosive ejection of the outer layers of a star with the formation of planetary nebula. The core of the star, in which thermonuclear reactions stop, cools down and turns into a helium white dwarf, as a rule, having a mass of up to 0.5-0.6 solar masses and a diameter of the order of the diameter of the Earth.

For massive and supermassive stars (with a mass of five solar masses or more), the processes occurring in their core, as gravitational compression increases, lead to an explosion supernova with the release of enormous energy. The explosion is accompanied by the ejection of a significant mass of the star's matter into interstellar space. This substance is further involved in the formation of new stars, planets or satellites. It is thanks to supernovae that the Universe as a whole and each galaxy in particular chemically evolves. The core of the star left after the explosion can end its evolution as a neutron star (pulsar), if the mass of the star in the later stages exceeds the Chandrasekhar limit (1.44 solar masses), or as a black hole, if the mass of the star exceeds the Oppenheimer-Volkov limit (estimated values ​​2 ,5-3 solar masses).

The process of stellar evolution in the Universe is continuous and cyclical - old stars die out, new ones are lit to replace them.

According to modern scientific concepts, the elements necessary for the emergence of planets and life on Earth were formed from stellar matter. Although there is no single generally accepted point of view on how life arose.

Stars, as you know, get their energy from thermonuclear fusion reactions, and sooner or later every star has a moment when thermonuclear fuel comes to an end. The higher the mass of a star, the faster it burns everything it can and goes to the final stage of its existence. Further events can go according to different scenarios, which one - first of all depends again on the mass.
At the time when the hydrogen in the center of the star “burns out”, a helium core is released in it, which contracts and releases energy. In the future, combustion reactions of helium and subsequent elements may begin in it (see below). The outer layers increase many times under the influence of increased pressure coming from the heated core, the star becomes a red giant.
Depending on the mass of the star, different reactions can take place in it. This determines what composition the star will have by the time the fusion fades.

white dwarfs

For stars with masses up to about 10 MC, the core weighs less than 1.5 MC. After the completion of thermonuclear reactions, the radiation pressure stops, and the nucleus begins to shrink under the influence of gravity. It is compressed until the pressure of the degenerate electron gas, due to the Pauli principle, begins to interfere. The outer layers are shed and dissipate, forming a planetary nebula. The first such nebula was discovered by French astronomer Charles Messier in 1764 and cataloged as M27.
What came out of the core is called a white dwarf. White dwarfs have a density greater than 10 7 g/cm 3 and a surface temperature of about 10 4 K. The luminosity is 2-4 orders of magnitude lower than that of the Sun. Thermonuclear fusion does not take place in it, all the energy emitted by it was accumulated earlier. Thus, white dwarfs slowly cool down and cease to be visible.
A white dwarf still has a chance to be active if it is part of a binary star and draws the mass of a companion onto itself (for example, the companion has become a red giant and filled its entire Roche lobe with its mass). In this case, either hydrogen synthesis can begin in the CNO cycle using the carbon contained in the white dwarf, ending with the shedding of the outer hydrogen layer (“new” star). Or the mass of a white dwarf can grow so much that its carbon-oxygen component will light up, a wave of explosive combustion coming from the center. As a result, heavy elements are formed with the release of a large amount of energy:

12 С + 16 O → 28 Si + 16.76 MeV
28 Si + 28 Si → 56 Ni + 10.92 MeV

The luminosity of the star increases strongly for 2 weeks, then rapidly decreases for another 2 weeks, after which it continues to fall by about 2 times in 50 days. The main energy (about 90%) is emitted in the form of gamma quanta from the nickel isotope decay chain. This phenomenon is called a type 1 supernova.
There are no white dwarfs with a mass of 1.5 or more solar masses. This is explained by the fact that for the existence of a white dwarf, it is necessary to balance the gravitational compression with the pressure of the electron gas, but this happens at masses no more than 1.4 M C , this limitation is called the Chandrasekhar limit. The value can be obtained as a condition of equality of pressure forces to gravitational contraction forces under the assumption that the momenta of electrons are determined by the uncertainty relation for the volume they occupy, and they move at a speed close to the speed of light.

neutron stars

In the case of more massive (> 10 M C) stars, things happen a little differently. The high temperature in the core activates energy-absorbing reactions, such as knocking out protons, neutrons and alpha particles from the nuclei, as well as e-capture of high-energy electrons that compensate for the mass difference two cores. The second reaction creates an excess of neutrons in the nucleus. Both reactions lead to its cooling and general contraction of the star. When the energy of nuclear fusion ends, the contraction turns into an almost free fall of the shell onto the contracting core. This sharply accelerates the rate of fusion in the outer falling layers, which leads to the emission of a huge amount of energy in a few minutes (comparable to the energy that light stars emit in their entire existence).
Due to the high mass, the collapsing nucleus overcomes the pressure of the electron gas and contracts further. In this case, reactions p + e - → n + ν e occur, after which there are almost no electrons that interfere with compression in the nucleus. Compression occurs to sizes of 10 − 30 km, corresponding to the density determined by the pressure of the neutron degenerate gas. The matter falling on the nucleus receives the shock wave reflected from the neutron nucleus and part of the energy released during its compression, which leads to a rapid ejection of the outer shell to the sides. The resulting object is called a neutron star. Most (90%) of the energy released from gravitational contraction is carried away by neutrinos in the first seconds after the collapse. The above process is called a Type II supernova explosion. The energy of the explosion is such that some of them are (rarely) visible to the naked eye even in daytime. The first supernova was recorded by Chinese astronomers in 185 AD. Currently, several hundred outbreaks are recorded per year.
The resulting neutron star has a density ρ ~ 10 14 − 10 15 g/cm 3 . The conservation of angular momentum during the contraction of the star leads to very short revolution periods, usually in the range from 1 to 1000 ms. For ordinary stars, such periods are impossible, because Their gravity will not be able to counteract the centrifugal forces of such rotation. A neutron star has a very large magnetic field, reaching 10 12 -10 13 gauss at the surface, which results in strong electromagnetic radiation. A magnetic axis that does not coincide with the axis of rotation leads to the fact that a neutron star sends periodic (with a rotation period) pulses of radiation in a given direction. Such a star is called a pulsar. This fact helped their experimental discovery and is being used for discovery. It is much more difficult to detect a neutron star by optical methods due to its low luminosity. The period of revolution gradually decreases due to the transition of energy into radiation.
The outer layer of a neutron star is composed of crystalline matter, mainly iron and its neighboring elements. Most of the rest of the mass is neutrons, pions and hyperons can be in the very center. The density of the star increases towards the center and can reach values ​​much greater than the density of nuclear matter. The behavior of matter at such densities is poorly understood. There are theories about free quarks, including not only the first generation, at such extreme densities of hadronic matter. Superconducting and superfluid states of neutron matter are possible.
There are 2 mechanisms for cooling a neutron star. One of them is the emission of photons, as everywhere else. The second mechanism is neutrino. It prevails as long as the core temperature is above 10 8 K. It usually corresponds to a surface temperature above 10 6 K and lasts 10 5 −10 6 years. There are several ways to emit neutrinos:

Black holes

If the mass of the original star exceeded 30 solar masses, then the core formed in the supernova explosion will be heavier than 3 M C . With such a mass, the pressure of the neutron gas can no longer restrain gravity, and the core does not stop at the stage of a neutron star, but continues to collapse (nevertheless, experimentally discovered neutron stars have masses no more than 2 solar masses, not three). This time, nothing will prevent the collapse, and a black hole is formed. This object has a purely relativistic nature and cannot be explained without GR. Despite the fact that the matter, according to the theory, collapsed into a point - a singularity, a black hole has a non-zero radius, called the Schwarzschild radius:

R W \u003d 2GM / c 2.

The radius denotes the boundary of the gravitational field of a black hole, which is insurmountable even for photons, called the event horizon. For example, the Schwarzschild radius of the Sun is only 3 km. Outside the event horizon, a black hole's gravitational field is the same as that of an ordinary object of its mass. A black hole can only be observed by indirect effects, since it itself does not radiate any noticeable energy.
Despite the fact that nothing can leave the event horizon, a black hole can still create radiation. In the quantum physical vacuum, virtual particle-antiparticle pairs are constantly born and disappear. The strongest gravitational field of a black hole can interact with them before they disappear and absorb the antiparticle. In the event that the total energy of the virtual antiparticle was negative, the black hole loses mass, and the remaining particle becomes real and receives energy sufficient to fly away from the black hole field. This radiation is called Hawking radiation and has a black body spectrum. It can be assigned a certain temperature:

The influence of this process on the mass of most black holes is negligible compared to the energy they receive even from the CMB. The exception is relic microscopic black holes, which could have formed in the early stages of the evolution of the Universe. Small sizes speed up the evaporation process and slow down the mass gain process. The last stages of evaporation of such black holes must end in an explosion. No explosions matching the description have ever been recorded.
Matter falling into a black hole heats up and becomes a source of x-rays, which serves as an indirect sign of the presence of a black hole. When matter with a large angular momentum falls into a black hole, it forms a rotating accretion disk around it, in which particles lose energy and angular momentum before falling into the black hole. In the case of a supermassive black hole, there are two distinct directions along the axis of the disk, in which the pressure of the emitted radiation and electromagnetic effects accelerate the particles knocked out of the disk. This creates powerful jets of matter in both directions, which can also be registered. According to one theory, this is how the active nuclei of galaxies and quasars are arranged.
A spinning black hole is a more complex object. With its rotation, it “captures” a certain region of space beyond the event horizon (“Lense-Thirring effect”). This area is called the ergosphere, its boundary is called the static limit. The static limit is an ellipsoid coinciding with the event horizon at the two poles of the black hole's rotation.
Rotating black holes have an additional mechanism of energy loss through its transfer to particles that have fallen into the ergosphere. This loss of energy is accompanied by a loss of angular momentum and slows down the rotation.

Bibliography

1. S.B. Popov, M.E. Prokhorov "Astrophysics of single neutron stars: radio-quiet neutron stars and magnetars" SAI MSU, 2002
2. William J. Kaufman "The Cosmic Frontiers of Relativity" 1977
3. Other Internet sources

December 20 10 y.

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

Encyclopedic YouTube

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

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✪ Surdin V.G. Star Evolution Part 1

✪ S. A. Lamzin - "Star 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 the evolution of a star depend almost entirely on its mass, and only at the very end of the star's evolution can its chemical composition play a role.

Young low mass stars

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

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

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

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

Young stars of intermediate mass

Young stars of intermediate mass (from 2 to 8 solar masses) [ ] evolve qualitatively in exactly the same way as their smaller sisters and brothers, 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 are irregular variables of spectral type B-F0. They also have discs and bipolar jets. The rate of outflow of matter from the surface, the luminosity, and the effective temperature are significantly higher than for T Tauri, so they effectively heat and disperse the remnants of the protostellar cloud.

Young stars with a mass greater than 8 solar masses

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

mid-life cycle of a star

Stars come in a wide variety of colors and sizes. They range in spectral type from hot blues to cool reds, and in mass from 0.0767 to about 300 solar masses, according to recent estimates. The luminosity and color of a star depend on the temperature of its surface, which, in turn, is determined by its mass. All new stars "take their place" on the main sequence according to their chemical composition and mass. This, of course, is not about the physical movement of the star - only about its position on the indicated diagram, which depends on the parameters of the star. In fact, the movement of a star along the diagram corresponds only to a change in the parameters of the star.

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

Final stages of stellar evolution

Old stars with low mass

At present, it is not known for certain what happens to light stars after the depletion of the supply of hydrogen in their interiors. Because the age of the Universe is 13.7 billion years, which is not enough to deplete the supply of hydrogen fuel in such stars, current theories are based on computer simulations of the processes occurring in such stars.

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

A star with a mass of less than 0.5 solar mass is not able to convert helium even after reactions involving hydrogen cease in its core - the mass of such a star is too small to provide a new phase of gravitational compression to a degree sufficient for "ignition" helium. These stars include red dwarfs, such as Proxima Centauri, whose main sequence lifespan ranges from tens of billions to tens of trillions of years. After the termination of thermonuclear reactions in their nuclei, they, gradually cooling down, will continue to radiate weakly in the infrared and microwave ranges of the electromagnetic spectrum.

medium sized stars

Upon reaching a medium-sized star (from 0.4 to 3.4 solar masses) [ ] of the red giant phase, hydrogen ends in its core, and the reactions of carbon synthesis from helium begin. This process occurs at higher temperatures and therefore the energy flux from the core increases and, as a result, the outer layers of the star begin to expand. The beginning of carbon synthesis marks a new stage in the life of a star and continues for some time. For a star close to the size of the Sun, this process can take about a billion years.

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

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

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

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

supermassive stars

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

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

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

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

blast wave and jets of neutrinos carry matter away from a dying star [ ] into interstellar space. Subsequently, as it cools and travels through space, this supernova material may collide with other space junk and possibly participate in the formation of new stars, planets, or satellites.

The processes that take place during the formation of a supernova are still being studied, and so far this issue is not clear. Also in question is the moment what actually remains of the original star. However, two options are being considered: neutron stars and black holes.

neutron stars

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

Such stars, known as neutron stars, are extremely small - no more than big 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 make 600 revolutions per second. For some of them, the angle between the radiation vector and the axis of rotation may be such that the Earth falls into the cone formed by this radiation; in this case, it is possible to record a radiation pulse that repeats at time intervals equal to the rotation period of the star. Such neutron stars were called "pulsars", and became the first discovered neutron stars.

Black holes

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

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