On what parameter does the evolutionary path of a star depend? The process of studying and the scheme of the evolution of stars. Deep in the bowels of a shrinking region begins the evolution of stars

If enough matter accumulates somewhere in the Universe, it shrinks into a dense lump, in which a thermonuclear reaction begins. This is how stars light up. The first flared up in the darkness of the young Universe 13.7 billion (13.7 * 10 9) years ago, and our Sun - only some 4.5 billion years ago. The lifetime of a star and the processes that occur at the end of this period depend on the mass of the star.

As long as the thermonuclear reaction of converting hydrogen into helium continues in the star, it is on the main sequence. The time a star spends on the main sequence depends on the mass: the largest and heaviest ones quickly reach the stage of a red giant, and then leave the main sequence as a result of a supernova explosion or the formation of a white dwarf.

The fate of the giants

The largest and most massive stars burn out quickly and explode in supernovae. After a supernova explosion, a neutron star or a black hole remains, and around them is matter ejected by the colossal energy of the explosion, which then becomes the material for new stars. Of our closest stellar neighbors, such a fate awaits, for example, Betelgeuse, but when it explodes, it is impossible to calculate.

A nebula formed by the ejection of matter from a supernova explosion. At the center of the nebula is a neutron star.

The neutron star is a terrible physical phenomenon. The core of an exploding star is compressed - much like the gas in an internal combustion engine, only in a very large and efficient one: a ball with a diameter of hundreds of thousands of kilometers turns into a ball from 10 to 20 kilometers in diameter. The compression force is so great that the electrons fall on the atomic nuclei, forming neutrons - hence the name.

NASA Neutron star (artist's vision)

The density of matter under such compression increases by about 15 orders of magnitude, and the temperature rises to unimaginable 10 12 K at the center of the neutron star and 1,000,000 K at the periphery. Some of this energy is emitted in the form of photon radiation, and some is carried away by the neutrinos that form in the core of the neutron star. But even due to very effective neutrino cooling, a neutron star cools very slowly: it takes 10 16 or even 10 22 years to completely exhaust the energy. It is difficult to say what will remain in the place of a cooled neutron star, but it is impossible to observe: the world is too young for this. There is an assumption that a black hole is again formed in the place of a cooled star.

Black holes are created by the gravitational collapse of very massive objects, such as supernova explosions. Perhaps in trillions of years, cooled neutron stars will turn into black holes.

The fate of medium scale stars

Other, less massive stars stay on the main sequence longer than the largest ones, but when they leave it, they die much faster than their neutron relatives. More than 99% of the stars in the Universe will never explode and will not turn into either black holes or neutron stars - their cores are too small for such cosmic dramas. Instead, medium-mass stars turn into red giants at the end of their lives, which, depending on the mass, turn into white dwarfs, explode, completely dissipate, or become neutron stars.

White dwarfs now make up 3 to 10% of the stellar population of the universe. Their temperature is very high - more than 20,000 K, more than three times the temperature of the surface of the Sun - but still less than that of neutron stars, and due to the lower temperature and larger area white dwarfs cool faster - in 10 14 - 10 15 years. This means that in the next 10 trillion years - when the universe will be a thousand times older than it is now - there will appear in the universe new type object: black dwarf, cooling product of a white dwarf.

So far, there are no black dwarfs in space. Even the oldest cooling stars to date have lost a maximum of 0.2% of their energy; for a white dwarf with a temperature of 20,000 K, this means cooling down to 19,960 K.

For the little ones

Even less is known about what happens when the smallest stars, such as our nearest neighbor, the red dwarf Proxima Centauri, cool down than about supernovae and black dwarfs. Thermonuclear fusion in their cores is slow, and they remain on the main sequence longer than the others - according to some calculations, up to 10 12 years, and after that, presumably, they will continue their lives as white dwarfs, that is, they will shine for another 10 14 - 10 15 years before the transformation into a black dwarf.

Thermonuclear fusion in the interior of stars

At this time, for stars with a mass greater than 0.8 solar masses, the core becomes transparent to radiation, and radiative energy transfer in the core will prevail, while the shell at the top remains convective. No one knows for sure what kind of stars of smaller mass arrive on the main sequence, since the time these stars spend in the category of young ones exceeds the age of the Universe. All our ideas about the evolution of these stars are based on numerical calculations.

As the star shrinks, the pressure of the degenerate electron gas begins to increase, and at some radius of the star, this pressure stops the growth of the central temperature, and then begins to lower it. And for stars less than 0.08, this turns out to be fatal: the energy released during nuclear reactions never enough to cover the cost of radiation. Such under-stars are called brown dwarfs, and their fate is a constant contraction until the pressure of the degenerate gas stops it, and then a gradual cooling with a stop to all nuclear reactions.

Young stars of intermediate mass

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

Objects of this type are associated with the so-called. Ae\Be Herbit stars are irregular variables of spectral type B-F5. They also have bipolar jet disks. The exhaust velocity, luminosity, and effective temperature are substantially greater than for τ Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.

Young stars with a mass greater than 8 solar masses

In fact, these are already normal stars. While the mass of the hydrostatic core was accumulating, the star managed to skip all the intermediate stages and heat up the nuclear reactions to such an extent that they compensate for the losses due to radiation. For these stars, the outflow of mass and luminosity is so high that it not only stops the collapse of the remaining outer regions, but pushes them back. Thus, the mass of the formed star is noticeably less mass protostellar cloud. Most likely, this explains the absence in our galaxy of stars more than 100-200 solar masses.

mid-life cycle of a star

Among the formed stars there is a huge variety of colors and sizes. They range in spectral type from hot blues to cool reds, and in mass from 0.08 to more than 200 solar masses. The luminosity and color of a star depends on the temperature of its surface, which, in turn, is determined by its mass. Everything, new stars "take their place" on the main sequence according to their chemical composition and mass. We are not talking about the physical movement of the star - only about its position on the indicated diagram, which depends on the parameters of the star. That is, we are talking, in fact, only about changing the parameters of the star.

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

Later years and the death of stars

Old stars with low mass

To date, it is not known for certain what happens to light stars after the depletion of the hydrogen supply. Since the universe is 13.7 billion years old, which is not enough to deplete the supply of hydrogen fuel, current theories are based on computer simulations of the processes occurring in such stars.

Some stars can only fuse helium in certain active regions, which causes instability and strong solar winds. In this case, the formation of a planetary nebula does not occur, and the star only evaporates, becoming even smaller than a brown dwarf.

But a star with a mass of less than 0.5 solar mass will never be able to synthesize helium even after reactions involving hydrogen cease in the core. Their stellar shell is not massive enough to overcome the pressure produced by the core. Such stars include red dwarfs (such as Proxima Centauri), whose main sequence lifetimes are hundreds of billions of years. After the termination of thermonuclear reactions in their core, they, gradually cooling down, will continue to weakly radiate in the infrared and microwave ranges of the electromagnetic spectrum.

medium sized stars

When a star reaches an average size (from 0.4 to 3.4 solar masses) of the red giant phase, its outer layers continue to expand, the core contracts, and reactions of carbon synthesis from helium begin. The fusion releases a lot of energy, giving the star a temporary reprieve. For a star similar in size to the Sun, this process can take about a billion years.

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

Helium combustion reactions are very sensitive to temperature. Sometimes this leads to great instability. Violent pulsations occur, which eventually impart enough kinetic energy to the outer layers to be ejected and become a planetary nebula. In the center of the nebula, the core of the star remains, which, cooling down, turns into a helium white dwarf, as a rule, having a mass of up to 0.5-0.6 solar and a diameter of the order of the diameter of the Earth.

white dwarfs

The vast majority of stars, including the Sun, end their evolution by shrinking 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 that of water, the star is called a white dwarf. It is deprived of sources of energy and, gradually cooling down, becomes dark and invisible.

In stars more massive than the Sun, the pressure of degenerate electrons cannot contain the compression of the core, and it continues until most of the particles turn into neutrons, packed so densely that the size of the star is measured in kilometers, and the density is 100 million times greater than the density water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.

supermassive stars

After the outer layers of the star, with a mass greater than five solar masses, have scattered to form a red supergiant, the core begins to shrink due to gravitational forces. As the compression increases, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, heavy elements are synthesized, which temporarily restrains the collapse of the nucleus.

Ultimately, as more and more heavy elements of the periodic system are formed, iron -56 is synthesized from silicon. Up to this point, the synthesis of elements released a large number of energy, however, it is the iron -56 nucleus that has the maximum mass defect and the formation of heavier nuclei is unfavorable. Therefore, when the iron core of a star reaches a certain value, the pressure in it is no longer able to withstand the colossal force of gravity, and an immediate collapse of the core occurs with the neutronization of its matter.

What happens next is not entirely clear. But whatever it is, in a matter of seconds, it leads to the explosion of a supernova of incredible force.

The accompanying burst of neutrinos provokes a shock wave. 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 escaping from the nucleus, 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.

The blast wave and jets of neutrinos carry material away from the dying star and into interstellar space. Subsequently, moving through space, this supernova material can collide with other space debris, 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. It is also questionable what actually remains of the original star. However, two options are being considered:

neutron stars

In some supernovae, the strong gravity in the supergiant's interior is known to cause electrons to fall into the atomic nucleus, where they fuse with protons to form neutrons. The electromagnetic forces separating nearby nuclei disappear. The core of the 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 conservation of angular momentum). Some make 600 revolutions per second. When the axis connecting the north and south magnetic pole of this rapidly rotating star, points to the Earth, it is possible to fix a radiation pulse that repeats at intervals equal to the period of revolution of the star. Such neutron stars were called "pulsars", and became the first discovered neutron stars.

Black holes

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

The existence of black holes was predicted by the general theory of relativity. According to general relativity, matter and information cannot leave a black hole under any circumstances. However, quantum mechanics makes exceptions to this rule possible.

A number of open questions remain. Chief among them: "Are there any black holes at all?" Indeed, in order to say for sure that a given object is a black hole, it is necessary to observe its event horizon. All attempts to do so ended in failure. But there is still hope, since some objects cannot be explained without involving accretion, moreover, accretion onto an object without a solid surface, but the very existence of black holes does not prove this.

Questions are also open: is it possible for a star to collapse directly into a black hole, bypassing a supernova? Are there supernovae that will eventually become black holes? What is the exact influence of the initial mass of a star on the formation of objects at the end of its life cycle?

Studying stellar evolution it is impossible to observe only one star - many changes in the 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

1 / 5

✪ Stars and stellar evolution (says astrophysicist Sergey Popov)

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

✪ Star evolution. The evolution of the blue giant in 3 minutes

✪ 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 contraction. Until hydrostatic equilibrium is established, the luminosity of the star 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 contraction stops, which leads to a halt in the further temperature increase in the star's core caused by the contraction, and then to its decrease. For stars less than 0.0767 solar masses, this does not happen: the energy released during nuclear reactions will never be enough to balance the internal pressure and 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 Taurus , so they effectively heat and scatter the remnants of the protostellar cloud.

Young stars with a mass greater than 8 solar masses

Stars with such masses already have the characteristics normal stars, because they went through all the intermediate stages and were able to achieve such a rate of nuclear reactions that compensated for the loss of energy for radiation, while mass was accumulated to achieve hydrostatic equilibrium of the nucleus. For these stars, the outflow of mass and luminosity are so great that they not only stop the gravitational collapse of the outer regions that have not yet become part of the star molecular cloud, 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. Since 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 lifetime 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 weakly radiate 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 takes place 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 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 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 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 Earth's diameter.

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 “scrap” 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 larger than a major city - and have unimaginably high densities. 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,

The lifetime of stars consists of several stages, passing through which for millions and billions of years the luminaries are steadily striving for the inevitable finale, turning into bright flashes or gloomy black holes.

The lifetime of a star of any type is an incredibly long and complex process, accompanied by phenomena on a cosmic scale. Its versatility is simply impossible to fully trace and study, even using the entire arsenal 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 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 is derived from imagery. This nebula is one of the largest and closest cradles of stars to us. Try to find protostars in this nebula, since the resolution of this panorama allows you to do this.

Episode II. young stars

Fomalhaut, image from the DSS catalog. There is still a protoplanetary disk around this star.

The next stage or cycle of a star's life is the period of its cosmic childhood, which, in turn, is divided into three stages: the young luminaries of the small (<3), промежуточной (от 2 до 8) и массой больше восьми солнечных единиц. На первом отрезке образования подвержены конвекции, которая затрагивает абсолютно все области молодых звезд. На промежуточном этапе такое явление не наблюдается. В конце своей молодости объекты уже во всей полноте наделены качествами, присущими взрослой звезде. Однако любопытно то, что на данной стадии они обладают колоссально сильной светимостью, которая замедляет или полностью прекращает процесс коллапса в еще не сформировавшихся солнцах.

Episode III. The heyday of the life path of a star

Sun shot in H line alpha. Our star is in its prime.

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

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

Old luminaries, like their young counterparts, are divided into several types: low-mass, medium-sized, supermassive stars, and. As for objects with a small mass, it is still impossible to say exactly what processes take place with them in the last stages of existence. All such phenomena are hypothetically described using computer simulations, and not based on careful observations of them. After the final burnout of carbon and oxygen, the atmospheric shell of the star increases and its gas component rapidly loses. At the end of their evolutionary path, the luminaries are repeatedly compressed, while their density, on the contrary, increases significantly. Such a star is considered to be a white dwarf. Then, in its life phase, the period of a red supergiant follows. The last in the life cycle of a star is its transformation, as a result of a very strong compression, into a neutron star. However, not all such cosmic bodies become such. Some, most often the largest in terms of parameters (more than 20-30 solar masses), pass into the category of black holes as a result of collapse.

Interesting facts from the life cycles of stars

One of the most peculiar and remarkable information from the stellar life of the cosmos is that the vast majority of the luminaries in ours are at the stage of red dwarfs. Such objects have a mass much less than that of the Sun.

It is also quite interesting that the magnetic attraction of neutron stars is billions of times higher than the similar radiation of the earthly body.

Effect of mass on a star

Another no less entertaining fact is the duration of the existence of the largest known types of stars. Due to the fact that their mass is capable of hundreds of times greater than the solar mass, their release of energy is also many times greater, sometimes even millions of times. Consequently, their life span is much shorter. In some cases, their existence fits into just a few million years, against the billions of years of the life of stars with a small mass.

An interesting fact is also the opposite of black holes to white dwarfs. It is noteworthy that the former arise from the most gigantic stars in terms of mass, and the latter, on the contrary, from the smallest.

In the Universe there is a huge number of unique phenomena that can be talked about endlessly, because the cosmos is extremely poorly studied and explored. All human knowledge about stars and their life cycles, which modern science has, is mainly obtained from observations and theoretical calculations. Such little-studied phenomena and objects give rise to constant work for thousands of researchers and scientists: astronomers, physicists, mathematicians, chemists. Thanks to their continuous work, this knowledge is constantly accumulated, supplemented and changed, thus becoming more accurate, reliable and comprehensive.

Contemplating the clear night sky away from city lights, it is easy to see that the universe is full of stars. How did nature manage to create a myriad of these objects? After all, according to estimates, there are about 100 billion stars in the Milky Way alone. In addition, stars are still being born today, 10-20 billion years after the formation of the Universe. How are stars formed? What changes does a star undergo before it reaches a steady state, like our Sun?

From the point of view of physics, a star is a ball of gas

From the point of view of physics, it is a gas ball. The heat and pressure generated in nuclear reactions - mainly in the reactions of fusion of helium from hydrogen - prevent the star from collapsing under its own gravity. The life of this relatively simple object follows a well-defined scenario. First, a star is born from a diffuse cloud of interstellar gas, then there is a long doomsday. But eventually, when all the nuclear fuel is exhausted, it will turn into a faintly luminous white dwarf, neutron star or black hole.

This description may give the impression that a detailed analysis of the formation and early stages of stellar evolution should not cause significant difficulties. But the interplay of gravity and thermal pressure causes stars to behave in unpredictable ways.
Consider, for example, the evolution of luminosity, that is, the change in the amount of energy emitted by the stellar surface per unit time. The internal temperature of a young star is too low for the fusion of hydrogen atoms, so its luminosity must be relatively low. It can increase when nuclear reactions begin, and only then can it gradually fall. In fact, a very young star is extremely bright. Its luminosity decreases with age, reaching a temporary minimum during the burning of hydrogen.

In the early stages of evolution, various physical processes take place in stars.

In the early stages of evolution, a variety of physical processes take place in stars, some of which are still poorly understood. Only in the last two decades have astronomers begun to build a detailed picture of the evolution of stars on the basis of advances in theory and observation.
Stars are born from large, invisible clouds located in the disks of spiral galaxies. Astronomers call these objects giant molecular complexes. The term "molecular" reflects the fact that the gas in the complexes is primarily composed of hydrogen in molecular form. Such clouds are the largest formations in the Galaxy, sometimes reaching more than 300 sv. years across.

In a more thorough analysis of the evolution of the star

A closer analysis reveals that stars form from individual condensations—compact zones—in a giant molecular cloud. Astronomers have studied the properties of compact zones with large radio telescopes, the only instruments capable of detecting faint millimoclouds. It follows from observations of this radiation that a typical compact zone has a diameter of several light months, a density of 30,000 hydrogen molecules per cm^, and a temperature of 10 Kelvin.
Based on these values, it was concluded that the pressure of the gas in compact zones is such that it can withstand compression under the action of self-gravitational forces.

Therefore, in order for a star to form, the compact zone must contract from an unstable state, such that the gravitational forces exceed the internal gas pressure.
It is not yet clear how compact zones condense from the initial molecular cloud and acquire such an unstable state. Nevertheless, even before the discovery of compact zones, astrophysicists had the opportunity to simulate the process of star formation. As early as the 1960s, theorists used computer simulations to determine how clouds compress in an unstable state.
Although a wide range of initial conditions was used for theoretical calculations, the results obtained coincided: for a cloud that is too unstable, the inner part contracts first, that is, the substance in the center is subjected to free fall first, while the peripheral regions remain stable. Gradually, the compression region expands outward, covering the entire cloud.

Deep in the bowels of a shrinking region begins the evolution of stars

Deep in the bowels of the shrinking region, star formation begins. The diameter of a star is only one light second, that is, one millionth of the diameter of the compact zone. For such relatively small sizes, the general pattern of cloud compression is not significant, and the main role here is played by the velocity of matter falling onto the star

The rate of fall of matter can be different, but it directly depends on the temperature of the cloud. The higher the temperature, the faster the speed. Calculations show that a mass equal to the mass of the Sun can accumulate in the center of a collapsing compact zone over a period of 100 thousand to 1 million years. A body formed in the center of a collapsing cloud is called a protostar. Using computer simulations, astronomers have developed a model that describes the structure of a protostar.
It turned out that the falling gas hits the surface of the protostar at a very high speed. Therefore, a powerful shock front is formed (a sharp transition to a very high pressure). Within the shock front, the gas heats up to almost 1 million Kelvin, then, during radiation near the surface, it rapidly cools to about 10,000 K, forming a protostar layer by layer.

The presence of a shock front explains the high brightness of young stars

The presence of a shock front explains the high brightness of young stars. If the mass of a protosis-star is equal to one solar mass, then its luminosity can exceed the solar one by ten times. But it is caused not by thermonuclear fusion reactions, as in ordinary stars, but by the kinetic energy of matter acquired in the gravitational field.
Protostars can be observed, but not with conventional optical telescopes.
All interstellar gas, including the one from which stars are formed, contains "dust" - a mixture of solid submicron particles. The radiation of the shock front encounters on its way a large number of these particles, which, together with the gas, fall on the surface of the protostar.
Cold dust particles absorb photons emitted by the shock front and re-emit them with longer wavelengths. This long-wavelength radiation is in turn absorbed and then re-emitted by even more distant dust. Therefore, while a photon makes its way through clouds of dust and gas, its wavelength is in the infrared range of the electromagnetic spectrum. But already at a distance of several light-hours from the protostar, the wavelength of the photon becomes too large, so that the dust cannot absorb it, and it can finally rush unhindered to Earth-sensitive telescopes that are sensitive to infrared radiation.
Despite the wide capabilities of modern detectors, astronomers cannot claim that telescopes actually register the radiation of protostars. Apparently, they are deeply hidden in the bowels of the compact zones registered in the radio range. The uncertainty in registration is due to the fact that detectors cannot distinguish a protostar from older stars interspersed in gas and dust.
For reliable identification, an infrared or radio telescope must detect a Doppler shift in the spectral emission lines of a protostar. The Doppler shift would show the true movement of the gas falling on its surface.
As soon as, as a result of the fall of matter, the mass of the protostar reaches several tenths of the mass of the Sun, the temperature in the center becomes sufficient for the start of thermonuclear fusion reactions. However, thermonuclear reactions in protostars are fundamentally different from reactions in middle-aged stars. The energy source of such stars is the reactions of thermonuclear fusion of helium from hydrogen.

Hydrogen is the most common chemical element in the universe

Hydrogen is the most abundant chemical element in the universe. At the birth of the Universe (Big Bang), this element was formed in its usual form with a nucleus consisting of one proton. But two out of every 100,000 nuclei are deuterium nuclei, made up of a proton and a neutron. This isotope of hydrogen is present in the modern era in the interstellar gas from which it enters the stars.
It is noteworthy that this meager admixture plays a dominant role in the life of protostars. The temperature in their depths is insufficient for the reactions of ordinary hydrogen, which occur at 10 million Kelvin. But as a result of gravitational compression, the temperature in the center of the protostar can easily reach 1 million Kelvin, when the fusion of deuterium nuclei begins, at which colossal energy is also released.

The opacity of protostellar matter is too great

The opacity of protostellar matter is too great for this energy to be transmitted by radiative transfer. Therefore, the star becomes convectively unstable: gas bubbles heated by "nuclear fire" float to the surface. These ascending flows are balanced by cold gas flows descending towards the center. Similar convective movements, but on a much smaller scale, take place in a steam-heated room. In a protostar, convective vortices carry deuterium from the surface to its interior. Thus, the fuel needed for thermonuclear reactions reaches the core of the star.
Despite the very low concentration of deuterium nuclei, the heat released during their merger has a strong effect on the protostar. The main consequence of deuterium combustion reactions is the "swelling" of the protostar. Due to the efficient transfer of heat by convection as a result of the "burning" of deuterium, the protostar increases in size, which depends on its mass. A protostar of one solar mass has a radius equal to five solar masses. With a mass equal to three solar, the protostar swells up to a radius equal to 10 solar.
The mass of a typical compact zone is greater than the mass of its generated star. Therefore, there must be some mechanism that removes excess mass and stops the fall of matter. Most astronomers are convinced that a strong stellar wind is responsible for this, escaping from the surface of the protostar. The stellar wind blows the incident gas backwards and eventually disperses the compact zone.

stellar wind idea

The "idea of ​​a stellar wind" does not follow from theoretical calculations. And astonished theorists were given evidence of this phenomenon: observations of molecular gas flows moving from infrared radiation sources. These flows are associated with the protostellar wind. Its origin is one of the deepest mysteries of young stars.
When the compact zone dissipates, an object that can be observed in the optical range is exposed - a young star. Like a protostar, it has a high luminosity that is more determined by gravity than by fusion. The pressure in the interior of the star prevents a catastrophic gravitational collapse. However, the heat responsible for this pressure is radiated from the stellar surface, so the star shines very brightly and contracts slowly.
As it contracts, its internal temperature gradually rises and eventually reaches 10 million Kelvin. Then the fusion reactions of hydrogen nuclei begin with the formation of helium. The heat released creates pressure that prevents compression, and the star will shine for a long time until nuclear fuel runs out in its depths.
Our Sun, a typical star, took about 30 million years to shrink from protostellar to modern size. Thanks to the heat released during thermonuclear reactions, it has retained these dimensions for about 5 billion years.
This is how stars are born. But despite such obvious successes of scientists who have allowed us to learn one of the many secrets of the universe, many more known properties of young stars are not yet fully understood. This refers to their irregular variability, colossal stellar wind, unexpected bright flashes. There are no definite answers to these questions yet. But these unresolved problems should be seen as breaks in a chain, the main links of which have already been soldered. And we will be able to close this chain and complete the biography of young stars if we find the key created by nature itself. And this key flickers in the clear sky above us.

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