Life cycle of a star - description, diagram and interesting facts. How stars die

Life cycle stars depends on their mass. Large stars burn their fuel more intensively and burn out over several tens of millions of years. Small ones can “smolder” for hundreds of billions of years.

When the hydrogen runs out, the nucleosynthesis reactions stop. Gravity begins to compress the star until the rising temperature triggers secondary fusion, which converts helium from the star's core into carbon. In his heart remains a crystal of pure carbon - a thousand-carat diamond. During the secondary combustion of helium in the core of a star, so much energy is released that the star begins to swell and turns into red giant , since its outer layer cools down to the red part of the spectrum. The diameter of the star increases by more than 100 times. When the fuel for secondary thermonuclear fusion runs out, gravitational forces again begin to compress the star and it turns into a degenerate white dwarf , which will radiate residual heat into space until it cools down completely. During the evolution of a star from a red giant to a white dwarf, most of its outer layers are dumped into the interstellar medium and become material for the subsequent formation of new stars.

Such a final is prepared for the middle stars, such as our Sun.

Stars more than 8 times more massive than the Sun die according to a different scenario. After burning helium in them, their huge mass during compression heats up the core and shell so much that subsequent nucleosynthesis reactions are triggered, as a result of which carbon is obtained first, then silicon, magnesium and the following elements with increasing nuclear masses. Moreover, with the beginning of each new reaction in the core of the star, the previous one continues in its shell. All chemical elements, of which the Universe is composed, were formed precisely as a result of nucleosynthesis in the depths of dying large stars. As soon as the turn comes to the formation of iron, the end of the star comes. During its synthesis, energy is not released, but only absorbed. In a short period of time, the fuel runs out, thermonuclear reactions stop, gravitational forces bring down the shell of the star to its center. The collision energy of the outer shell with the nucleus is very high. She blows up the star.

If the star was more than 30 times heavier than the Sun, then after its explosion, like a supernova, the gravitational collapse does not stop - it formsblack hole. It has a density such as the Earth would have if it was compressed to a diameter of 5 cm. Therefore, the gravitational force of black holes tends to infinity. Such an attractive force cannot be overcome even by particles of light with their limiting speeds. Therefore, the black hole does not reflect the light falling on it, it absorbs it. Hence the name.

Scientists suggest that the laws of physics do not apply in black holes, space and time cease to exist, but information remains in the form of holographic projections. The edge of a black hole event horizon is the boundary of time and space. The center of a black hole singularity – physical uncertainty. A black hole consumes stars and nebulae as long as there is room for them. And then throws out a powerful stream of gas - quasar outside the galaxy. The width of the quasar is greater than the diameter solar system. Outside the galaxy, new stars and new galaxies begin to form. Black holes guide the evolution of the universe.

The death of the stars gives construction material for the universe. All chemical elements - gold, silver, platinum, iron and others are formed inside dying stars and fly into space during their explosions.

The first stars were massive (several thousand times larger than the Sun) and unstable. They were born quickly and died quickly, leaving behind cosmic dust rich in various chemical elements.

The first stars were formed from cosmic nebulae, thanks to the energy of the Big Bang. In later stages and now, stars continue to be born. But this only happens after the explosion of another supernova. Its blast wave gives impetus to the interaction of cosmic dust particles, as a result of which they begin to move and interlock. Linking into one object, they increase it more and more in size, thereby increasing its gravity, which attracts other particles even more, and then larger space objects.

A young star and its circumstellar space initial stage it is a raging element with a large number of chaotically rotating minor planets. Colliding with each other, some of them crumble, while others grow, absorbing the remains of the first. From such collisions, for example, Mercury's upper crust flew off and only the core remained.

After 500 million years, the number of planets decreases and their size increases.

The sun belongs to the small stars. His death in 5 - 6 billion years will take place according to the first scenario. Right now, 80% of the stars in the universe are no bigger than the Sun.

Photo from CSO website:At a distance of 35 million light years from Earth, in the constellation Eridanus (Eridanus), lies spiral galaxy NGC 1637. In 1999, its serene beauty was shattered by a very bright supernova. Image taken with ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile.

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.

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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 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: nuclear reactions there is never enough energy 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 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 those that have not yet become part of the star outer areas 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, modern theories are based on computer simulation 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 a strong infrared radiation source star in such shells, ideal conditions are formed 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 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,

It is quite natural that stars are not living beings, but they also go through evolutionary stages similar to birth, life and death. Like a person, a star undergoes radical changes throughout its life. But it should be noted that they obviously live longer - millions and even billions of Earth years.

How are stars born? Initially, or rather after the Big Bang, matter in the universe was unevenly distributed. Stars began to form in nebulae, giant clouds of interstellar dust and gases, mostly hydrogen. This matter is affected by gravity, and part of the nebula is compressed. Then round and dense gas and dust clouds are formed - Bok globules. As such a globule continues to thicken, its mass increases due to the attraction of matter from the nebula towards itself. In the inner part of the globule, the gravitational force is strongest, and it begins to heat up and rotate. This is already a protostar. Hydrogen atoms begin to bombard each other and thereby generate a large amount of energy. Eventually the temperature of the central part reaches a temperature of the order of fifteen million degrees Celsius, the core of a new star is formed. The newborn flares up, begins to burn and glow. How long this will continue depends on what was the mass of the born star. What I said at our last meeting. The larger the mass, the shorter the life of the star.
By the way, it depends on the mass whether a protostar can become a star. According to calculations, in order for this contracting celestial body to turn into a star, its mass must be at least 8% of the mass of the Sun. A smaller globule, condensing, will gradually cool down and turn into a transitional object, something in between a star and a planet. Such objects are called brown dwarfs.

The planet Jupiter, for example, is too small to be a star. If Jupiter were more massive, perhaps thermonuclear reactions would begin in its depths, and our solar system would be a binary star system. But it's all poetry...

So, the main stage of the life of a star. Most of its existence, the star is in an equilibrium state. The force of gravity tends to compress the star, and the energy released as a result of thermonuclear reactions occurring in the star forces the star to expand. These two forces create a stable position of equilibrium - so stable that the star lives like this for millions and billions of years. This phase of a star's life secures its place in the main sequence. -

After shining for millions of years, a large star, that is, a star at least six times heavier than the Sun, begins to burn out. When the core runs out of hydrogen, the star expands and cools, turning into a red supergiant. This supergiant will then contract until it finally explodes in a monstrous and dramatic blazing explosion known as a supernova. It should be noted here that very massive blue supergiants bypass the stage of transformation into a red supergiant and explode much faster in a supernova.
If the remaining supernova core is small, then its catastrophic contraction (gravitational collapse) into a very dense neutron star begins, and if it is large enough, it will contract even more, forming a black hole.

A slightly different demise ordinary star. Such a star lives longer and dies a more peaceful death. The sun, for example, will burn for another five billion years before the hydrogen in its core runs out. Its outer layers will then expand and cool; a red giant is formed. In this form, a star can exist for about 100 million years on the helium formed during its lifetime in its core. But helium also burns out. To top it off, the outer layers will be blown away - they form a planetary nebula, and a dense white dwarf will shrink from the core. Although the white dwarf is hot enough, eventually it will cool down, turning into a dead star, which is called a black dwarf.

Stars, like people, can be newborn, young, old. Every moment some stars die and others are formed. Usually the youngest of them are similar to the Sun. They are at the stage of formation and actually represent protostars. Astronomers call them T-Taurus stars, after their prototype. By their properties - for example, luminosity - protostars are variable, since their existence has not yet entered a stable phase. Around many of them is a large amount of matter. Powerful wind currents emanate from T-type stars.

Protostars: the beginning of the life cycle

If matter falls on the surface of a protostar, it quickly burns out and turns into heat. As a result, the temperature of protostars is constantly increasing. When it rises so much that nuclear reactions are triggered in the center of the star, the protostar acquires the status of an ordinary one. With the onset of nuclear reactions, the star has a constant source of energy that supports its vital activity for a long time. How long the life cycle of a star in the universe will be depends on its initial size. However, it is believed that stars with a diameter of the Sun have enough energy to exist comfortably for about 10 billion years. Despite this, it also happens that even more massive stars live only a few million years. This is due to the fact that they burn their fuel much faster.

Stars of normal size

Each of the stars is a bunch of hot gas. In their depths, the process of generating nuclear energy is constantly going on. However, not all stars are like the Sun. One of the main differences is in color. Stars are not only yellow, but also bluish, reddish.

Brightness and luminosity

They also differ in such features as brilliance, brightness. How bright a star observed from the surface of the Earth will be depends not only on its luminosity, but also on the distance from our planet. Given the distance to the Earth, the stars can have completely different brightness. This indicator ranges from one ten-thousandth of the brilliance of the Sun to a brightness comparable to more than a million Suns.

Most of the stars are in the lower segment of this spectrum, being dim. In many ways, the Sun is an average, typical star. However, compared to others, it has a much greater brightness. A large number of dim stars can be observed even with the naked eye. The reason stars differ in brightness is because of their mass. Color, brilliance and change in brightness over time is determined by the amount of substance.

Attempts to explain the life cycle of stars

People have long tried to trace the life of the stars, but the first attempts of scientists were rather timid. The first advance was the application of Lane's law to the Helmholtz-Kelvin hypothesis of gravitational contraction. This brought a new understanding to astronomy: theoretically, the temperature of a star should increase (its temperature is inversely proportional to the radius of the star) until the increase in density slows down the contraction processes. Then the energy consumption will be higher than its income. At this point, the star will begin to cool rapidly.

Hypotheses about the life of stars

One of the original hypotheses about the life cycle of a star was proposed by astronomer Norman Lockyer. He believed that stars arise from meteoric matter. At the same time, the provisions of his hypothesis were based not only on the available in astronomy theoretical conclusions, but also on the data of the spectral analysis of stars. Lockyer was convinced that the chemical elements that take part in the evolution of celestial bodies are composed of elementary particles- "protoelements". Unlike modern neutrons, protons and electrons, they have not a general, but an individual character. For example, according to Lockyer, hydrogen breaks down into what is called "protohydrogen"; iron becomes "proto-iron". Other astronomers also tried to describe the life cycle of a star, for example, James Hopwood, Yakov Zeldovich, Fred Hoyle.

Giant and dwarf stars

Stars large sizes are the hottest and brightest. They are usually white or bluish in appearance. Despite the fact that they have giant size, the fuel inside them burns out so quickly that they lose it in just a few million years.

Small stars, in contrast to giant ones, are usually not as bright. They have a red color, live long enough - for billions of years. But among the brightest stars in the sky there are also red and orange ones. An example is the star Aldebaran - the so-called "bull's eye", located in the constellation Taurus; as well as in the constellation Scorpio. Why are these cool stars able to compete in brightness with hot stars like Sirius?

This is due to the fact that once they expanded very much, and in their diameter they began to exceed the huge red stars (supergiants). The huge area allows these stars to radiate an order of magnitude more energy than the Sun. And this despite the fact that their temperature is much lower. For example, the diameter of Betelgeuse, located in the constellation Orion, is several hundred times larger than the diameter of the Sun. And the diameter of ordinary red stars is usually not even a tenth of the size of the Sun. Such stars are called dwarfs. Each celestial body can go through these types of the life cycle of stars - the same star at different segments of its life can be both a red giant and a dwarf.

As a rule, luminaries like the Sun support their existence due to the hydrogen inside. It turns into helium inside the nuclear core of the star. The sun disposes huge amount fuel, but even it is not infinite - over the past five billion years, half the reserve has been used up.

Lifetime of stars. Life cycle of stars

After the reserves of hydrogen inside the star are exhausted, serious changes come. The remaining hydrogen begins to burn not inside its core, but on the surface. In this case, the lifetime of the star is decreasing more and more. The cycle of stars, at least most of them, in this segment passes into the stage of a red giant. The size of the star becomes larger, and its temperature, on the contrary, becomes smaller. This is how most red giants, as well as supergiants, appear. This process is part of the overall sequence of changes that occur with the stars, which scientists called the evolution of stars. The life cycle of a star includes all its stages: in the end, all stars grow old and die, and the duration of their existence is directly determined by the amount of fuel. Large stars end their lives with a huge, spectacular explosion. More modest ones, on the contrary, die, gradually shrinking to the size of white dwarfs. Then they just fade away.

How long does it live middle star? The life cycle of a star can last from less than 1.5 million years to 1 billion years or more. All this, as was said, depends on its composition and size. Stars like the Sun live between 10 and 16 billion years. Very bright stars, like Sirius, live for a relatively short time - only a few hundred million years. The life cycle diagram of a star includes the following stages. This is a molecular cloud - the gravitational collapse of the cloud - the birth of a supernova - the evolution of a protostar - the end of the protostellar phase. Then the stages follow: the beginning of the stage of a young star - the middle of life - maturity - the stage of a red giant - a planetary nebula - the stage of a white dwarf. The last two phases are characteristic of small stars.

The nature of planetary nebulae

So, we have briefly considered the life cycle of a star. But what is it? Turning from a huge red giant into a white dwarf, sometimes stars shed their outer layers, and then the core of the star becomes naked. The gas envelope begins to glow under the influence of energy emitted by the star. This stage got its name due to the fact that the luminous gas bubbles in this shell often look like disks around planets. But in fact, they have nothing to do with the planets. The life cycle of stars for children may not include all the scientific details. One can only describe the main phases of the evolution of the heavenly bodies.

star clusters

Astronomers are very fond of exploring. There is a hypothesis that all luminaries are born precisely in groups, and not one by one. Since the stars belonging to the same cluster have similar properties, the differences between them are true, and not due to the distance to the Earth. Whatever changes these stars make, they begin at the same time and at the same time. equal conditions. Especially a lot of knowledge can be obtained by studying the dependence of their properties on mass. After all, the age of stars in clusters and their distance from the Earth are approximately equal, so they differ only in this indicator. The clusters will be of interest not only to professional astronomers - every amateur will be happy to make beautiful photo, admire them exclusively beautiful view in the planetarium.

Let us briefly consider the main stages in the evolution of stars.

Change in physical characteristics, internal structure and the chemical composition of the star over time.

Fragmentation of matter. .

It is assumed that stars are formed during the gravitational compression of fragments of a gas and dust cloud. So, the so-called globules can be the places of star formation.

A globule is a dense opaque molecular dust (gas and dust) interstellar cloud, which is observed against the background of luminous clouds of gas and dust in the form of a dark round formation. It consists mainly of molecular hydrogen (H 2) and helium ( He ) with an admixture of molecules of other gases and solid interstellar dust particles. Temperature of the gas in the globule (mainly the temperature of molecular hydrogen) T≈ 10 h 50K, average density n~ 10 5 particles / cm 3, which is several orders of magnitude greater than in the densest ordinary gas and dust clouds, diameter D~ 0.1 h one . Mass of globules M≤ 10 2 × M ⊙ . Some globules contain young types T Taurus.

The cloud is compressed by its own gravity due to gravitational instability, which can occur either spontaneously or as a result of the interaction of the cloud with a shock wave from a supersonic stellar wind stream from another nearby source of star formation. Other reasons for the emergence of gravitational instability are also possible.

Theoretical studies show that under the conditions that exist in ordinary molecular clouds (T≈ 10 ÷ 30K and n ~ 10 2 particles / cm 3), the initial one can occur in cloud volumes with mass M≥ 10 3 × M ⊙ . In such a contracting cloud, further decay into less massive fragments is possible, each of which will also be compressed under the influence of its own gravity. Observations show that in the Galaxy, in the process of star formation, not one, but a group of stars with different masses, for example, an open star cluster, is born.

When compressed into central regions cloud density increases, resulting in a moment when the substance of this part of the cloud becomes opaque to its own radiation. In the bowels of the cloud, a stable dense condensation occurs, which astronomers call oh.

Fragmentation of matter - the decay of a molecular dust cloud into smaller parts, the further of which leads to the appearance.

is an astronomical object that is in the stage , from which after some time (for the solar mass this time T~ 10 8 years) normal is formed.

With a further fall of matter from the gaseous envelope onto the nucleus (accretion), the mass of the latter, and consequently, the temperature and increase so much that the gas and radiant pressure are compared with the forces . Kernel compression stops. The formed one is surrounded by a gas-dust shell that is opaque for optical radiation, passing only infrared and longer-wave radiation to the outside. Such an object (-cocoon) is observed as a powerful source of radio and infrared radiation.

With a further increase in the mass and temperature of the core, light pressure stops accretion, and the remains of the shell disperse into outer space. A young one appears physical characteristics which depend on its mass and initial chemical composition.

The main source of energy for a star being born is, apparently, the energy released during gravitational contraction. This assumption follows from the virial theorem: in a stationary system, the sum potential energy E p all members of the system and twice the kinetic energy 2 E to of these terms is zero:

E p + 2 E c = 0. (39)

The theorem is valid for systems of particles moving in a limited region of space under the action of forces whose magnitude is inversely proportional to the square of the distance between the particles. It follows that the thermal (kinetic) energy is equal to half of the gravitational (potential) energy. When a star is compressed, the total energy of the star decreases, while the gravitational energy decreases: half of the change in gravitational energy leaves the star through radiation, due to the second half it increases thermal energy stars.

Young low mass stars(up to three solar masses), which are on the way to the main sequence, are completely convective; the process of convection covers all areas of the star. These are still, in fact, protostars, in the center of which nuclear reactions are just beginning, and all the radiation occurs mainly due to. It has not yet been established whether stars decrease at a constant effective temperature. In the Hertzsprung-Russell diagram, such stars form an almost vertical track, called the Hayashi track. As the compression slows down, the young one approaches the main sequence.

As the star contracts, the pressure of the degenerate electron gas begins to increase, and when a certain radius of the star is reached, the compression stops, which leads to a halt in further growth. central temperature, caused by 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 . Such "understars" radiate more energy than is formed during nuclear reactions, and belong to the so-called; their fate is a constant contraction until the pressure of the degenerate gas stops it, and then a gradual cooling with the cessation of all nuclear reactions that have begun.

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

Stars with mass greater than 8 solar massesalready have the characteristics of normal stars, because they have gone through all the intermediate stages and were able to achieve such a rate of nuclear reactions that they compensate for the energy loss by radiation while the mass of the nucleus accumulates. In these stars, the outflow of mass is so great that it not only stops the collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, thaws them away. Thus, the mass of the formed star is noticeably less than the mass of the protostellar cloud.

Main sequence

The temperature of the star rises until in the central regions it reaches values ​​sufficient to turn on thermonuclear reactions, which then become the main source of energy for the star. For massive stars ( M > 1 ÷ 2 × M ⊙ ) is the "combustion" of hydrogen in the carbon cycle; for stars with a mass equal to or less than the mass of the Sun, energy is released in a proton-proton reaction. passes into the equilibrium stage and takes its place on the main sequence of the Hertzsprung-Russell diagram: in a star of large mass, the temperature in the core is very high ( T ≥ 3 × 107 K ), energy production is very intense, - on the main sequence it occupies a place above the Sun in the region of early ( O … A , (F )); in a star of small mass, the temperature in the core is relatively low ( T ≤ 1.5 × 107 K ), energy production is not so intense, - on the main sequence it takes place near or below the Sun in the region of late (( F ), G , K , M ).

It spends up to 90% of the time allotted by nature for its existence on the main sequence. The time a star spends in the main sequence stage also depends on the mass. Yes, with mass M ≈ 10 ÷ 20 × M ⊙ O or B is in the main sequence stage for about 10 7 years, while the red dwarf K 5 with mass M ≈ 0.5 × M ⊙ is in the main sequence stage for about 10 11 years, that is, a time comparable to the age of the Galaxy. Massive hot stars quickly pass into the next stages of evolution, cold dwarfs are in the main sequence stage all the time of the existence of the Galaxy. It can be assumed that red dwarfs are the main type of the population of the Galaxy.

Red giant (supergiant).

The rapid burning of hydrogen in the central regions of massive stars leads to the appearance of a helium core in them. With a fraction of the mass of hydrogen of a few percent in the nucleus, the carbon reaction of the conversion of hydrogen into helium almost completely stops. The core contracts, which leads to an increase in its temperature. As a result of the heating caused by the gravitational contraction of the helium core, hydrogen "lights up" and energy release begins in a thin layer located between the core and the extended shell of the star. The shell expands, the radius of the star increases, the effective temperature decreases and grows. "leaves" the main sequence and passes into the next stage of evolution - into the stage of a red giant or, if the mass of the star M > 10 × M⊙ , into the red supergiant stage.

With an increase in temperature and density, helium begins to “burn” in the core. At T ~ 2 × 10 8 K and r ~ 10 3 ¸ 10 4 g / cm 3 starts a thermonuclear reaction, which is called triple a -process: out of three a -particles (helium nuclei 4 He ) one stable carbon nucleus 12 C is formed. With the mass of the star's core M< 1,4 × M ⊙ тройной a - the process leads to the explosive nature of the energy release - a helium flash, which for a particular star can be repeated many times.

In the central regions of massive stars that are in the giant or supergiant stage, an increase in temperature leads to the successive formation of carbon, carbon-oxygen, and oxygen cores. After carbon burnout, reactions occur, as a result of which heavier chemical elements are formed, possibly also iron nuclei. Further evolution of a massive star can lead to shell ejection, a flare of a star as a Nova or, with the subsequent formation of objects that are the final stage in the evolution of stars: a white dwarf, a neutron star, or a black hole.

The final stage of evolution is the stage of evolution of all normal stars after these have exhausted their thermonuclear fuel; cessation of thermonuclear reactions as a source of energy for the star; the transition of a star, depending on its mass, to the stage of a white dwarf, or a black hole.

White dwarfs are the last stage in the evolution of all normal stars with mass M< 3 ÷ 5 × M ⊙ after exhaustion of thermonuclear fuel by these mi. Having passed the stage of a red giant (or subgiant), such a shell sheds and exposes the core, which, cooling down, becomes a white dwarf. Small radius (R b.c ~ 10 -2 × R ⊙ ) and white or blue-white (T b.c ~ 10 4 K) determined the name of this class of astronomical objects. The mass of a white dwarf is always less than 1.4×M⊙ - it is proved that white dwarfs with large masses cannot exist. With a mass comparable to that of the Sun and dimensions comparable to those of major planets solar system, white dwarfs have a huge average density: ρ b.c ~ 10 6 g / cm 3, that is, a weight of 1 cm 3 of white dwarf matter weighs a ton! Acceleration free fall on the surface g b.c ~ 10 8 cm / s 2 (compare with acceleration on the surface of the Earth - g c ≈980 cm/s 2). With such a gravitational load on the inner regions of the star equilibrium state the white dwarf is supported by the pressure of the degenerate gas (mainly the degenerate electron gas, since the contribution of the ionic component is small). Recall that a gas is called degenerate if there is no Maxwellian velocity distribution of particles. In such a gas, at certain values ​​of temperature and density, the number of particles (electrons) having any speed in the range from v = 0 to v = v max will be the same. v max is determined by the density and temperature of the gas. With a white dwarf mass M b.c > 1.4 × M ⊙ maximum speed electrons in a gas is comparable to the speed of light, the degenerate gas becomes relativistic and its pressure is no longer able to resist gravitational contraction. The radius of the dwarf tends to zero - "collapses" into a point.

The thin, hot atmospheres of white dwarfs are either composed of hydrogen, with virtually no other elements found in the atmosphere; or from helium, while there is hundreds of thousands of times less hydrogen in the atmosphere than in the atmospheres of normal stars. According to the type of spectrum, white dwarfs belong to spectral classes O, B, A, F. To “distinguish” white dwarfs from normal stars, the letter D is placed in front of the designation (DOVII, DBVII, etc. D is the first letter in English word Degenerate - degenerate). The radiation source of a white dwarf is the supply of thermal energy that the white dwarf received while being the core of the parent star. Many white dwarfs inherited from their parent a strong magnetic field, the strength of which H ~ 10 8 O. It is believed that the number of white dwarfs is about 10% of total number stars of the Galaxy.

On fig. 15 is a photograph of Sirius - the brightest star sky (α Big Dog; m v = -1 m ,46; class A1V). The disk visible in the picture is the result of photographic irradiation and diffraction of light on the telescope lens, that is, the disk of the star itself is not resolved in the photograph. The rays coming from the photographic disk of Sirius are traces of the distortion of the wave front of the light flux on the elements of the telescope's optics. Sirius is located at a distance of 2.64 from the Sun, the light from Sirius takes 8.6 years to reach the Earth - thus, it is one of the stars closest to the Sun. Sirius is 2.2 times more massive than the Sun; his M v = +1 m ,43, that is, our neighbor radiates 23 times more energy than the Sun.

The uniqueness of the photograph lies in the fact that, together with the image of Sirius, it was possible to obtain an image of his satellite - the satellite “glows” with a bright dot to the left of Sirius. Sirius - telescopically: Sirius itself is denoted by the letter A, and its satellite by the letter B. The apparent magnitude of Sirius B m v \u003d +8 m,43, that is, it is almost 10,000 times weaker than Sirius A. The mass of Sirius B is almost exactly equal to the mass of the Sun, the radius is about 0.01 of the radius of the Sun, the surface temperature is about 12000K, but Sirius B radiates 400 times less than the Sun . Sirius B is a typical white dwarf. Moreover, this is the first white dwarf discovered, by the way, by Alven Clark in 1862 during visual observation through a telescope.

Sirius A and Sirius B revolve around in common with a period of 50 years; the distance between components A and B is only 20 AU.

According to the apt remark of V.M. Lipunov, “they “ripen” inside massive stars (with a mass of more than 10×M⊙ )”. The nuclei of stars evolving into a neutron star have 1.4× M ⊙ ≤ M ≤ 3 × M ⊙ ; after the sources of thermonuclear reactions run out and the parent ejects a significant part of the matter with a flash, these nuclei will become independent objects of the stellar world with very specific characteristics. The compression of the core of the parent star stops at a density comparable to the nuclear one (ρ n. h ~ 10 14 h 10 15 g/cm3). With such a mass and density, the radius of the born only 10 consists of three layers. The outer layer (or outer crust) is formed by a crystal lattice of iron atomic nuclei ( Fe ) with a possible small admixture of atomic nuclei of other metals; the thickness of the outer crust is only about 600 m with a radius of 10 km. Beneath the outer crust is another inner hard crust, composed of iron atoms ( Fe ), but these atoms are overenriched with neutrons. The thickness of this bark≈ 2 km. The inner crust borders on the liquid neutron core, the physical processes in which are determined by the remarkable properties of the neutron liquid - superfluidity and, in the presence of free electrons and protons, superconductivity. It is possible that in the very center the matter may contain mesons and hyperons.

They rotate rapidly around an axis - from one to hundreds of revolutions per second. Such rotation in the presence of a magnetic field ( H ~ 10 13 h 10 15 Oe) often leads to the observed effect of pulsation of the star's radiation in different ranges electromagnetic waves. We saw one of these pulsars inside the Crab Nebula.

Total number the rotation speed is already insufficient for the ejection of particles, so this cannot be a radio pulsar. However, it is still large, and captured magnetic field the surrounding neutron star cannot fall, that is, the accretion of matter does not occur.

Accretor (X-ray pulsar). The rotation speed is reduced to such an extent that now nothing prevents the matter from falling onto such a neutron star. The plasma, falling, moves along the lines of the magnetic field and hits a solid surface in the region of the poles, heating up to tens of millions of degrees. A substance heated to such high temperatures glows in the X-ray range. The area in which the falling matter stops with the surface of the star is very small - only about 100 meters. This hot spot, due to the rotation of the star, periodically disappears from view, which the observer perceives as pulsations. Such objects are called X-ray pulsars.

Georotator. The rotation speed of such neutron stars is low and does not prevent accretion. But the dimensions of the magnetosphere are such that the plasma is stopped by the magnetic field before it is captured by gravity.

If it is a component of a close binary system, then there is a “transfer” of matter from a normal star (the second component) to a neutron one. The mass may exceed the critical one (M > 3×M⊙ ), then the gravitational stability of the star is violated, nothing can resist gravitational contraction, and “leaves” under its gravitational radius

r g = 2 × G × M/c 2 , (40)

turning into a black hole. In the above formula for r g: M is the mass of the star, c is the speed of light, G is the gravitational constant.

A black hole is an object whose gravitational field is so large that neither a particle, nor a photon, nor any material body can reach the second cosmic velocity and escape into outer space.

A black hole is a singular object in the sense that the nature of the flow physical processes inside it is still inaccessible to a theoretical description. The existence of black holes follows from theoretical considerations, in reality they can be located in the central regions of globular clusters, quasars, giant galaxies, including the center of our galaxy.