Shooting star of evolution. Life cycle of a star - description, diagram and interesting facts

Although stars seem eternal on the human time scale, they, like everything in nature, are born, live and die. According to the generally accepted gas-dust cloud hypothesis, a star is born as a result of gravitational compression of an interstellar gas-dust cloud. As such a cloud thickens, it first forms protostar, the temperature at its center steadily increases until it reaches the limit necessary for the speed of thermal motion of particles to exceed the threshold after which protons are able to overcome the macroscopic forces of mutual electrostatic repulsion ( cm. Coulomb's Law) and enter into a thermonuclear fusion reaction ( cm. Nuclear decay and fusion).

As a result of a multi-stage thermonuclear fusion reaction, four protons ultimately form a helium nucleus (2 protons + 2 neutrons) and a whole fountain of various elementary particles is released. In the final state, the total mass of the formed particles is less the masses of the four initial protons, which means that free energy is released during the reaction ( cm. Theory of relativity). Because of this, the internal core of the newborn star quickly heats up to ultra-high temperatures, and its excess energy begins to splash towards its less hot surface - and out. At the same time, the pressure in the center of the star begins to increase ( cm. Equation of state of an ideal gas). Thus, by “burning” hydrogen in the process of a thermonuclear reaction, the star does not allow the forces of gravitational attraction to compress itself to a super-dense state, countering the gravitational collapse with continuously renewed internal thermal pressure, resulting in a stable energy equilibrium. Stars actively burning hydrogen are said to be in the "primary phase" of their life cycle or evolution ( cm. Hertzsprung-Russell diagram). The transformation of one chemical element into another inside a star is called nuclear fusion or nucleosynthesis.

In particular, the Sun has been at the active stage of burning hydrogen in the process of active nucleosynthesis for about 5 billion years, and the reserves of hydrogen in the core for its continuation should be enough for our luminary for another 5.5 billion years. The more massive the star, the greater the supply of hydrogen fuel it has, but to counteract the forces of gravitational collapse, it must burn hydrogen at an intensity that exceeds the growth rate of hydrogen reserves as the mass of the star increases. Thus, the more massive the star, the shorter its lifetime, determined by the depletion of hydrogen reserves, and the largest stars literally burn out in “some” tens of millions of years. The smallest stars, on the other hand, live comfortably for hundreds of billions of years. So, on this scale, our Sun belongs to the “strong middle class”.

Sooner or later, however, any star will use up all the hydrogen suitable for combustion in its thermonuclear furnace. What's next? It also depends on the mass of the star. The sun (and all stars not exceeding its mass by more than eight times) end my life in a very banal way. As the reserves of hydrogen in the bowels of the star are depleted, the forces of gravitational compression, which have been patiently waiting for this hour since the very moment of the birth of the star, begin to gain the upper hand - and under their influence the star begins to shrink and become denser. This process has a twofold effect: The temperature in the layers immediately around the star's core rises to a level at which the hydrogen contained there finally undergoes thermonuclear fusion to form helium. At the same time, the temperature in the core itself, now consisting almost entirely of helium, rises so much that the helium itself—a kind of “ash” of the fading primary nucleosynthesis reaction—enters into a new thermonuclear fusion reaction: from three helium nuclei, one carbon nucleus is formed. This process of secondary thermonuclear fusion reaction, fueled by the products of the primary reaction, is one of the key moments in the life cycle of stars.

During the secondary combustion of helium in the star's core, so much energy is released that the star literally begins to inflate. In particular, the shell of the Sun at this stage of life will expand beyond the orbit of Venus. In this case, the total energy of the star's radiation remains approximately at the same level as during the main phase of its life, but since this energy is now emitted through a much larger surface area, the outer layer of the star cools down to the red part of the spectrum. The star turns into red giant.

For solar-class stars, after the fuel feeding the secondary nucleosynthesis reaction has been depleted, the stage of gravitational collapse begins again—this time final. The temperature inside the core is no longer able to rise to the level necessary to initiate the next level of thermonuclear reaction. Therefore, the star contracts until the forces of gravitational attraction are balanced by the next force barrier. His role is played by degenerate electron gas pressure(cm. Chandrasekhar limit). Electrons, which until this stage played the role of unemployed extras in the evolution of the star, not participating in nuclear fusion reactions and freely moving between nuclei in the process of fusion, at a certain stage of compression find themselves deprived of “living space” and begin to “resist” further gravitational compression of the star. The state of the star stabilizes, and it turns into a degenerate white dwarf, which will radiate residual heat into space until it cools completely.

Stars more massive than the Sun face a much more spectacular end. After the combustion of helium, their mass during compression turns out to be sufficient to heat the core and shell to the temperatures necessary to launch the next nucleosynthesis reactions - carbon, then silicon, magnesium - and so on, as the nuclear masses grow. Moreover, with the beginning of each new reaction in the core of the star, the previous one continues in its shell. In fact, all the chemical elements, including iron, that make up the Universe, were formed precisely as a result of nucleosynthesis in the depths of dying stars of this type. But iron is the limit; it cannot serve as fuel for nuclear fusion or decay reactions at any temperature or pressure, since both its decay and the addition of additional nucleons to it require an influx of external energy. As a result, a massive star gradually accumulates an iron core inside itself, which cannot serve as fuel for any further nuclear reactions.

Once the temperature and pressure inside the nucleus reach a certain level, electrons begin to interact with the protons of the iron nuclei, resulting in the formation of neutrons. And in a very short period of time - some theorists believe that this takes a matter of seconds - the electrons free throughout the previous evolution of the star literally dissolve in the protons of the iron nuclei, the entire substance of the star’s core turns into a solid bunch of neutrons and begins to rapidly compress in gravitational collapse , since the counteracting pressure of the degenerate electron gas drops to zero. The outer shell of the star, from under which all support is knocked out, collapses towards the center. The energy of the collision of the collapsed outer shell with the neutron core is so high that it bounces off at tremendous speed and scatters in all directions from the core - and the star literally explodes in a blinding flash supernova stars. In a matter of seconds, a supernova explosion can release more energy into space than all the stars in the galaxy put together during the same time.

After a supernova explosion and the expansion of the shell of stars with a mass of about 10-30 solar masses, the ongoing gravitational collapse leads to the formation of a neutron star, the matter of which is compressed until it begins to make itself felt pressure of degenerate neutrons - in other words, now neutrons (just as electrons did earlier) begin to resist further compression, requiring to myself living space. This usually occurs when the star reaches a size of about 15 km in diameter. The result is a rapidly rotating neutron star, emitting electromagnetic pulses at the frequency of its rotation; such stars are called pulsars. Finally, if the star's core mass exceeds 30 solar masses, nothing can stop its further gravitational collapse, and a supernova explosion results in

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 are actually protostars. Astronomers call them T-Taurus stars, after their prototype. In terms of their properties - for example, luminosity - protostars are variable, since their existence has not yet entered a stable phase. Many of them have large amounts of matter around them. Powerful wind currents emanate from T-type stars.

Protostars: the beginning of their life cycle

If matter falls onto the surface of a protostar, it quickly burns and turns into heat. As a consequence, the temperature of protostars is constantly increasing. When it rises so high that nuclear reactions are triggered in the center of the star, the protostar acquires the status of an ordinary one. With the start of nuclear reactions, the star has a constant source of energy that supports its life for a long time. How long a star's life cycle in the universe will depend on its original size. However, it is believed that stars the 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.

Normal sized stars

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

Brightness and Luminosity

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

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

Attempts to explain the life cycle of stars

People have long tried to trace the life of 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 indicator is inversely proportional to the radius of the star) until an increase in density slows down the compression processes. Then the energy consumption will be higher than its income. At this moment, the star will begin to rapidly cool down.

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. Moreover, the provisions of his hypothesis were based not only on theoretical conclusions available in astronomy, but also on data from spectral analysis of stars. Lockyer was convinced that the chemical elements that take part in the evolution of celestial bodies consist of elementary particles - “protoelements”. Unlike modern neutrons, protons and electrons, they do not have a general, but an individual character. For example, according to Lockyer, hydrogen decays 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 stars and dwarf stars

Larger stars are the hottest and brightest. They are usually white or bluish in appearance. Despite the fact that they are gigantic in size, the fuel inside them burns so quickly that they are deprived of it in just a few million years.

Small stars, as opposed to giant ones, are usually not so bright. They are red in color and live long enough - for billions of years. But among the bright stars in the sky there are also red and orange ones. An example is the star Aldebaran - the so-called “eye of the bull”, located in the constellation Taurus; and also 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 they once expanded very much, and in their diameter they began to exceed huge red stars (supergiants). The huge area allows these stars to emit an order of magnitude more energy than the Sun. This is 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 the size of the Sun. Such stars are called dwarfs. Each celestial body can go through these types of star life cycles - the same star at different stages 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 contained within. It turns into helium inside the star's nuclear core. The sun has a huge amount of fuel, but even it is not infinite - over the past five billion years, half of the supply has been used up.

Lifetime of stars. Life cycle of stars

Once the supply of hydrogen inside a star is depleted, major changes occur. The remaining hydrogen begins to burn not inside its core, but on the surface. At the same time, the lifespan of a star is increasingly shortened. The cycle of stars, at least most of them, enters the red giant stage during this period. The size of the star becomes larger, and its temperature, on the contrary, decreases. This is how most red giants and supergiants appear. This process is part of the general sequence of changes occurring in stars, which scientists call stellar evolution. The life cycle of a star includes all its stages: ultimately, all stars age and die, and the duration of their existence is directly determined by the amount of fuel. Big 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 the average star live? The life cycle of a star can last from less than 1.5 million years to 1 billion years or more. All this, as has been said, depends on its composition and size. Stars like the Sun live between 10 and 16 billion years. Very bright stars, like Sirius, have relatively short lives - only a few hundred million years. The star life cycle diagram includes the following stages. This is a molecular cloud - gravitational collapse of the cloud - the birth of a supernova - the evolution of a protostar - the end of the protostellar phase. Then follow the stages: the beginning of the young star stage - mid-life - maturity - red giant stage - planetary nebula - white dwarf stage. The last two phases are characteristic of small stars.

The nature of planetary nebulae

So, we briefly looked at the life cycle of a star. But what is Transforming from a huge red giant to a white dwarf, sometimes stars shed their outer layers, and then the core of the star becomes exposed. The gas shell begins to glow under the influence of energy emitted by the star. This stage got its name due to the fact that luminous gas bubbles in this shell often look like disks around planets. But in reality they have nothing to do with 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 celestial bodies.

Star clusters

Astronomers love to explore. There is a hypothesis that all luminaries are born in groups, and not individually. Since 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 occur to these stars, they originate at the same time and under equal conditions. Especially a lot of knowledge can be obtained by studying the dependence of their properties on mass. After all, the age of the stars in the 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 take a beautiful photograph and admire their exceptionally beautiful view in the planetarium.

The birth of stars and entire galaxies occurs permanently, as does their death. The disappearance of one star compensates for the appearance of another, so it seems to us that the same luminaries are constantly in the sky.

Stars owe their birth to the process of compression of the interstellar cloud, which is affected by a strong drop in gas pressure. Depending on the mass of the compressed gas, the number of stars being born changes: if it is small, then one star is born, if it is large, then the formation of an entire cluster is possible.

Stages of the emergence of a star

Here it is necessary to distinguish two main stages - the fast compression of the protostar and the slow one. In the first case, the distinguishing feature is gravity: the matter of the protostar undergoes an almost free fall towards its center. At this stage, the temperature of the gas remains unchanged, its duration is about 100 thousand years, and during this time the size of the protostar decreases very significantly.

And if at the first stage the excess heat was constantly leaving, then the protostar becomes denser. Heat removal does not occur at such a high rate; the gas continues to compress and heat up quickly. The slow compression of the protostar lasts even longer - more than ten million years. Upon reaching an ultra-high temperature (more than a million degrees), thermonuclear reactions take their toll, leading to the cessation of compression. After which a new star is formed from the protostar.

Life cycle of a star

Stars are like living organisms: they are born, reach their peak of development, and then die. Major changes begin when the central part of the star runs out of hydrogen. It begins to burn out already in the shell, gradually increasing its size, and the star can turn into a red giant or even a supergiant.

All stars have completely different life cycles, it all depends on their mass. Those that weigh more live longer and eventually explode. Our sun is not a massive star, so celestial bodies of this type face a different end: they gradually fade away and become a dense structure called a white dwarf.

Red giant

Stars that have used up their hydrogen supply can acquire colossal sizes. Such luminaries are called red giants. Their distinguishing feature, in addition to their size, is their extended atmosphere and very low surface temperature. Research has shown that not all stars go through this stage of development. Only those stars with significant mass become red giants.

The most striking representatives are Arcturus and Antare, the visible layers of which have a relatively low temperature, and the discharged shell has a considerable extent. A process of ignition of helium occurs inside the bodies, characterized by the absence of sharp fluctuations in luminosity.

White dwarf

Small stars in size and mass turn into white dwarfs. Their density is extremely high (about a million times higher than the density of water), which is why the substance of the star passes into a state called “degenerate gas.” No thermonuclear reactions are observed inside the white dwarf, and only the fact of cooling gives it light. The size of the star in this state is extremely small. For example, many white dwarfs are similar in size to Earth.

Star-- a celestial body in which thermonuclear reactions are occurring, have occurred, or will occur. Stars are massive luminous balls of gas (plasma). Formed from a gas-dust environment (hydrogen and helium) as a result of gravitational compression. The temperature of matter in the interior of stars is measured in millions of kelvins, and on their surface - in thousands of kelvins. The energy of the vast majority of stars is released as a result of thermonuclear reactions converting hydrogen into helium, occurring at high temperatures in the internal regions. Stars are often called the main bodies of the Universe, since they contain the bulk of luminous matter in nature. Stars are huge, spherical objects made of helium and hydrogen, as well as other gases. The energy of a star is contained in its core, where helium interacts with hydrogen every second. Like everything organic in our universe, stars arise, develop, change and disappear - this process takes billions of years and is called the process of “Star Evolution”.

1. Evolution of stars

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

2. Thermonuclear fusion in the interior of stars

By 1939, it was established that the source of stellar energy is thermonuclear fusion occurring in the bowels of stars. Most stars radiate because in their core four protons combine through a series of intermediate steps into one alpha particle. This transformation can occur in two main ways, called the proton-proton, or p-p, cycle, and the carbon-nitrogen, or CN, cycle. In low-mass stars, energy release is mainly provided by the first cycle, in heavy stars - by the second. The supply of nuclear fuel in a star is limited and is constantly spent on radiation. The process of thermonuclear fusion, which releases energy and changes the composition of the star's matter, in combination with gravity, which tends to compress the star and also releases energy, as well as radiation from the surface, which carries away the released energy, are the main driving forces of stellar evolution. The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in a galaxy actually contains between 0.1 and 1 molecule per cm?. The molecular cloud has a density of about a million molecules per cm?. The mass of such a cloud exceeds the mass of the Sun by 100,000-10,000,000 times due to its size: from 50 to 300 light years in diameter. While the cloud rotates freely around the center of its home galaxy, nothing happens. However, due to the inhomogeneity of the gravitational field, disturbances may arise in it, leading to local concentrations of mass. Such disturbances cause gravitational collapse of the cloud. One of the scenarios leading to this is the collision of two clouds. Another event causing collapse could be the passage of a cloud through the dense arm of a spiral galaxy. Also a critical factor could be the explosion of a nearby supernova, the shock wave of which will collide with the molecular cloud at enormous speed. It is also possible that galaxies collide, which could cause a burst of star formation as the gas clouds in each galaxy are compressed by the collision. In general, any inhomogeneities in the forces acting on the mass of the cloud can initiate the process of star formation. Due to the inhomogeneities that have arisen, the pressure of the molecular gas can no longer prevent further compression, and the gas begins to gather around the center of the future star under the influence of gravitational attraction forces. Half of the released gravitational energy goes to heating the cloud, and half goes to light radiation. In clouds, pressure and density increase towards the center, and the collapse of the central part occurs faster than the periphery. As compression progresses, the mean free path of photons decreases, and the cloud becomes less and less transparent to its own radiation. This leads to a faster rise in temperature and an even faster rise in pressure. As a result, the pressure gradient balances the gravitational force, and a hydrostatic core is formed, with a mass of about 1% of the mass of the cloud. This moment is invisible. The further evolution of the protostar is the accretion of matter that continues to fall onto the “surface” of the core, which due to this grows in size. The mass of freely moving matter in the cloud is exhausted, and the star becomes visible in the optical range. This moment is considered the end of the protostellar phase and the beginning of the young star phase. The process of star formation can be described in a unified way, but the subsequent stages of a star's development depend almost entirely on its mass, and only at the very end of stellar evolution can chemical composition play a role.

3. Mid-life cycle of a star

Stars come in a wide variety of colors and sizes. Their spectral class ranges from hot blue to cool red, and their mass ranges from 0.0767 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. All 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, depending on the parameters of the star. In fact, the movement of a star along the diagram corresponds only to a change in the parameters of the star. Small, cool red dwarfs slowly burn up their hydrogen reserves and remain on the main sequence for hundreds of billions of years, while massive supergiants will leave the main sequence within a few million years of formation. Medium-sized stars like the Sun remain on the main sequence for an average of 10 billion years. It is believed that the Sun is still on it as it is in the middle of its life cycle. Once a star runs out of hydrogen in its core, it leaves the main sequence. After a certain time - from a million to tens of billions of years, depending on the initial mass - the star depletes the hydrogen resources of the core. In large and hot stars this happens much faster than in small and cooler ones. Depletion of the hydrogen supply leads to the stopping of thermonuclear reactions. Without the pressure generated by these reactions to balance the star's own gravitational pull, the star begins to contract again, as it did earlier during its formation. Temperature and pressure rise again, but, unlike the protostar stage, to a higher level. The collapse continues until thermonuclear reactions involving helium begin at a temperature of approximately 100 million K. The thermonuclear combustion of matter resumed at a new level causes a monstrous expansion of the star. The star “looses” and its size increases approximately 100 times. Thus, the star becomes a red giant, and the helium burning phase lasts about several million years. Almost all red giants are variable stars. What happens next again depends on the mass of the star.

4. Later years and death of stars

Old stars with low mass

To date, it is not known for certain what happens to light stars after their hydrogen supply is depleted. 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 simulations of the processes occurring in such stars. Some stars can only synthesize helium in certain active zones, causing instability and strong stellar winds. In this case, the formation of a planetary nebula does not occur, and the star only evaporates, becoming even smaller than a brown dwarf. Stars with masses less than 0.5 solar are not able to convert helium even after reactions involving hydrogen cease in the core - their mass is too small to provide a new phase of gravitational compression to the extent that initiates the “ignition” of helium . These stars include red dwarfs such as Proxima Centauri, which have tens of billions to tens of trillions of years on the main sequence. After the cessation of thermonuclear reactions in their core, they, gradually cooling, will continue to weakly emit in the infrared and microwave ranges of the electromagnetic spectrum.

Medium sized stars

When a star of average size (from 0.4 to 3.4 solar masses) reaches the red giant phase, its core runs out of hydrogen and reactions of carbon synthesis from helium begin. This process occurs at higher temperatures and therefore the flow of energy from the core increases, which leads to the fact that the outer layers of the star begin to expand. The beginning of carbon synthesis marks a new stage in the life of a star and continues for some time. For a star similar in size to the Sun, this process can take about a billion years. Changes in the amount of energy emitted cause the star to go through periods of instability, including changes in size, surface temperature and energy output. Energy output shifts towards low frequency radiation. All this is accompanied by increasing mass loss due to strong stellar winds and intense pulsations. Stars in this phase are called late-type stars, OH-IR stars, or Mira-like stars, depending on their exact characteristics. The ejected gas is relatively rich in heavy elements produced in the star's interior, 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 central star, ideal conditions for the activation of masers are formed in such shells. Helium combustion reactions are very temperature sensitive. Sometimes this leads to great instability. Strong pulsations arise, which ultimately impart sufficient acceleration to the outer layers to be thrown off and turn into a planetary nebula. In the center of the nebula, the bare core of the star remains, in which thermonuclear reactions stop, and as it cools, it turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar and a diameter on the order of the diameter of the Earth.

White dwarfs

Soon after the helium flash, carbon and oxygen “ignite”; each of these events causes a serious restructuring of the star and its rapid movement along the Hertzsprung-Russell diagram. The size of the star's atmosphere increases even more, and it begins to intensively lose gas in the form of scattering streams of stellar wind. The fate of the central part of a star depends entirely on its initial mass: the core of a star can end its evolution as a white dwarf (low-mass stars); if its mass in the later stages of evolution exceeds the Chandrasekhar limit - like a neutron star (pulsar); if the mass exceeds the Oppenheimer limit - Volkov - like a black hole. In the last two cases, the completion of the evolution of stars is accompanied by catastrophic events - supernova explosions. The vast majority of stars, including the Sun, end their evolution by contracting until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a hundred times, and the density becomes a million times higher than the density of water, the star is called a white dwarf. It is deprived of energy sources and, gradually cooling down, becomes dark and invisible. In stars more massive than the Sun, the pressure of degenerate electrons cannot stop further compression of the core, and electrons begin to be “pressed” into atomic nuclei, which leads to the transformation of protons into neutrons, between which there are no electrostatic repulsion forces. Such neutronization of matter leads to the fact that the size of the star, which, in fact, now represents one huge atomic nucleus, is measured in several kilometers, and the density is 100 million times greater than the density of water. Such an object is called a neutron star.

Supermassive stars

After a star with a mass greater than five times the sun enters the red supergiant stage, its core begins to shrink under the influence of gravity. As compression increases, temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, increasingly heavier elements are synthesized: helium, carbon, oxygen, silicon and iron, which temporarily restrains the collapse of the core. Ultimately, as heavier and heavier elements of the periodic table are formed, iron-56 is synthesized from silicon. At this stage, further thermonuclear fusion becomes impossible, since the iron-56 nucleus has a maximum mass defect and the formation of heavier nuclei with the release of energy is impossible. Therefore, when the iron core of a star reaches a certain size, the pressure in it is no longer able to withstand the gravity of the outer layers of the star, and immediate collapse of the core occurs with neutronization of its matter. What happens next is not yet completely clear, but, in any case, the processes taking place in a matter of seconds lead to the explosion of a supernova of incredible force. The accompanying burst of neutrinos provokes a shock wave. Strong jets of neutrinos and a rotating magnetic field push out much of the star's accumulated material - so-called seed elements, including iron and lighter elements. The exploding matter is bombarded by neutrons emitted from the nucleus, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and perhaps even californium). Thus, supernova explosions explain the presence of elements heavier than iron in interstellar matter, which, however, is not the only possible way of their formation; for example, this is demonstrated by technetium stars. The blast wave and neutrino jets carry matter away from the dying star into interstellar space. Subsequently, as it cools and moves through space, this supernova material can collide with other space “junk”, and possibly participate in the formation of new stars, planets or satellites. The processes occurring during the formation of a supernova are still being studied, and so far there is no clarity on this issue. Also questionable is what actually remains of the original star. However, two options are being considered: neutron stars and black holes.

Neutron stars

It is known that in some supernovae, strong gravity in the depths of the supergiant forces electrons to be absorbed by the atomic nucleus, where they merge with protons to form neutrons. This process is called neutronization. The electromagnetic forces separating nearby nuclei disappear. The star's core is now a dense ball of atomic nuclei and individual neutrons. Such stars, known as neutron stars, are extremely small—no larger than the size of a large city—and have an unimaginably high density. Their orbital period becomes extremely short as the size of the star decreases (due to conservation of angular momentum). Some 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 detect a radiation pulse repeating at intervals equal to the star’s orbital period. Such neutron stars were called “pulsars” and became the first neutron stars to be discovered.

Black holes

Not all supernovae become neutron stars. If the star has a large enough 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. After this, the star becomes a black hole. The existence of black holes was predicted by the general theory of relativity. According to this theory, matter and information cannot leave a black hole under any conditions. However, quantum mechanics probably makes exceptions to this rule possible. A number of open questions remain. Chief among them: “Are there black holes at all?” After all, in order to say for sure that a given object is a black hole, it is necessary to observe its event horizon. This is impossible purely by defining the horizon, but using ultra-long-baseline radio interferometry it is possible to determine the metric near an object, as well as record fast, millisecond variability. These properties, observed in one object, should definitively prove the existence of black holes.

Life cycle of stars

A typical star releases energy by fusing hydrogen into helium in a nuclear furnace at its core. After the star uses up the hydrogen in the center, it begins to burn out in the shell of the star, which increases in size and swells. The size of the star increases, its temperature decreases. This process gives rise to red giants and supergiants. The lifespan of each star is determined by its mass. Massive stars end their life cycle with an explosion. Stars like the Sun shrink, becoming dense white dwarfs. During the process of transforming from a red giant to a white dwarf, a star can shed its outer layers as a light gaseous envelope, exposing the core.