Most of a star's life is occupied by processes. Life cycle of a star
Astrophysics has already made sufficient progress in studying the evolution of stars. Theoretical models are supported by reliable observations, and although there are some gaps, the general picture of a star's life cycle has long been known.
Birth
It all starts with a molecular cloud. These are huge regions of interstellar gas that are dense enough for hydrogen molecules to form in them.
Then an event occurs. Perhaps it will be caused by a shock wave from a supernova that exploded nearby, or perhaps by natural dynamics inside the molecular cloud. However, there is only one outcome - gravitational instability leads to the formation of a center of gravity somewhere inside the cloud.
Yielding to the temptation of gravity, the surrounding matter begins to rotate around this center and layers on its surface. Gradually, a balanced spherical core with increasing temperature and luminosity is formed - a protostar.
The disk of gas and dust around the protostar rotates faster and faster, due to its growing density and mass, more and more particles collide in its depths, and the temperature continues to rise.
As soon as it reaches millions of degrees, the first thermonuclear reaction occurs in the center of the protostar. Two hydrogen nuclei overcome the Coulomb barrier and combine to form a helium nucleus. Then another two nuclei, then another... until the chain reaction covers the entire region in which the temperature allows hydrogen to synthesize helium.
The energy of thermonuclear reactions then rapidly reaches the surface of the star, sharply increasing its brightness. So a protostar, if it has enough mass, turns into a full-fledged young star.
Active star forming region N44 / ©ESO, NASA
No childhood, no adolescence, no youth
All protostars that warm up enough to trigger a thermonuclear reaction in their cores then enter the longest and most stable period, occupying 90% of their entire existence.
All that happens to them at this stage is the gradual burning of hydrogen in the zone of thermonuclear reactions. Literally "burning through life." The star will very slowly - over billions of years - become hotter, the intensity of thermonuclear reactions will increase, as will the luminosity, but nothing more.
Of course, events are possible that accelerate stellar evolution - for example, a close proximity or even a collision with another star, but this does not depend in any way on the life cycle of an individual star.
There are also peculiar “stillborn” stars that cannot reach the main sequence - that is, they are not able to cope with the internal pressure of thermonuclear reactions.
These are low-mass (less than 0.0767 of the mass of the Sun) protostars - the same ones that are called brown dwarfs. Due to insufficient gravitational compression, they lose more energy than is formed as a result of hydrogen synthesis. Over time the thermo nuclear reactions in the depths of these stars cease, and all that remains for them is a long but inevitable cooling.
Artist's impression of a brown dwarf / ©ESO/I. Crossfield/N. Risinger
Troubled old age
Unlike people, the most active and interesting phase in the “life” of massive stars begins towards the end of their existence.
The further evolution of each individual luminary that reached the end main sequence- that is, the point when there is no more hydrogen left for thermonuclear fusion in the center of the star - directly depends on the mass of the star and its chemical composition.
The less mass a star has on the main sequence, the longer its “life” will be, and the less grandiose its ending will be. For example, stars with a mass less than half the mass of the Sun - those called red dwarfs - have never “died” at all since big bang. According to calculations and computer simulations, such stars, due to the weak intensity of thermonuclear reactions, can quietly burn hydrogen for tens of billions to tens of trillions of years, and at the end of their journey they will probably go out in the same way as brown dwarfs.
Stars with an average mass of half to ten solar masses, after burning out hydrogen in the center, are able to burn heavier chemical elements in their composition - first helium, then carbon, oxygen and then, depending on the mass, up to iron-56 (an isotope of iron, which is sometimes called "thermonuclear combustion ash").
For such stars, the phase following the main sequence is called the red giant stage. Launching helium thermonuclear reactions, then carbon ones, etc. each time leads to significant transformations of the star.
In a sense, this is the death throes. The star then expands hundreds of times and turns red, then contracts again. The luminosity also changes - it increases thousands of times, then decreases again.
At the end of this process, the red giant's outer shell is shed, forming a spectacular planetary nebula. What remains at the center is an exposed core - a white helium dwarf with a mass of approximately half the Sun and a radius approximately equal to the radius of the Earth.
White dwarfs have a fate similar to red dwarfs - quietly burning out over billions to trillions of years, unless, of course, there is a companion star nearby, due to which the white dwarf can increase its mass.
The KOI-256 system, consisting of red and white dwarfs / ©NASA/JPL-Caltech
Extreme old age
If the star is particularly lucky with its mass, and it is approximately 12 solar or more, then the final stages of its evolution are characterized by much more extreme events.
If the mass of the red giant's core exceeds the Chandrasekhar limit of 1.44 solar masses, then the star not only sheds its shell in the finale, but releases the accumulated energy in a powerful thermonuclear explosion - a supernova.
In the heart of the remnants of a supernova, which scatters stellar matter with enormous force for many light years around, in this case what remains is not a white dwarf, but a super-dense neutron star, with a radius of only 10-20 kilometers.
However, if the mass of the red giant is more than 30 solar masses (or rather, already a supergiant), and the mass of its core exceeds the Oppenheimer-Volkov limit, equal to approximately 2.5-3 solar masses, then neither a white dwarf nor a neutron star is formed.
In the center of the supernova remnant, something much more impressive appears - a black hole, since the core of the exploding star is compressed so much that even neutrons begin to collapse, and nothing else, including light, can leave the newborn black hole - or rather, its event horizon.
Particularly massive stars - blue supergiants - can bypass the red supergiant stage and also explode in a supernova.
Supernova SN 1994D in the galaxy NGC 4526 (bright point in the lower left corner) / ©NASA
What awaits our Sun?
The Sun is a medium-mass star, so if you carefully read the previous part of the article, you yourself can predict exactly what path our star is on.
However, humanity will face a series of astronomical shocks even before the Sun turns into a red giant. Life on Earth will become impossible within a billion years, when the intensity of thermonuclear reactions at the center of the Sun becomes sufficient to evaporate the Earth's oceans. In parallel with this, conditions for life on Mars will improve, which at some point may make it suitable for habitation.
In about 7 billion years, the Sun will warm up enough to trigger a thermonuclear reaction in its outer regions. The radius of the Sun will increase by about 250 times, and the luminosity will increase by 2700 times - it will transform into a red giant.
Due to the increased solar wind, the star at this stage will lose up to a third of its mass, but will have time to absorb Mercury.
The mass of the solar core, due to the burning of hydrogen around it, will then increase so much that a so-called helium flare will occur, and thermonuclear fusion of helium nuclei into carbon and oxygen will begin. The radius of the star will decrease significantly, to 11 standard solar.
Solar activity / ©NASA/Goddard/SDO
However, 100 million years later, the reaction with helium will move to the outer regions of the star, and it will again increase to the size, luminosity and radius of a red giant.
The solar wind at this stage will become so strong that it will blow the outer regions of the star into space, and they form a vast planetary nebula.
And where the Sun was, there will remain a white dwarf the size of the Earth. At first extremely bright, but as time goes on it becomes dimmer and dimmer.
INTRODUCTION
CHAPTER 1. Evolution of stars
CHAPTER 2.Thermonuclear fusion in the interior of stars and the birth of stars
CHAPTER 3. Mid-life cycle of a star
CHAPTER 4. Later years and death of stars
CONCLUSION
Literature
INTRODUCTION
Modern scientific sources indicate that the universe consists of 98% stars, which “in turn” are the main element of the galaxy. Information sources give various definitions this concept, here are some of them:
Star - heavenly body, in which thermonuclear reactions are taking place, have taken place or will take place. 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, which occurs when high temperatures in the interior areas. Stars are often called the main bodies of the Universe, since they contain the bulk of luminous matter in nature.
Stars are huge, spherical objects made of helium and hydrogen, as well as other gases. The energy of a star is contained in its core, where helium interacts with hydrogen every second.
Like everything organic in our universe, stars arise, develop, change and disappear - this process takes billions of years and is called the process of “Star Evolution”.
CHAPTER 1. Evolution of stars
Evolution of stars- the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years while it emits light and heat.
A star begins its life as a cold, rarefied cloud of interstellar gas (a rarefied gaseous medium that fills all the space between stars), compressing under its own gravity and gradually taking the shape of a ball. During compression, gravitational energy (universal fundamental interaction between all material bodies) turns into heat, and the temperature of the object increases. When the temperature in the center reaches 15-20 million K, thermonuclear reactions begin and compression stops. The object becomes a full-fledged star. The first stage of a star's life is similar to the solar one - it is dominated by reactions of the hydrogen cycle. It remains in this state for most of its life, being on the main sequence of the Hertzsprung-Russell diagram (Fig. 1) (showing the relationship between absolute magnitude, luminosity, spectral class and surface temperature of the star, 1910), until the fuel reserves in its core. When all the hydrogen in the center of the star is converted into helium, a helium core is formed, and thermonuclear burning of hydrogen continues at its periphery. During this period, the structure of the star begins to change. Its luminosity increases, its outer layers expand, and its surface temperature decreases - the star becomes a red giant, which form a branch on the Hertzsprung-Russell diagram. The star spends significantly less time on this branch than on the main sequence. When the accumulated mass of the helium core becomes significant, it cannot 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).
Rice. 1. Hertzsprung-Russell diagram
Evolution of a class G star using the example of the Sun
CHAPTER 2. Thermonuclear fusion in the interior of stars
By 1939, it was established that the source of stellar energy was thermonuclear fusion occurring in the bowels of stars. Most stars emit radiation because in their core four protons combine through a series of intermediate steps into a single alpha particle. This transformation can occur in two main ways, called the proton-proton, or p-p, cycle, and the carbon-nitrogen, or CN, cycle. In low-mass stars, energy release is mainly provided by the first cycle, in heavy stars - by the second. Stock 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 stellar evolution.
The Birth of Stars
The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in a galaxy actually contains between 0.1 and 1 molecule per cm³. The molecular cloud has a density of about a million molecules per cm³. The mass of such a cloud exceeds the mass of the Sun by 100,000-10,000,000 times due to its size: from 50 to 300 light years in diameter.
While the cloud rotates freely around the center of its home galaxy, nothing happens. However, due to the 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 under the influence of gravitational attraction forces future star. Half of the released gravitational energy goes to heating the cloud, and half goes to light radiation. In clouds, pressure and density increase towards the center, and the collapse of the central part occurs faster than the periphery. As it contracts, the mean free path of photons decreases, and the cloud becomes less and less transparent to its own radiation. This leads to 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.
Stellar evolution in astronomy is the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years while it emits light and heat. Over such enormous periods of time, the changes are quite significant.
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 3 . A molecular cloud has a density of about a million molecules per cm 3 . The mass of such a cloud exceeds the mass of the Sun by 100,000–10,000,000 times due to its size: from 50 to 300 light years across.
The evolution of a star begins in a giant molecular cloud, also called a stellar cradle.
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 that causes 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 trigger the process of star formation.
any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.
During this process, the inhomogeneities of the molecular cloud will compress under the influence of their own gravity and gradually take the shape of a ball. When compressed, gravitational energy 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.
Subsequent stages of a star's evolution depend almost entirely on its mass, and only at the very end of a star's evolution can its chemical composition play a role.
The first stage of a star's life is similar to the sun's - it is dominated by hydrogen cycle reactions.
It remains in this state for most of its life, being on the main sequence of the Hertzsprung–Russell diagram, until the fuel reserves in its core run out. When all the hydrogen in the center of the star is converted into helium, a helium core is formed, and thermonuclear burning of hydrogen continues at the periphery of the core.
Small, cool red dwarfs slowly burn up their hydrogen reserves and remain on the main sequence for tens of billions of years, while massive supergiants leave the main sequence within a few tens of millions (and some just a few million) years after formation.
At present, it is not known for certain what happens to light stars after the supply of hydrogen in their cores is depleted. Since the age of the universe is 13.8 billion years, which is not enough for such stars to deplete the supply of hydrogen fuel, modern theories are based on computer modeling processes occurring in such stars.
According to theoretical concepts, some of the light stars, losing their matter (stellar wind), will gradually evaporate, becoming smaller and smaller. Others, red dwarfs, will slowly cool over billions of years while continuing to emit faint emissions in the infrared and microwave ranges of the electromagnetic spectrum.
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.
Once a star runs out of hydrogen in its core, it leaves the main sequence.
Without the pressure that arose during thermonuclear reactions and balanced the internal gravity, the star begins to shrink again, as it had previously during the process of its formation.
Temperature and pressure rise again, but, unlike the protostar stage, to a much higher level.
The collapse continues until, at a temperature of approximately 100 million K, thermonuclear reactions involving helium begin, during which helium is converted into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally – silicon to iron).
The collapse continues until thermonuclear reactions involving helium begin at a temperature of approximately 100 million K
The thermonuclear “burning” of matter, resumed at a new level, causes a monstrous expansion of the star. The star "swells", becoming very "loose", and its size increases approximately 100 times.
The star becomes a red giant, and the helium burning phase lasts about several million years.
What happens next also depends on the mass of the star.
At the stars average size The reaction of thermonuclear burning of helium can lead to explosive release outer layers stars forming from them planetary nebula. The core of the star, in which thermonuclear reactions stop, cools down and turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar masses and a diameter on the order of the diameter of the Earth.
For massive and supermassive stars (with a mass of five solar masses or more), the processes occurring in their core as gravitational compression increases lead to an explosion supernova with the release of enormous energy. The explosion is accompanied by the ejection of a significant mass of star matter into interstellar space. This substance subsequently participates in the formation of new stars, planets or satellites. It is thanks to supernovae that the Universe as a whole, and each galaxy in particular, chemically evolves. The stellar core remaining after the explosion may end up evolving as a neutron star (pulsar) if the star's late-stage mass exceeds the Chandrasekhar limit (1.44 Solar masses), or as a black hole if the star's mass exceeds the Oppenheimer–Volkoff limit (estimated values of 2 .5-3 Solar masses).
The process of stellar evolution in the Universe is continuous and cyclical - old stars fade away and new ones light up to replace them.
According to modern scientific concepts, the elements necessary for the emergence of planets and life on Earth were formed from stellar matter. Although there is no single generally accepted point of view on how life arose.
If enough matter accumulates somewhere in the Universe, it is compressed into a dense lump, in which a thermonuclear reaction begins. This is how stars light up. The first ones 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 lifespan of a star and the processes occurring at the end of this period depend on the mass of the star.
While the thermonuclear reaction of converting hydrogen into helium continues in a star, it is on the main sequence. The time a star spends on the main sequence depends on its mass: the largest and heaviest ones quickly reach the red giant stage, and then leave the main sequence as a result of a supernova explosion or the formation of a white dwarf.
Fate of the Giants
The largest and most massive stars burn out quickly and explode as supernovae. After a supernova explosion, what remains is a neutron star or black hole, and around them is matter ejected by the colossal energy of the explosion, which then becomes material for new stars. Of our closest stellar neighbors, such a fate awaits, for example, Betelgeuse, but it is impossible to calculate when it will explode.
A nebula formed as a result of the ejection of matter during a supernova explosion. At the center of the nebula is a neutron star.
A neutron star is a scary physical phenomenon. The core of an exploding star is compressed, much like gas in an engine. internal combustion, only in a very large and effective way: 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 strong that electrons fall onto atomic nuclei, forming neutrons - hence the name.
NASA
Neutron star (artist's vision)
The density of matter during such compression increases by about 15 orders of magnitude, and the temperature rises to an incredible 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, while some is carried away by neutrinos produced in the core of a neutron star. But even due to very efficient neutrino cooling, a neutron star cools very slowly: it takes 10 16 or even 10 22 years to completely exhaust its energy. It is difficult to say what will remain in the place of the cooled neutron star, and impossible to observe: the world is too young for that. There is an assumption that a black hole will again form in place of the cooled star.
Black holes arise from the gravitational collapse of very massive objects, such as supernova explosions. Perhaps, after trillions of years, cooled neutron stars will turn into black holes.
The fate of medium-sized stars
Other, less massive stars remain on the main sequence longer than the largest ones, but once they leave it, they die much faster than their neutron relatives. More than 99% of the stars in the Universe will never explode and turn into either black holes or neutron stars - their cores are too small for such cosmic dramas. Instead, intermediate-mass stars become red giants at the end of their lives, which, depending on their mass, become white dwarfs, explode and dissipate completely, or become neutron stars.
White dwarfs now make up from 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, both due to their 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 is a thousand times older than it is now—there will be new type object: a black dwarf, a product of the cooling of a white dwarf.
There are no black dwarfs in space yet. 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 to 19,960 K.
For the little ones
Science knows even less 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 proceeds slowly, and they remain on the main sequence longer than others - according to some calculations, up to 10 12 years, and after that, presumably, they will continue to live as white dwarfs, that is, they will shine for another 10 14 - 10 15 years before transformation into a black dwarf.
Star evolution is change over time. physical characteristics, internal structure and chemical composition of stars. The modern theory of stellar evolution is capable of explaining the general course of stellar development in satisfactory agreement with the data of astronomical observations. The course of a star's evolution depends on its mass and initial chemical composition. The stars of the first generation were formed from matter, the composition of which was determined by cosmological conditions (about 70% hydrogen, 30% helium, an insignificant admixture of deuterium and lithium). During the evolution of first-generation stars, heavy elements were formed that were ejected into interstellar space as a result of the outflow of matter from stars or during stellar explosions. Stars of subsequent generations were formed from matter containing 3–4% heavy elements.
The birth of a star is the formation of an object whose radiation is supported by its own energy sources. The process of star formation continues continuously, and it continues to this day.
To explain the structure of the megaworld, the most important is gravitational interaction. In gas and dust nebulae, under the influence of gravitational forces, unstable inhomogeneities are formed, due to which diffuse matter breaks up into a series of condensations. If such condensations persist long enough, then over time they turn into stars. It is important to note that the birth process is not of an individual star, but of stellar associations. The resulting gas bodies are attracted to each other, but do not necessarily combine into one huge body. They usually begin to rotate relative to each other, and the centrifugal forces of this movement counteract the attractive forces leading to further concentration.
Young stars are those that are still in the stage of initial gravitational compression. The temperature at the center of such stars is not yet sufficient for thermonuclear reactions to occur. The glow of stars occurs only due to the conversion of gravitational energy into heat. Gravitational compression is the first stage in the evolution of stars. It leads to heating of the central zone of the star to the temperature at which the thermonuclear reaction begins (10 – 15 million K) – the transformation of hydrogen into helium.
The enormous energy emitted by stars is generated as a result of nuclear processes occurring inside stars. The energy generated inside a star allows it to emit light and heat for millions and billions of years. For the first time, the assumption that the source of stellar energy is thermonuclear reactions of the synthesis of helium from hydrogen was put forward in 1920 by the English astrophysicist A.S. Eddington. In the interior of stars, two types of thermonuclear reactions involving hydrogen are possible, called the hydrogen (proton-proton) and carbon (carbon-nitrogen) cycles. In the first case, only hydrogen is required for the reaction to occur; in the second, the presence of carbon is also necessary, serving as a catalyst. The starting material is protons, from which helium nuclei are formed as a result of nuclear fusion.
Since the transformation of four protons into a helium nucleus produces two neutrinos, 1.8∙10 38 neutrinos are generated every second in the depths of the Sun. Neutrinos interact weakly with matter and have great penetrating power. Having passed through a huge thickness of solar matter, neutrinos retain all the information that they received in thermonuclear reactions in the depths of the Sun. The flux density of solar neutrinos falling on the Earth's surface is 6.6∙10 10 neutrinos per 1 cm 2 per 1 s. Measuring the flux of neutrinos falling on the Earth makes it possible to judge the processes occurring inside the Sun.
Thus, the source of energy for most stars is hydrogen thermonuclear reactions in the central zone of the star. As a result of a thermonuclear reaction, an outward flow of energy occurs in the form of radiation over a wide range of frequencies (wavelengths). The interaction between radiation and matter results in a steady state of equilibrium: the pressure of outward radiation is balanced by the pressure of gravity. Further compression of the star stops while sufficient quantity energy. This state is quite stable, and the size of the star remains constant. Hydrogen is the main one component cosmic matter and the most important type of nuclear fuel. The star's hydrogen reserves last for billions of years. This explains why stars are so stable long time. Until all the hydrogen in the central zone burns out, the properties of the star change little.
The hydrogen burnout field in the central zone of the star forms a helium core. Hydrogen reactions continue to occur, but only in thin layer near the surface of the nucleus. Nuclear reactions move to the periphery of the star. The structure of the star at this stage is described by models with a layered energy source. The burnt-out core begins to shrink, and the outer shell begins to expand. The shell swells to colossal sizes, the external temperature becomes low. The star enters the red giant stage. From this moment on, the star's life begins to decline. Red giants are different low temperatures and huge sizes (from 10 to 1000 R c). Average density the substance in them does not reach 0.001 g/cm 3 . Their luminosity is hundreds of times higher than the luminosity of the Sun, but the temperature is much lower (about 3000 - 4000 K).
It is believed that our Sun, when transitioning to the red giant stage, can increase so much that it fills the orbit of Mercury. True, the Sun will become a red giant in 8 billion years.
The red giant is characterized by low external temperatures, but very high internal temperatures. As it increases, increasingly heavier nuclei are included in thermonuclear reactions. At a temperature of 150 million K, helium reactions begin, which are not only a source of energy, but during them the synthesis of heavier chemical elements is carried out. After the formation of carbon in the helium core of a star, the following reactions are possible:
It should be noted that the synthesis of the next heavier nucleus requires higher and higher energies. By the time magnesium is formed, all the helium in the star's core is depleted, and in order for further nuclear reactions to become possible, the star must contract again and its temperature rise. However, this is not possible for all stars, only for large ones whose mass exceeds the mass of the Sun by more than 1.4 times (the so-called Chandrasekhar limit). In stars of lower mass, reactions end at the stage of magnesium formation. In stars whose mass exceeds the Chandrasekhar limit, due to gravitational compression, the temperature rises to 2 billion degrees, reactions continue, forming heavier elements - up to iron. Elements heavier than iron are formed when stars explode.
As a result of increasing pressure, pulsations and other processes, the red giant continuously loses matter, which is ejected into interstellar space in the form of stellar wind. When the internal thermonuclear energy sources are completely depleted, the further fate of the star depends on its mass.
With a mass less than 1.4 solar masses, the star enters a stationary state with a very high density (hundreds of tons per 1 cm 3). Such stars are called white dwarfs. In the process of transforming a red giant into a white dwarf, a race can shed its outer layers like a light shell, exposing the core. The gas shell glows brightly under the influence of powerful radiation from the star. This is how planetary nebulae are formed. At high densities of matter inside a white dwarf, the electron shells of atoms are destroyed, and the matter of the star is an electron-nuclear plasma, and its electron component is a degenerate electron gas. White dwarfs are in an equilibrium state due to the equality of forces between gravity (compression factor) and the pressure of degenerate gas in the bowels of the star (expansion factor). White dwarfs can exist for billions of years.
The thermal reserves of the star are gradually depleted, the star is slowly cooling, which is accompanied by ejections of the stellar envelope into interstellar space. The star gradually changes its color from white to yellow, then to red, finally it stops emitting, becoming a small lifeless object, a dead cold star, the size of which smaller sizes Earth, and the mass is comparable to the mass of the Sun. The density of such a star is billions of times greater than the density of water. Such stars are called black dwarfs. This is how most stars end their existence.
When the mass of the star is more than 1.4 solar masses, the stationary state of the star without internal energy sources becomes impossible, because the pressure inside the star cannot balance the force of gravity. Gravitational collapse begins - compression of matter towards the center of the star under the influence of gravitational forces.
If the repulsion of particles and other reasons stop the collapse, then a powerful explosion occurs ─ a supernova explosion with the ejection of a significant part of the matter into the surrounding space and the formation of gas nebulae. The name was proposed by F. Zwicky in 1934. A supernova explosion is one of the intermediate stages in the evolution of stars before their transformation into white dwarfs, neutron stars or black holes. During an explosion, energy is released in the amount of 10 43 ─ 10 44 J with a radiation power of 10 34 W. In this case, the brightness of the star increases by tens of magnitudes in a few days. The luminosity of a supernova can exceed the luminosity of the entire galaxy in which it exploded.
The gas nebula formed during a supernova explosion consists partly of elements ejected by the explosion. upper layers stars, and partly from interstellar matter, compacted and heated by the scattering products of the explosion. The most famous gas nebula is the Crab Nebula in the constellation Taurus - a remnant of the supernova of 1054. Young supernova remnants are expanding at speeds of 10-20 thousand km/s. The collision of the expanding shell with stationary interstellar gas generates a shock wave in which the gas is heated to millions of Kelvin and becomes a source of X-ray radiation. The propagation of a shock wave in a gas leads to the appearance of fast charged particles ( cosmic rays), which, moving in an interstellar magnetic field compressed and enhanced by the same wave, emit in the radio range.
Astronomers recorded supernova explosions in 1054, 1572, 1604. In 1885, a supernova was observed in the Andromeda nebula. Its brilliance exceeded the brilliance of the entire Galaxy and turned out to be 4 billion times more intense than the brilliance of the Sun.
By 1980, more than 500 supernova explosions had been discovered, but not a single one had been observed in our Galaxy. Astrophysicists have calculated that in our Galaxy, supernovae explode with a period of 10 million years in the immediate vicinity of the Sun. On average, a supernova explosion occurs in the Metagalaxy every 30 years.
Doses cosmic radiation on Earth, they can exceed normal levels by 7,000 times. This will lead to serious mutations in living organisms on our planet. Some scientists explain the sudden death of dinosaurs this way.
Part of the mass of an exploding supernova may remain in the form of a superdense body - a neutron star or black hole. The mass of neutron stars is (1.4 – 3) M s, the diameter is about 10 km. The density of a neutron star is very high, higher than the density atomic nuclei─ 10 15 g/cm 3 . With increasing compression and pressure, the reaction of absorption of electrons by protons becomes possible 
As a result, all the matter of the star will consist of neutrons. Neutronization of a star is accompanied by a powerful burst of neutrino radiation. During the supernova explosion SN1987A, the duration of the neutrino burst was 10 s, and the energy carried away by all neutrinos reached 3∙10 46 J. The temperature of the neutron star reaches 1 billion K. Neutron stars cool very quickly, their luminosity weakens. But they intensely emit radio waves in a narrow cone in the direction of the magnetic axis. Stars whose magnetic axis does not coincide with the axis of rotation are characterized by radio emission in the form of repeating pulses. That's why neutron stars are called pulsars. The first pulsars were discovered in 1967. The frequency of radiation pulsations, determined by the rotation speed of the pulsar, is from 2 to 200 Hz, which indicates their small size. For example, the pulsar in the Crab Nebula has a pulse emission period of 0.03 s. Hundreds of neutron stars are currently known. A neutron star may appear as a result of the so-called “silent collapse”. If a white dwarf enters dual system from nearby stars, the phenomenon of accretion occurs when matter from a neighboring star flows onto the white dwarf. The mass of the white dwarf grows and at a certain point exceeds the Chandrasekhar limit. A white dwarf turns into a neutron star.
If the final mass of the white dwarf exceeds 3 solar masses, then the degenerate neutron state is unstable and gravitational contraction continues until the formation of an object called a black hole. The term “black hole” was introduced by J. Wheeler in 1968. However, the idea of such objects arose several centuries earlier, after the discovery of the law of universal gravitation by I. Newton in 1687. In 1783, J. Mitchell suggested that dark stars should exist in nature, the gravitational field of which is so strong that light cannot escape from them. In 1798, the same idea was expressed by P. Laplace. In 1916, physicist Schwarzschild, solving Einstein's equations, came to the conclusion about the possibility of the existence of objects with unusual properties, later called black holes. A black hole is a region of space in which the gravitational field is so strong that the second escape velocity for bodies located in this area must exceed the speed of light, i.e. Nothing can fly out of a black hole - neither particles nor radiation. In accordance with general theory relativity, the characteristic size of a black hole is determined by the gravitational radius: R g =2GM/c 2, where M is the mass of the object, c is the speed of light in vacuum, G is the gravitational constant. The gravitational radius of the Earth is 9 mm, the Sun is 3 km. The boundary of the region beyond which light does not escape is called the event horizon of a black hole. Rotating black holes have an event horizon radius smaller than the gravitational radius. Of particular interest is the possibility of a black hole capturing bodies arriving from infinity.
The theory allows the existence of black holes with a mass of 3–50 solar masses, formed in the late stages of the evolution of massive stars with a mass of more than 3 solar masses, supermassive black holes in the cores of galaxies weighing millions and billions of solar masses, primary (relict) black holes formed in the early stages of the evolution of the Universe. Relic black holes weighing more than 10 15 g (the mass of an average mountain on Earth) should have survived to this day due to the mechanism of quantum evaporation of black holes proposed by S.W. Hawking.
Astronomers detect black holes with powerful x-ray radiation. An example of this type of star is the powerful X-ray source Cygnus X-1, whose mass exceeds 10 M s. Black holes often occur in X-ray binaries star systems. Dozens of stellar-mass black holes have already been discovered in such systems (m black holes = 4-15 M s). Based on the effects of gravitational lensing, several single black holes of stellar mass have been discovered (m black holes = 6-8 M s). In the case of a close binary star, the phenomenon of accretion is observed - the flow of plasma from the surface of an ordinary star under the influence of gravitational forces onto a black hole. Matter flowing into a black hole has angular momentum. Therefore, the plasma forms a rotating disk around the black hole. The temperature of the gas in this rotating disk can reach 10 million degrees. At this temperature the gas emits X-rays. This radiation can be used to determine the presence of a black hole in a given location.
Of particular interest are supermassive black holes in the nuclei of galaxies. Based on the study of the X-ray image of the center of our Galaxy, obtained using the CHANDRA satellite, the presence of a supermassive black hole, the mass of which is 4 million times the mass of the Sun, has been established. As a result of recent research, American astronomers have discovered a unique superheavy black hole located in the center of a very distant galaxy, the mass of which is 10 billion times the mass of the Sun. In order to reach such unimaginably enormous size and density, the black hole must have formed over many billions of years, continuously attracting and absorbing matter. Scientists estimate its age at 12.7 billion years, i.e. it began to form approximately one billion years after the Big Bang. To date, more than 250 supermassive black holes have been discovered in the nuclei of galaxies (m black holes = (10 6 – 10 9) M s).
Closely related to the evolution of stars is the question of the origin of chemical elements. If hydrogen and helium are the elements left over from early stages evolution of the expanding Universe, then heavier chemical elements could only be formed in the interior of stars during thermonuclear reactions. Inside stars, thermonuclear reactions can produce up to 30 chemical elements (iron inclusive).
Based on their physical state, stars can be divided into normal and degenerate. The former consist mainly of low-density matter; thermonuclear fusion reactions take place in their depths. Degenerate stars include white dwarfs and neutron stars; they represent the final stage of stellar evolution. The fusion reactions in them have ended, and the equilibrium is maintained by the quantum mechanical effects of degenerate fermions: electrons in white dwarfs and neutrons in neutron stars. White dwarfs, neutron stars and black holes are collectively called “compact remnants”.
At the end of evolution, depending on the mass, the star either explodes or more quietly dumps matter already enriched with heavy chemical elements. In this case, the remaining elements are formed periodic table. Stars of the next generations are formed from the interstellar medium enriched with heavy elements. For example, the Sun is a second-generation star, formed from matter that has already been in the bowels of stars and was enriched with heavy elements. Therefore, the age of stars can be judged by their chemical composition, determined by spectral analysis.