Life cycle of stars. The evolution of stars of different masses
Evolution of Stars of Different Masses
Astronomers cannot observe the life of one star from beginning to end, because even the shortest-lived stars exist for millions of years - longer life of all mankind. Change over time physical characteristics and chemical composition of stars, i.e. stellar evolution, astronomers study by comparing the characteristics of many stars at different stages of evolution.
The physical laws that connect the observed characteristics of stars are reflected in the color-luminosity diagram - the Hertzsprung-Russell diagram, on which the stars form separate groups - sequences: the main sequence of stars, sequences of supergiants, bright and weak giants, subgiants, subdwarfs and white dwarfs.
For most of its life, any star is on the so-called main sequence color-luminosity diagrams. All other stages of the evolution of a star before the formation of a compact remnant take no more than 10% of this time. That is why most of the stars observed in our Galaxy are modest red dwarfs with the mass of the Sun or less. The main sequence includes about 90% of all observed stars.
The lifespan of a star and what it turns into at the end life path, is completely determined by its mass. Stars with a mass greater than the sun's mass live much less than the Sun, and the lifetime of the most massive stars is only millions of years. For the vast majority of stars, the lifetime is about 15 billion years. After the star exhausts its sources of energy, it begins to cool and shrink. The end product of the evolution of stars are compact massive objects, the density of which is many times greater than that of ordinary stars.
Stars different weight eventually come to one of three states: white dwarfs, neutron stars, or black holes. If the star's mass is small, then the gravitational forces are relatively weak and the star's compression (gravitational collapse) stops. It enters the stable state of a white dwarf. If the mass exceeds a critical value, compression continues. At very high density, electrons combine with protons to form neutrons. Soon, almost the entire star consists of only neutrons and has such an enormous density that a huge stellar mass is concentrated in a very small ball with a radius of several kilometers and the compression stops - a neutron star. If the mass of the star is so great that even the formation of a neutron star does not stop the gravitational collapse, then the final stage in the evolution of the star will be a black hole.
At the beginning of the 20th century, Hertzsprung and Russell plotted various stars on the "Absolute Magnitude" - "spectral class" diagram, and it turned out that most of them were grouped along a narrow curve. Later, this diagram (now called the Hertzsprung-Russell diagram) turned out to be the key to understanding and studying the processes occurring inside the star.
The diagram makes it possible (although not very accurately) to find the absolute value of the spectral class. especially for spectral classes O-F. For later classes, this is complicated by the need to make a choice between a giant and a dwarf. However, certain differences in the intensity of some lines allow us to confidently make this choice.
Most of the stars (about 90%) are located on the diagram along a long narrow strip called main sequence. It stretched from the upper left corner (from the blue supergiants) to the lower right corner (to the red dwarfs). The main sequence stars include the Sun, whose luminosity is taken as unity.
The points corresponding to giants and supergiants are located above the main sequence on the right, and those corresponding to white dwarfs are in the lower left corner, below the main sequence.
It has now become clear that main-sequence stars are normal stars, similar to the Sun, in which hydrogen is burned in thermonuclear reactions. The main sequence is a sequence of stars of different masses. The largest stars in terms of mass are located in the upper part of the main sequence and are blue giants. The smallest mass stars are dwarfs. They are located at the bottom of the main sequence. Parallel to the main sequence, but slightly below it, are subdwarfs. They differ from main sequence stars in their lower metal content.
A star spends most of its life on the main sequence. During this period, its color, temperature, luminosity and other parameters hardly change. But before the star reaches this steady state, while still in the protostar state, it is red and for a short time more luminous than it would be on the main sequence.
Stars of large mass (supergiants) spend their energy generously, and the evolution of such stars lasts only hundreds of millions of years. Therefore, blue supergiants are young stars.
The stages of star evolution after the main sequence are also short. In this case, typical stars become red giants, and very massive stars become red supergiants. The star rapidly increases in size and its luminosity increases. It is these phases of evolution that are reflected in the Hertzsprung-Russell diagram.
Each star spends about 90% of its life on the main sequence. During this period, the main sources of energy for the star are thermonuclear reactions of the conversion of hydrogen into helium at its center. Having exhausted this source, the star moves to the region of giants, where it spends about 10% of its life. At this time, the main source of stellar energy release is the conversion of hydrogen into helium in the layer surrounding the dense helium core. This so-called red giant stage.
The birth of the stars
The evolution of a star begins in a giant molecular cloud, also called a stellar cradle, in which, as a result of gravitational instability, the primary density fluctuation begins to grow. Most of the "empty" space in the galaxy actually contains between 0.1 and 1 molecule per cm3. A molecular cloud, on the other hand, has a density of about a million molecules per cm³. The mass of such a cloud exceeds the mass of the Sun by 100,000-10,000,000 times due to its size: from 50 to 300 light-years across.
During the collapse, the molecular cloud is divided into parts, forming smaller and smaller clumps. Fragments with a mass less than ~100 solar masses are capable of forming a star. In such formations, the gas heats up as it contracts due to the release of gravitational potential energy, and the cloud becomes a protostar, transforming into a rotating spherical object.
Stars at the initial stage of their existence, as a rule, are hidden from view inside a dense cloud of dust and gas. Often the silhouettes of such star-forming cocoons can be observed against the background of bright radiation from the surrounding gas. Such formations are called Bok's globules.
A very small proportion of protostars does not reach a temperature sufficient for thermonuclear fusion reactions. Such stars are called "brown dwarfs", their mass does not exceed one tenth of the sun. Such stars die quickly, gradually cooling down over several hundred million years. In some of the most massive protostars, the temperature due to strong compression can reach 10 million K, making it possible to fuse helium from hydrogen. Such a star begins to glow. The onset of thermonuclear reactions establishes hydrostatic equilibrium, preventing the core from further gravitational collapse. Further, the star can exist in a stable state.
The initial stage of the evolution of stars
On the Hertzsprung-Russell diagram, the emerging star occupies a point in the upper right corner: it has a high luminosity and low temperature. The main radiation occurs in infrared range. Radiation from the cold dust shell reaches us. In the process of evolution, the position of the star on the diagram will change. The only source of energy at this stage is gravitational contraction. Therefore, the star moves quite quickly parallel to the y-axis.
The surface temperature does not change, but the radius and luminosity decrease. The temperature in the center of the star rises, reaching a value at which reactions begin with light elements: lithium, beryllium, boron, which quickly burn out, but manage to slow down the compression. The track rotates parallel to the y-axis, the temperature on the surface of the star rises, and the luminosity remains almost constant. Finally, in the center of the star, the reactions of the formation of helium from hydrogen (hydrogen combustion) begin. The star enters the main sequence.
The duration of the initial stage is determined by the mass of the star. For stars like the Sun, it is about 1 million years, for a star with a mass of 10 M ☉ about 1000 times smaller, and for a star with a mass of 0.1 M ☉ a thousand times more.
Main sequence stage
At the main sequence stage, the star shines due to the release of energy in the nuclear reactions of the conversion of hydrogen into helium. The supply of hydrogen provides the luminosity of a star with a mass of 1M ☉ for about 10 10 years. Stars greater mass consume hydrogen faster: for example, a star with a mass of 10 M ☉ will use up hydrogen in less than 10 7 years (luminosity is proportional to the fourth power of mass).
low mass stars
As the hydrogen burns out, the central regions of the star are strongly compressed.
Stars of high mass
After entering the main sequence, the evolution of a large-mass star (>1.5 M ☉ ) is determined by the conditions of combustion of nuclear fuel in the interior of the star. At the main sequence stage, this is the burning of hydrogen, but unlike low-mass stars, reactions of the carbon-nitrogen cycle dominate in the core. In this cycle, the C and N atoms play the role of catalysts. The rate of energy release in the reactions of such a cycle is proportional to T 17 . Therefore, a convective core is formed in the core, surrounded by a zone in which energy transfer is carried out by radiation.
The luminosity of large mass stars is much higher than the luminosity of the Sun, and hydrogen is consumed much faster. This is due to the fact that the temperature in the center of such stars is also much higher.
As the proportion of hydrogen in the substance of the convective core decreases, the rate of energy release decreases. But since the rate of release is determined by the luminosity, the core begins to shrink, and the rate of energy release remains constant. At the same time, the star expands and passes into the region of red giants.
Star maturity stage
low mass stars
By the time the hydrogen is completely burned out, a small helium core forms in the center of a low-mass star. In the core, the density of matter and temperature reach values of 10 9 kg/m 3 and 10 8 K, respectively. Hydrogen combustion occurs on the surface of the nucleus. As the temperature in the core rises, the rate of hydrogen burning increases, and the luminosity increases. The radiant zone gradually disappears. And because of the increase in the speed of convective currents, the outer layers of the star swell. Its size and luminosity increase - the star turns into a red giant.
Stars of high mass
When the hydrogen of a large mass star is completely exhausted, a triple helium reaction begins in the core and, simultaneously, the reaction of oxygen production (3He=>C and C+He=>O). At the same time, hydrogen begins to burn on the surface of the helium core. The first layer source appears.
The supply of helium is exhausted very quickly, since in the described reactions, relatively little energy is released in each elementary act. The picture repeats itself, and two layer sources appear in the star, and the C + C => Mg reaction begins in the core.
At the same time, the evolutionary track turns out to be very complicated. In the Hertzsprung-Russell diagram, the star moves along the sequence of giants or (for very large masses in the supergiant region) periodically becomes a Cepheid.
Final stages of stellar evolution
Old low-mass stars
In a star of low mass, in the end, the speed of the convective flow at some level reaches the second space velocity, the shell breaks off, and the star turns into a white dwarf, surrounded by a planetary nebula.
Death of high mass stars
At the end of its evolution, a large-mass star has a very complex structure. Each layer has its own chemical composition, nuclear reactions proceed in several layered sources, and an iron core is formed in the center.
Nuclear reactions with iron do not proceed, since they require the expenditure (rather than release) of energy. Therefore, the iron core is rapidly compressed, the temperature and density in it increase, reaching fantastic values - a temperature of 10 9 K and a density of 10 9 kg/m3.
At this point, two critical process, going in the nucleus at the same time and very quickly (apparently, in minutes). The first is that during nuclear collisions, iron atoms decay into 14 helium atoms, the second is that electrons are “pressed” into protons, forming neutrons. Both processes are associated with the absorption of energy, and the temperature in the core (also the pressure) drops instantly. The outer layers of the star begin to fall towards the center.
The fall of the outer layers leads to a sharp increase in temperature in them. Hydrogen, helium, carbon begin to burn. This is accompanied by a powerful stream of neutrons that comes from the central core. As a result, the most powerful nuclear explosion, throwing off the outer layers of the star, which already contain all the heavy elements, up to californium. According to modern views, all atoms of heavy chemical elements (i.e., heavier than helium) were formed in the Universe precisely in supernova explosions. In place of the exploded supernova, depending on the mass of the exploded star, either a neutron star or black hole.
Stellar evolution in astronomy is the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years, while it radiates light and heat. during such colossal periods of time, the changes are very significant.
The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in the galaxy actually contains 0.1 to 1 molecule per cm3. A molecular cloud, on the other hand, has a density of about a million molecules per cm3. The mass of such a cloud exceeds the mass of the Sun by 100,000–10,000,000 times due to its size: from 50 to 300 light-years across.
The evolution of a star begins in a giant molecular cloud, also called a stellar cradle.
As long as the cloud circulates freely around the center of the native galaxy, nothing happens. However, due to the inhomogeneity of the gravitational field, disturbances can arise in it, leading to local mass concentrations. Such perturbations cause the gravitational collapse of the cloud. One of the scenarios leading to this is the collision of two clouds. Another collapse-causing event could be the passage of a cloud through the dense arm of a spiral galaxy. Also a critical factor may be the explosion of a nearby supernova, the shock wave of which will collide with the molecular cloud at great speed. In addition, a collision of galaxies is possible, capable of causing a burst of star formation, as the gas clouds in each of the galaxies are compressed by the collision. In general, any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.
any inhomogeneities in the forces acting on the mass of the cloud can trigger the process of star formation.
In the course of this process, the inhomogeneities of the molecular cloud will be compressed under the influence of their own gravity and gradually take the shape of a ball. When compressed, the gravitational energy is converted into heat, and the temperature of the object increases.
When the temperature in the center reaches 15–20 million K, thermonuclear reactions begin and the compression stops. The object becomes a full-fledged star.
The subsequent stages of a star's evolution depend almost entirely on its mass, and only at the very end of a star's evolution can its chemical composition play its role.
The first stage of a star's life is similar to that of the sun - it is dominated by the reactions of the hydrogen cycle.
It remains in this state for most of its life, being on the main sequence of the Hertzsprung-Russell diagram, until the fuel reserves in its core run out. When all the hydrogen in the center of the star turns into helium, a helium core is formed, and the thermonuclear combustion of hydrogen continues on the periphery of the core.
Small and cold red dwarfs slowly burn their hydrogen reserves and remain on the main sequence for tens of billions of years, while massive supergiants leave the main sequence after only a few tens of millions (and some only a few million) years after formation.
At present, it is not known for certain what happens to light stars after the depletion of the supply of hydrogen in their interiors. Since the universe is 13.8 billion years old, which is not enough to deplete the supply of hydrogen fuel in such stars, modern theories are based on computer simulation of the processes occurring in such stars.
According to theoretical concepts, some of the light stars, losing their substance (stellar wind), will gradually evaporate, becoming smaller and smaller. Others, red dwarfs, will slowly cool down over billions of years, continuing to radiate weakly in the infrared and microwave ranges of the electromagnetic spectrum.
Medium-sized stars like the Sun stay on the main sequence for an average of 10 billion years.
It is believed that the Sun is still on it, since it is in the middle of its life cycle. As soon as the star depletes the supply of hydrogen in the core, it leaves the main sequence.
As soon as the star depletes the supply of hydrogen in the core, it leaves the main sequence.
Without the pressure generated by the fusion reactions to balance the internal gravity, the star begins to contract again, as it did earlier in the process of its formation.
The temperature and pressure rise again, but, unlike in the protostar stage, to a much higher level.
The collapse continues until, at a temperature of approximately 100 million K, thermonuclear reactions involving helium begin, during which helium is converted into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally silicon to iron).
The collapse continues until, at a temperature of approximately 100 million K, thermonuclear reactions involving helium begin.
The thermonuclear "burning" of matter resumed at a new level causes a monstrous expansion of the star. The star "swells up", becoming very "loose", and its size increases by about 100 times.
The star becomes a red giant, and the helium burning phase continues for about several million years.
What happens next also depends on the mass of the star.
By the stars medium size the reaction of thermonuclear combustion of helium can lead to an explosive ejection of the outer layers of a star with the formation of planetary nebula. The core of the star, in which thermonuclear reactions stop, cools down and turns into a helium white dwarf, as a rule, having a mass of up to 0.5-0.6 solar masses and a diameter of the order of the diameter of the Earth.
For massive and supermassive stars (with a mass of five solar masses or more), the processes occurring in their core, as gravitational compression increases, lead to an explosion supernova with the release of enormous energy. The explosion is accompanied by the ejection of a significant mass of the star's matter into interstellar space. This substance is further involved in the formation of new stars, planets or satellites. It is thanks to supernovae that the Universe as a whole and each galaxy in particular chemically evolves. The core of the star left after the explosion can end its evolution as a neutron star (pulsar), if the mass of the star in the later stages exceeds the Chandrasekhar limit (1.44 solar masses), or as a black hole, if the mass of the star exceeds the Oppenheimer-Volkov limit (estimated values 2 ,5-3 solar masses).
The process of stellar evolution in the Universe is continuous and cyclical - old stars die out, new ones are lit to replace them.
According to modern scientific ideas, 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.
Astrophysics has already advanced enough in the study of the evolution of stars. Theoretical models are supported by reliable observations, and despite some gaps, the overall 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 dense enough for hydrogen molecules to form.
Then an event occurs. Perhaps it will be caused by a shock wave from a supernova that exploded nearby, or maybe by the 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 is layered on its surface. Gradually, a balanced spherical core with increasing temperature and luminosity is formed - a protostar.
The gas and dust disk around the protostar rotates faster and faster, due to its growing density and mass, more and more particles collide in its depths, 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 - the other two nuclei, then - the other ... 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 formation region N44 / ©ESO, NASA
No childhood, no adolescence, no youth
All protostars that heat up enough to start a thermonuclear reaction in their interiors then enter the longest and most stable period, taking up 90% of their entire lifetime.
Everything that happens to them this stage, this is the gradual burnout of hydrogen in the zone of thermonuclear reactions. Literally "burning life". The star very slowly - over billions of years - will 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 neighborhood or even a collision with another star, but this does not depend 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 very ones that are called brown dwarfs. Due to insufficient gravitational compression, they lose more energy than is formed as a result of hydrogen fusion. Over time, thermonuclear reactions in the interiors of these stars cease, and all that remains for them is a prolonged but inevitable cooling.
An artist's view 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 star that has reached the end of the 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 smaller the mass of a star on the main sequence, the longer its "life" will be, and the less grandiose its finale will be. For example, stars with masses less than half that of the Sun - such as are called red dwarfs - have never "died" at all since the Big Bang. According to calculations and computer simulations, due to the low intensity of thermonuclear reactions, such stars can easily burn hydrogen from tens of billions to tens of trillions of years, and at the end of their journey, they will probably go out just like 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 its composition - first helium, then carbon, oxygen, and then, how lucky with the mass, up to iron-56 (an isotope of iron, which is sometimes called the "ash of thermonuclear combustion").
For such stars, the phase following the main sequence is called the red giant stage. Starting helium thermonuclear reactions, then carbon, etc. each time leads to significant transformations of the star.
In a way, this is death throes. The star either 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 off, forming a spectacular planetary nebula. A naked core remains in the center - a white helium dwarf with a mass of approximately half the solar mass and a radius approximately equal to the radius of the Earth.
White dwarfs have a fate similar to red dwarfs - a quiet burnout for 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 a star is especially lucky with its mass, and it is about 12 solar masses or more, then the final stages of its evolution are characterized by much more extreme events.
If the mass of the core of a red giant exceeds the Chandrasekhar limit of 1.44 solar masses, then the star does not just shed its shell in the final, but releases the accumulated energy in a powerful thermonuclear explosion- supernova.
In the heart of the remnants of a supernova, which scatters stellar matter with great force for many light years around, in this case it is no longer a white dwarf, but a superdense neutron star with a radius of only 10-20 kilometers.
However, if the mass of a red giant is more than 30 solar masses (or rather, already a supergiant), and the mass of its core exceeds the Oppenheimer-Volkov limit, which is approximately 2.5-3 solar masses, then neither a white dwarf nor a neutron star is formed.
Something much more impressive appears in the center of the remnants of a supernova - a black hole, as the core of the exploded star is compressed so much that even neutrons begin to collapse, and nothing else, including light, can leave the limits of 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 dot in the lower left corner) / ©NASA
And what about our Sun?
The sun belongs to the stars of medium mass, so if you carefully read the previous part of the article, then you yourself can predict exactly which path our star is on.
However, even before the transformation of the Sun into a red giant, humanity is waiting for a number of astronomical upheavals. Life on Earth will become impossible in a billion years, when the intensity of thermonuclear reactions in the center of the Sun becomes sufficient to evaporate the Earth's oceans. In parallel with this, the conditions for life on Mars will improve, which at some point may make it habitable.
In about 7 billion years, the Sun will have warmed up enough for a thermonuclear reaction to start in its outer regions. The radius of the Sun will increase by about 250 times, and the luminosity by 2700 times - there will be a transformation 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 it 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 the so-called helium flash will occur, and the 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, already 100 million years later, the reaction with helium will go 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 be a white dwarf the size of the Earth. Extremely bright at first, but as time goes on, it gets dimmer and dimmer.
It occupies a point in the upper right corner: it has a high luminosity and a low temperature. The main radiation occurs in the infrared range. Radiation from the cold dust shell reaches us. In the process of evolution, the position of the star on the diagram will change. The only source of energy at this stage is gravitational contraction. Therefore, the star moves quite quickly parallel to the y-axis.
The surface temperature does not change, but the radius and luminosity decrease. The temperature in the center of the star rises, reaching a value at which reactions begin with light elements: lithium, beryllium, boron, which quickly burn out, but manage to slow down the compression. The track turns parallel to the y-axis, the temperature on the surface of the star rises, and the luminosity remains almost constant. Finally, in the center of the star, the reactions of the formation of helium from hydrogen (hydrogen combustion) begin. The star enters the main sequence.
The duration of the initial stage is determined by the mass of the star. For stars like the Sun, it is about 1 million years, for a star with a mass of 10 M☉ about 1000 times smaller, and for a star with a mass of 0.1 M☉ thousands of times more.
Young low mass stars
At the beginning of its evolution, a low-mass star has a radiant core and a convective envelope (Fig. 82, I).
At the main sequence stage, the star shines due to the release of energy in the nuclear reactions of the conversion of hydrogen into helium. The supply of hydrogen ensures the luminosity of a star of mass 1 M☉ Approximately within 10 10 years. Stars of greater mass consume hydrogen faster: for example, a star with a mass of 10 M☉ will use up hydrogen in less than 10 7 years (luminosity is proportional to the fourth power of mass).
low mass stars
As the hydrogen burns out, the central regions of the star are strongly compressed.
Stars of high mass
After entering the main sequence, the evolution of a large-mass star (>1.5 M☉) is determined by the conditions of combustion of nuclear fuel in the interior of the star. At the main sequence stage, this is the combustion of hydrogen, but unlike low-mass stars, reactions of the carbon-nitrogen cycle dominate in the core. In this cycle, the C and N atoms play the role of catalysts. The rate of energy release in the reactions of such a cycle is proportional to T 17 . Therefore, a convective core is formed in the core, surrounded by a zone in which energy transfer is carried out by radiation.
The luminosity of large mass stars is much higher than the luminosity of the Sun, and hydrogen is consumed much faster. This is due to the fact that the temperature in the center of such stars is also much higher.
As the proportion of hydrogen in the substance of the convective core decreases, the rate of energy release decreases. But since the rate of release is determined by the luminosity, the core begins to shrink, and the rate of energy release remains constant. At the same time, the star expands and passes into the region of red giants.
low mass stars
By the time the hydrogen is completely burned out, a small helium core is formed in the center of a low-mass star. In the core, the matter density and temperature reach 10 9 kg/m and 10 8 K, respectively. Hydrogen combustion occurs on the surface of the nucleus. As the temperature in the core rises, the rate of hydrogen burning increases, and the luminosity increases. The radiant zone gradually disappears. And because of the increase in the speed of convective flows, the outer layers of the star swell. Its size and luminosity increase - the star turns into a red giant (Fig. 82, II).
Stars of high mass
When the hydrogen of a star of large mass is completely exhausted, a triple helium reaction begins in the core and at the same time the reaction of oxygen production (3He=>C and C+He=>0). At the same time, hydrogen begins to burn on the surface of the helium core. The first layer source appears.
The supply of helium is exhausted very quickly, since in the described reactions in each elementary act, relatively little energy is released. The picture repeats itself, and two layer sources appear in the star, and the C + C => Mg reaction begins in the core.
The evolutionary track in this case turns out to be very complex (Fig. 84). In the Hertzsprung-Russell diagram, the star moves along the sequence of giants or (with a very large mass in the supergiant region) periodically becomes a cephei.
Old low-mass stars
In a star of low mass, in the end, the speed of the convective flow at some level reaches the second cosmic velocity, the shell comes off, and the star turns into a white dwarf, surrounded by a planetary nebula.
The evolutionary track of a low-mass star on the Hertzsprung-Russell diagram is shown in Figure 83.
Death of high mass stars
At the end of evolution, a large mass star has a very complex structure. Each layer has its own chemical composition, nuclear reactions take place in several layer sources, and an iron core is formed in the center (Fig. 85).
Nuclear reactions with iron do not proceed, since they require the expenditure (and not release) of energy. Therefore, the iron core is rapidly compressed, the temperature and density in it increase, reaching fantastic values - a temperature of 10 9 K and a pressure of 10 9 kg / m 3. material from the site
At this moment, two most important processes begin, going on in the nucleus simultaneously and very quickly (apparently, in minutes). The first is that during the collision of nuclei, iron atoms decay into 14 helium atoms, the second is that electrons are “pressed” into protons, forming neutrons. Both processes are associated with the absorption of energy, and the temperature in the core (also pressure) drops instantly. The outer layers of the star begin to fall towards the center.
The fall of the outer layers leads to a sharp increase in temperature in them. Hydrogen, helium, carbon begin to burn. This is accompanied by a powerful stream of neutrons that comes from the central core. As a result, a powerful nuclear explosion occurs, throwing off the outer layers of the star, which already contain all the heavy elements, up to californium. According to modern views, all atoms of heavy chemical elements (i.e., heavier than helium) were formed in the Universe precisely in flares