Evolution of stars of different masses. Life cycle of a star - description, diagram and interesting facts
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 to deplete the supply of hydrogen fuel in such stars, modern theories are based on computer simulations of the 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, thermal nuclear reactions with the participation of helium, during which helium is converted into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally silicon into 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 combustion of helium can lead to the explosive release of the outer layers of the 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, 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.
Studying stellar evolution is impossible by observing just one star - many changes in stars occur too slowly to be noticed even after many centuries. Therefore, scientists study many stars, each of which is at a certain stage of its life cycle. Over the past few decades, modeling of the structure of stars using computer technology has become widespread in astrophysics.
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Thermonuclear fusion in the interior of stars
Young stars
The process of star formation can be described in a unified way, but the subsequent stages of a star's evolution depend almost entirely on its mass, and only at the very end of the star's evolution can its chemical composition play a role.
Young low mass stars
Young low-mass stars (up to three solar masses) [ ], which are approaching the main sequence, are completely convective - the convection process covers the entire body of the star. These are essentially protostars, in the centers of which nuclear reactions are just beginning, and all radiation occurs mainly due to gravitational compression. Until hydrostatic equilibrium is established, the star's luminosity decreases at a constant effective temperature. On the Hertzsprung-Russell diagram, such stars form an almost vertical track called the Hayashi track. As the compression slows, the young star approaches the main sequence. Objects of this type are associated with T Tauri stars.
At this time, for stars with a mass greater than 0.8 solar masses, the core becomes transparent to radiation, and radiative energy transfer in the core becomes predominant, since convection is increasingly hampered by the increasing compaction of stellar matter. In the outer layers of the star’s body, convective energy transfer prevails.
It is not known for certain what characteristics do stars of lower mass have at the moment they enter the main sequence, since the time these stars spent in the young category exceeds the age of the Universe [ ] . All ideas about the evolution of these stars are based only on numerical calculations and mathematical modeling.
As the star contracts, the pressure of the degenerate electron gas begins to increase and when a certain radius of the star is reached, the compression stops, which leads to a stop in the further increase in temperature in the core of the star caused by the compression, and then to its decrease. For stars smaller than 0.0767 solar masses, this does not happen: the energy released during nuclear reactions is never enough to balance the internal pressure and gravitational compression. Such “understars” emit more energy than is produced during thermonuclear reactions, and are classified as so-called brown dwarfs. Their fate is constant compression until the pressure of the degenerate gas stops it, and then gradual cooling with the cessation of all thermonuclear reactions that have begun.
Young intermediate mass stars
Young stars of intermediate mass (from 2 to 8 solar masses) [ ] evolve qualitatively in exactly the same way as their smaller sisters and brothers, with the exception that they do not have convective zones up to the main sequence.
Objects of this type are associated with the so-called. Ae\Be Herbig stars with irregular variables of spectral class B-F0. They also exhibit disks and bipolar jets. The rate of outflow of matter from the surface, luminosity and effective temperature are significantly higher than for T Taurus, so they effectively heat and disperse the remnants of the protostellar cloud.
Young stars with a mass greater than 8 solar masses
Stars with such masses already have the characteristics of normal stars, since they went through all the intermediate stages and were able to achieve such a rate of nuclear reactions that compensated for the energy lost to radiation while mass accumulated to achieve hydrostatic equilibrium of the core. For these stars, the outflow of mass and luminosity are so great that they not only stop the gravitational collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, disperse them away. Thus, the mass of the resulting star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars with a mass greater than about 300 solar masses.
Mid-life cycle of a star
Stars come in a wide variety of colors and sizes. By spectral type they range from hot blue to cool red, and by mass - from 0.0767 to about 300 solar masses, according to the latest estimates. The luminosity and color of a star depend on its surface temperature, 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. Naturally, 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.
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. So the star becomes a red giant, and the helium burning phase lasts about several million years. Almost all red giants are variable stars.
Final stages of stellar evolution
Old stars with low mass
At present, it is not known for certain what happens to light stars after the supply of hydrogen in their cores 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 [ ] .
A star with a mass less than 0.5 solar is not able to convert helium even after reactions involving hydrogen stop in its core - the mass of such a star is too small to provide a new phase of gravitational compression to a degree sufficient to “ignite” helium. Such stars include red dwarfs, such as Proxima Centauri, whose residence time on the main sequence ranges from tens of billions to tens of trillions of years. After the cessation of thermonuclear reactions in their cores, they, gradually cooling, will continue to weakly emit in the infrared and microwave ranges of the electromagnetic spectrum.
Medium sized stars
Upon reaching a medium-sized star (from 0.4 to 3.4 solar masses) [ ] of the red giant phase, hydrogen runs out in its core, and reactions of synthesis of carbon from helium begin. This process occurs at higher temperatures and therefore the energy flow from the core increases and, as a result, the outer layers of the star begin to expand. The beginning of carbon synthesis marks a new stage in the life of a star and continues for some time. For a star similar in size to the Sun, this process can take about a billion years.
Changes in the amount of energy emitted cause the star to go through periods of instability, including changes in size, surface temperature and energy release. 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” (also “retired stars”), OH -IR stars or World-like stars, depending on their exact specifications. The ejected gas is relatively rich in heavy elements produced in the interior of the star, such as oxygen and carbon. The gas forms an expanding shell and cools as it moves away from the star, allowing the formation of dust particles and molecules. With strong infrared radiation from the source star, ideal conditions to activate cosmic masers.
Thermonuclear combustion reactions of helium are very sensitive to temperature. Sometimes this leads to great instability. Strong pulsations arise, which as a result impart sufficient acceleration to the outer layers to be thrown off and turn into a planetary nebula. In the center of such a nebula, the bare core of the star remains, in which thermonuclear reactions stop, and as it cools, it 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.
The vast majority of stars, including the Sun, complete their evolution by contracting until the pressure of degenerate electrons balances gravity. In this state, when the size of the star decreases by a 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, becomes an invisible black dwarf.
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 turns protons into neutrons, between which there are no electrostatic repulsion forces. This neutronization of matter leads to the fact that the size of the star, which is now, in fact, one huge atomic nucleus, is measured in several kilometers, and the density is 100 million times higher than the density of water. Such an object is called a neutron star; its equilibrium is maintained by the pressure of the degenerate neutron matter.
Supermassive stars
After a star with a mass greater than five solar masses enters the red supergiant stage, its core begins to shrink under the influence of gravity. As the compression proceeds, the temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, increasingly heavier elements are synthesized: helium, carbon, oxygen, silicon and iron, which temporarily restrains the collapse of the core.
As a result, as increasingly heavier elements of the Periodic Table are formed, iron-56 is synthesized from silicon. At this stage, further exothermic thermonuclear fusion becomes impossible, since the iron-56 nucleus has a maximum mass defect and the formation of heavier nuclei with 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 weight of the overlying 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 a supernova explosion of incredible power.
Strong neutrino jets and a rotating magnetic field push out much of the star's accumulated material. [ ] - so-called seating elements, including iron and lighter elements. The exploding matter is bombarded by neutrons escaping from the stellar core, 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, but this is not the only possible way their formation, which, for example, is demonstrated by technetium stars.
blast wave And neutrino jets carry matter away from dying star [ ] into interstellar space. Subsequently, as it cools and moves through space, this supernova material can collide with other cosmic “salvage” 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 more 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 the conservation of angular momentum). Some neutron stars rotate 600 times 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 stars, after going through the supernova explosion phase, become neutron stars. If the star has a sufficiently large mass, then the collapse of such a star will continue, and the neutrons themselves will begin to fall inward until its radius becomes less than the Schwarzschild radius. 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,
Evolution of Stars of Different Masses
Astronomers cannot observe the life of a single star from beginning to end, because even the shortest-lived stars exist for millions of years - longer life of all humanity. Changes in the physical characteristics and chemical composition of stars over time, i.e. stellar evolution, astronomers study by comparing the characteristics of many stars located on different stages evolution.
Physical patterns connecting 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 faint giants, subgiants, subdwarfs and white dwarfs.
For most of its life, any star is on the so-called main sequence of the color-luminosity diagram. All other stages of the star's evolution before the formation of a compact remnant take no more than 10% of this time. This 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 contains 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 masses greater than the Sun 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 a star exhausts its energy sources, it begins to cool and contract. The final product evolutionary stars are compact massive objects whose density is many times greater than that of ordinary stars.
Stars of different masses end up in one of three states: white dwarfs, neutron stars or black holes. If the mass of the star is small, then the gravitational forces are relatively weak and the compression of the star (gravitational collapse) stops. It transitions to a stable white dwarf state. If the mass exceeds a critical value, compression continues. At very high densities, electrons combine with protons to form neutrons. Soon, almost the entire star consists of only neutrons and has such an enormous density that the huge stellar mass is concentrated in a very small ball with a radius of several kilometers and the compression stops - a neutron star is formed. If the mass of the star is so great that even the formation of a neutron star will not stop the gravitational collapse, then the final stage of the star’s evolution will be a black hole.
The Universe is a constantly changing macrocosm, where every object, substance or matter is in a state of transformation and change. These processes last for billions of years. Compared to duration human life this incomprehensible period of time is enormous. On a cosmic scale, these changes are quite fleeting. The stars that we now see in the night sky were the same thousands of years ago, when they could be seen egyptian pharaohs, however, in fact, all this time the change in the physical characteristics of the celestial bodies did not stop for a second. Stars are born, live and certainly age - the evolution of stars goes on as usual.
Position of constellation stars Big Dipper to different historical periods in the interval 100,000 years ago - our time and after 100 thousand years
Interpretation of the evolution of stars from the point of view of the average person
For the average person, space appears to be a world of calm and silence. In fact, the Universe is a giant physical laboratory where enormous transformations occur, during which the chemical composition changes, physical characteristics and the structure of stars. The life of a star lasts as long as it shines and gives off heat. However, such a brilliant state does not last forever. The bright birth is followed by a period of star maturity, which inevitably ends with the aging of the celestial body and its death.
Formation of a protostar from a gas and dust cloud 5-7 billion years ago
All our information about stars today fits within the framework of science. Thermodynamics gives us an explanation of the processes of hydrostatic and thermal equilibrium in which stellar matter resides. Nuclear and quantum physics allow us to understand the complex process of nuclear fusion that allows a star to exist, emitting heat and giving light to the surrounding space. At the birth of a star, hydrostatic and thermal equilibrium is formed, maintained by its own energy sources. At the end of a brilliant stellar career, this balance is disrupted. A series of irreversible processes begins, the result of which is the destruction of the star or collapse - a grandiose process of instant and brilliant death of the heavenly body.
A supernova explosion is a bright finale to the life of a star born in the early years of the Universe.
Changes in the physical characteristics of stars are due to their mass. The rate of evolution of objects is influenced by their chemical composition and, to some extent, by existing astrophysical parameters - rotation speed and state magnetic field. It is not possible to talk exactly about how everything actually happens due to the enormous duration of the processes described. The rate of evolution and the stages of transformation depend on the time of birth of the star and its location in the Universe at the time of birth.
The evolution of stars from a scientific point of view
Any star is born from a clump of cold interstellar gas, which, under the influence of external and internal gravitational forces, is compressed to the state of a gas ball. The process of compression of the gaseous substance does not stop for a moment, accompanied by a colossal release of thermal energy. The temperature of the new formation increases until thermonuclear fusion starts. From this moment, the compression of stellar matter stops, and a balance is reached between the hydrostatic and thermal states of the object. The Universe has been replenished with a new full-fledged star.
The main stellar fuel is the hydrogen atom as a result of a launched thermonuclear reaction.
In the evolution of stars, their sources of thermal energy are of fundamental importance. The radiant and thermal energy escaping into space from the surface of the star is replenished by cooling the inner layers of the celestial body. Constantly occurring thermonuclear reactions and gravitational compression in the bowels of the star make up for the loss. As long as there is sufficient nuclear fuel in the bowels of the star, the star glows with bright light and emits heat. As soon as the process of thermonuclear fusion slows down or stops completely, the mechanism of internal compression of the star is activated to maintain thermal and thermodynamic equilibrium. At this stage, the object already emits thermal energy, which is visible only in the infrared range.
Based on the processes described, we can conclude that the evolution of stars represents a consistent change in sources of stellar energy. In modern astrophysics, the processes of transformation of stars can be arranged in accordance with three scales:
- nuclear timeline;
- thermal period of a star's life;
- dynamic segment (final) of the life of a luminary.
In each individual case, the processes that determine the age of the star, its physical characteristics and the type of death of the object are considered. The nuclear timeline is interesting as long as the object is powered by its own heat sources and emits energy that is a product of nuclear reactions. The duration of this stage is estimated by determining the amount of hydrogen that will be converted into helium during thermonuclear fusion. The greater the mass of the star, the greater the intensity of nuclear reactions and, accordingly, the higher the luminosity of the object.
Sizes and masses of various stars, ranging from a supergiant to a red dwarf
The thermal time scale defines the stage of evolution during which a star expends all its thermal energy. This process begins from the moment when the last reserves of hydrogen are used up and nuclear reactions stop. To maintain the object's balance, a compression process is started. Stellar matter falls towards the center. In this case, the kinetic energy is converted into thermal energy, which is spent on maintaining the necessary temperature balance inside the star. Some of the energy escapes into outer space.
Considering the fact that the luminosity of stars is determined by their mass, at the moment of compression of an object, its brightness in space does not change.
A star on its way to the main sequence
Star formation occurs according to a dynamic time scale. Stellar gas falls freely inward toward the center, increasing the density and pressure in the bowels of the future object. The higher the density at the center of the gas ball, the higher the temperature inside the object. From this moment on, heat becomes the main energy of the celestial body. The greater the density and the higher the temperature, the greater the pressure in the depths of the future star. The free fall of molecules and atoms stops, and the process of compression of stellar gas stops. This state of an object is usually called a protostar. The object is 90% molecular hydrogen. When the temperature reaches 1800K, hydrogen passes into the atomic state. During the decay process, energy is consumed, and the temperature increase slows down.
The Universe is 75% composed of molecular hydrogen, which during the formation of protostars turns into atomic hydrogen - the nuclear fuel of a star
In this state, the pressure inside the gas ball decreases, thereby giving freedom to the compression force. This sequence is repeated each time all the hydrogen is ionized first, and then the helium is ionized. At a temperature of 10⁵ K, the gas is completely ionized, the compression of the star stops, and hydrostatic equilibrium of the object arises. The further evolution of the star will occur in accordance with the thermal time scale, much slower and more consistent.
The radius of the protostar has been decreasing from 100 AU since the beginning of formation. up to ¼ a.u. The object is in the middle of a gas cloud. As a result of the accretion of particles from the outer regions of the stellar gas cloud, the mass of the star will constantly increase. Consequently, the temperature inside the object will increase, accompanying the process of convection - the transfer of energy from the inner layers of the star to its outer edge. Subsequently, with increasing temperature in the interior of the celestial body, convection is replaced by radiative transfer, moving towards the surface of the star. At this moment, the luminosity of the object rapidly increases, and the temperature of the surface layers of the stellar ball also increases.
Convection processes and radiative transfer in a newly formed star before the onset of thermonuclear fusion reactions
For example, for stars with a mass identical to the mass of our Sun, the compression of the protostellar cloud occurs in just a few hundred years. As for the final stage of the formation of the object, the condensation of stellar matter has been stretching for millions of years. The Sun is moving towards the main sequence quite quickly, and this journey will take hundreds of millions or billions of years. In other words, the greater the mass of the star, the longer the period of time spent on the formation of a full-fledged star. A star with a mass of 15 M will move along the path to the main sequence for much longer - about 60 thousand years.
Main sequence phase
Despite the fact that some thermonuclear fusion reactions start at lower temperatures, the main phase of hydrogen combustion starts at a temperature of 4 million degrees. From this moment the main sequence phase begins. Comes into play new form reproduction of stellar energy - nuclear. The kinetic energy released during the compression of an object fades into the background. Achieved balance ensures a long and quiet life for a star that finds itself in the initial phase of the main sequence.
The fission and decay of hydrogen atoms during a thermonuclear reaction occurring in the interior of a star
From this moment on, observation of the life of a star is clearly tied to the phase of the main sequence, which is an important part of the evolution of celestial bodies. It is at this stage that the only source of stellar energy is the result of hydrogen combustion. The object is in a state of equilibrium. As consumption nuclear fuel only the chemical composition of the object changes. The Sun's stay in the main sequence phase will last approximately 10 billion years. This is how long it will take for our native star to use up its entire supply of hydrogen. As for massive stars, their evolution occurs faster. By emitting more energy, a massive star remains in the main sequence phase for only 10-20 million years.
Less massive stars burn in the night sky for much longer. Thus, a star with a mass of 0.25 M will remain in the main sequence phase for tens of billions of years.
Hertzsprung–Russell diagram assessing the relationship between the spectrum of stars and their luminosity. Points on the diagram - location famous stars. The arrows indicate the displacement of stars from the main sequence into the giant and white dwarf phases.
To imagine the evolution of stars, just look at the diagram characterizing the path of a celestial body in the main sequence. Top part The graphics look less object-saturated since this is where the massive stars are concentrated. This location is explained by their short life cycle. Of the stars known today, some have a mass of 70M. Objects whose mass exceeds the upper limit of 100M may not form at all.
Heavenly bodies whose mass is less than 0.08 M do not have the opportunity to overcome the critical mass required for the onset of thermonuclear fusion and remain cold throughout their lives. The smallest protostars collapse and form planet-like dwarfs.
A planet-like brown dwarf compared to a normal star (our Sun) and the planet Jupiter
At the bottom of the sequence are concentrated objects dominated by stars with a mass equal to the mass of our Sun and slightly more. The imaginary boundary between the upper and lower parts of the main sequence are objects whose mass is – 1.5M.
Subsequent stages of stellar evolution
Each of the options for the development of the state of a star is determined by its mass and the length of time during which the transformation of stellar matter occurs. However, the Universe is a multifaceted and complex mechanism, so the evolution of stars can take other paths.
When traveling along the main sequence, a star with a mass approximately equal to the mass of the Sun has three main route options:
- live your life calmly and rest peacefully in the vast expanses of the Universe;
- enter the red giant phase and slowly age;
- become a white dwarf, explode as a supernova, and become a neutron star.
Possible options for the evolution of protostars depending on time, the chemical composition of objects and their mass
After the main sequence comes the giant phase. By this time, the reserves of hydrogen in the bowels of the star are completely exhausted, the central region of the object is a helium core, and thermonuclear reactions shift to the surface of the object. Under the influence of thermonuclear fusion, the shell expands, but the mass of the helium core increases. An ordinary star turns into a red giant.
Giant phase and its features
In stars with low mass, the core density becomes colossal, turning stellar matter into a degenerate relativistic gas. If the mass of the star is slightly more than 0.26 M, an increase in pressure and temperature leads to the beginning of helium synthesis, covering the entire central region of the object. From this moment on, the temperature of the star increases rapidly. main feature The process is that the degenerate gas does not have the ability to expand. Under influence high temperature only the rate of helium fission increases, which is accompanied by an explosive reaction. At such moments we can observe a helium flash. The brightness of the object increases hundreds of times, but the agony of the star continues. The star transitions to a new state, where all thermodynamic processes occur in the helium core and in the discharged outer shell.
The structure of a solar-type main sequence star and a red giant with an isothermal helium core and a layered nucleosynthesis zone
This condition is temporary and not stable. Stellar matter is constantly mixed, and a significant part of it is ejected into the surrounding space, forming a planetary nebula. A hot core remains at the center, called a white dwarf.
For stars with large masses, the processes listed above are not so catastrophic. Helium combustion is replaced by the nuclear fission reaction of carbon and silicon. In the end stellar core will turn into star iron. The giant phase is determined by the mass of the star. The greater the mass of an object, the lower the temperature at its center. This is clearly not enough to trigger the nuclear fission reaction of carbon and other elements.
The fate of a white dwarf - a neutron star or a black hole
Once in the white dwarf state, the object is in an extremely unstable state. The stopped nuclear reactions lead to a drop in pressure, the core goes into a state of collapse. Energy released in in this case, is spent on the decay of iron into helium atoms, which further decays into protons and neutrons. The running process is developing at a rapid pace. The collapse of a star characterizes the dynamic segment of the scale and takes a fraction of a second in time. The combustion of nuclear fuel residues occurs explosively, releasing a colossal amount of energy in a split second. This is quite enough to blow up the upper layers of the object. The final stage of a white dwarf is a supernova explosion.
The star's core begins to collapse (left). The collapse forms a neutron star and creates a flow of energy into the outer layers of the star (center). Energy released when the outer layers of a star are shed during a supernova explosion (right).
The remaining superdense core will be a cluster of protons and electrons, which collide with each other to form neutrons. The Universe has been replenished with a new object - a neutron star. Due to the high density, the core becomes degenerate, and the process of core collapse stops. If the star's mass were large enough, the collapse could continue until the remaining stellar matter finally fell into the center of the object, forming a black hole.
Explaining the final part of stellar evolution
For normal equilibrium stars, the described evolution processes are unlikely. However, the existence of white dwarfs and neutron stars proves the real existence of compression processes of stellar matter. The small number of such objects in the Universe indicates the transience of their existence. The final stage of stellar evolution can be represented as a sequential chain of two types:
- normal star - red giant - shedding of outer layers - white dwarf;
- massive star – red supergiant – supernova explosion – neutron star or black hole – nothingness.
Diagram of the evolution of stars. Options for the continuation of the life of stars outside the main sequence.
It is quite difficult to explain the ongoing processes from a scientific point of view. Nuclear scientists agree that in the case of the final stage of stellar evolution, we are dealing with fatigue of matter. As a result of prolonged mechanical and thermodynamic influence, matter changes its physical properties. The fatigue of stellar matter, depleted by long-term nuclear reactions, can explain the appearance of degenerate electron gas, its subsequent neutronization and annihilation. If all of the above processes take place from beginning to end, stellar matter ceases to be a physical substance - the star disappears in space, leaving nothing behind.
Interstellar bubbles and gas and dust clouds, which are the birthplace of stars, cannot be replenished only by disappeared and exploded stars. The Universe and galaxies are in an equilibrium state. Mass loss occurs constantly, the density of interstellar space decreases in one part outer space. Consequently, in another part of the Universe, conditions are created for the formation of new stars. In other words, the scheme works: if a certain amount of matter was lost in one place, in another place in the Universe the same amount of matter appeared in a different form.
Finally
By studying the evolution of stars, we come to the conclusion that the Universe is a gigantic rarefied solution in which part of the matter is transformed into hydrogen molecules, which are the building material for stars. The other part dissolves in space, disappearing from the sphere of material sensations. A black hole in this sense is the place of transition of all material into antimatter. It is quite difficult to fully comprehend the meaning of what is happening, especially if, when studying the evolution of stars, you rely only on the laws of nuclear, quantum physics and thermodynamics. The theory of relative probability should be included in the study of this issue, which allows for the curvature of space, allowing the transformation of one energy into another, one state into another.
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 a star.
The diagram makes it possible (although not very accurately) to find the absolute value by spectral class. Especially for spectral classes O-F. For later classes this is complicated by the need to choose between a giant and a dwarf. However, certain differences in the intensity of some lines allow us to confidently make this choice.
Most stars (about 90%) are located on the diagram along a long narrow strip called main sequence. It extends from the upper left corner (from blue supergiants) to the lower right corner (to red dwarfs). The main sequence stars include the Sun, whose luminosity is taken to be unity.
The points corresponding to giants and supergiants are located above the main sequence on the right, and the points 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 by mass are located at the top of the main sequence and are blue giants. The smallest stars by mass are dwarfs. They are located at the bottom of the main sequence. Subdwarfs are located parallel to the main sequence, but slightly below it. They differ from main sequence stars in their lower metal content.
The star spends most of its life on the main sequence. During this period, its color, temperature, luminosity and other parameters remain almost unchanged. But before the star reaches this stable state, while still in the protostar state, it has a red color and, for a short time, greater luminosity than it would have on the main sequence.
Large-mass stars (supergiants) expend 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. 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 converting 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 energy released by the star is the conversion of hydrogen into helium in the layer surrounding the dense helium core. This is the so-called red giant stage.
The Birth of 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 a galaxy actually contains between 0.1 and 1 molecule per cm³. A 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.
During 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 in the early stages of their existence are usually hidden from view within a dense cloud of dust and gas. These star-forming cocoons can often be seen silhouetted against the bright radiation of the surrounding gas. Such formations are called Bok globules.
Very small share protostars do not reach temperatures 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 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 synthesize 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 stellar evolution
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 the infrared range. The radiation from the cold dust shell reaches us. During the process of evolution, the position of the star on the diagram will change. The only source of energy at this stage is gravitational compression. Therefore, the star moves quite quickly parallel to the ordinate 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 ordinate axis, the temperature on the surface of the star increases, and the luminosity remains almost constant. Finally, in the center of the star, 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 less, 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 converting hydrogen into helium. The supply of hydrogen provides the luminosity of a star with a mass of 1M ☉ for about 10 10 years. Stars with a larger mass consume hydrogen faster: for example, a star with a mass of 10 M ☉ will consume hydrogen in less than 10 7 years (luminosity is proportional to the fourth power of mass).
Low mass stars
As hydrogen burns out, the central regions of the star are greatly compressed.
High mass stars
After entering the main sequence, the evolution of a high-mass star (>1.5 M ☉ ) is determined by the combustion conditions 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 reactions of such a cycle is proportional to T17. Therefore, a convective core is formed in the core, surrounded by a zone in which energy is transferred by radiation.
The luminosity of high-mass stars is much higher than the luminosity of the Sun, and hydrogen is consumed much faster. This is also due to the fact that the temperature in the center of such stars is also much higher.
As the proportion of hydrogen in the matter of the convective core decreases, the rate of energy release decreases. But since the rate of release is determined by luminosity, the core begins to contract, and the rate of energy release remains constant. At the same time, the star expands and moves 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 core. As the temperature in the core rises, the rate of hydrogen burnout increases and the luminosity increases. The radiant zone gradually disappears. And due to the increase in the speed of convective flows, the outer layers of the star inflate. Its size and luminosity increase - the star turns into a red giant.
High mass stars
When the hydrogen in a large-mass star is completely exhausted, a triple helium reaction begins to occur in the core and at the same time the reaction of oxygen formation (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 reactions described, relatively little energy is released in each elementary act. The picture repeats itself, and two layer sources appear in the star, and the reaction C+C=>Mg begins in the core.
The evolutionary track turns out to be very complex. In the Hertzsprung-Russell diagram, the star moves along the sequence of giants or (with very high mass in the supergiant region) periodically becomes a Cepheid.
The final stages of stellar evolution
Old low mass stars
For a low-mass star, eventually the speed of the convective flow at some level reaches the second escape velocity, the shell comes 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 high-mass star has a very complex structure. Each layer has its own chemical composition, nuclear reactions occur in several layer sources, and an iron core is formed in the center.
Nuclear reactions with iron do not occur, since they require the expenditure (and not the release) of energy. Therefore, the iron core quickly contracts, 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 moment two begin critical process, going in the core simultaneously 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 pressure) instantly drops. The outer layers of the star begin to fall toward the center.
The fall of the outer layers leads to a sharp increase in temperature in them. Hydrogen, helium, and carbon begin to burn. This is accompanied by powerful flow neutrons that comes from the central nucleus. As a result, a powerful nuclear explosion, throwing off the outer layers of the star, already containing 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. At the site of the exploding supernova, depending on the mass of the exploding star, either a neutron star or black hole.
