Although stars seem eternal on the human time scale, they, like everything in nature, are born, live and die. According to the generally accepted gas-dust cloud hypothesis, a star is born as a result of gravitational compression of an interstellar gas-dust cloud. As such a cloud thickens, it first forms protostar, the temperature at its center steadily increases until it reaches the limit necessary for the speed of thermal motion of particles to exceed the threshold after which protons are able to overcome the macroscopic forces of mutual electrostatic repulsion ( cm. Coulomb's Law) and react thermo nuclear fusion (cm. Nuclear decay and fusion).
As a result of a multi-stage thermonuclear fusion reaction, four protons ultimately form a helium nucleus (2 protons + 2 neutrons) and a whole fountain of various elementary particles is released. In the final state, the total mass of the formed particles is less masses of the four original protons, which means that during the reaction, free energy (cm. Theory of relativity). Because of this, the internal core of the newborn star quickly heats up to ultra-high temperatures, and its excess energy begins to splash towards its less hot surface - and out. At the same time, the pressure in the center of the star begins to increase ( cm. Equation of state of an ideal gas). Thus, by “burning” hydrogen in the process of a thermonuclear reaction, the star does not allow the forces of gravitational attraction to compress itself to a super-dense state, countering the gravitational collapse with continuously renewed internal thermal pressure, resulting in a stable energy equilibrium. Stars actively burning hydrogen are said to be in the "primary phase" of their life cycle or evolution ( cm. Hertzsprung-Russell diagram). The transformation of one chemical element into another inside a star is called nuclear fusion or nucleosynthesis.
In particular, the Sun has been at the active stage of burning hydrogen in the process of active nucleosynthesis for about 5 billion years, and the reserves of hydrogen in the core for its continuation should be enough for our luminary for another 5.5 billion years. The more massive the star, the greater the supply of hydrogen fuel it has, but to counteract the forces of gravitational collapse it must burn hydrogen at an intensity that exceeds the growth rate of hydrogen reserves as the mass of the star increases. Thus, the more massive the star, the shorter its lifetime, determined by the depletion of hydrogen reserves, and the largest stars literally burn out in “some” tens of millions of years. The smallest stars, on the other hand, live comfortably for hundreds of billions of years. So, on this scale, our Sun belongs to the “strong middle class”.
Sooner or later, however, any star will use up all the hydrogen suitable for combustion in its thermonuclear furnace. What's next? It also depends on the mass of the star. The sun (and all stars not exceeding its mass by more than eight times) end my life in a very banal way. As the reserves of hydrogen in the bowels of the star are depleted, the forces of gravitational compression, which have been patiently waiting for this hour since the very moment of the birth of the star, begin to gain the upper hand - and under their influence the star begins to shrink and become denser. This process has a twofold effect: The temperature in the layers immediately around the star's core rises to a level at which the hydrogen contained there finally undergoes thermonuclear fusion to form helium. At the same time, the temperature in the core itself, now consisting almost entirely of helium, rises so much that helium itself is a kind of “ash” of the decaying primary reaction nucleosynthesis - enters into a new thermonuclear fusion reaction: three helium nuclei form one carbon nucleus. This process of secondary thermonuclear fusion reaction, for which the products of the primary reaction serve as fuel, is one of key points life cycle of stars.
During the secondary combustion of helium in the core of the star, so much energy is released that the star literally begins to inflate. In particular, the shell of the Sun at this stage of life will expand beyond the orbit of Venus. In this case, the total energy of the star's radiation remains approximately at the same level as during the main phase of its life, but since this energy is now emitted through a much larger surface area, the outer layer of the star cools down to the red part of the spectrum. The star turns into red giant.
For solar-class stars, after the fuel feeding the secondary nucleosynthesis reaction has been depleted, the stage of gravitational collapse begins again—this time final. The temperature inside the core is no longer able to rise to the level necessary to initiate the next level of thermonuclear reaction. Therefore, the star contracts until the forces of gravitational attraction are balanced by the next force barrier. His role is played by degenerate electron gas pressure(cm. Chandrasekhar limit). Electrons, which until this stage played the role of unemployed extras in the evolution of the star, not participating in nuclear fusion reactions and freely moving between nuclei in the process of fusion, at a certain stage of compression find themselves deprived of “living space” and begin to “resist” further gravitational compression of the star. The state of the star stabilizes, and it turns into a degenerate white dwarf, which will radiate residual heat into space until it cools completely.
Stars more massive than the Sun face a much more spectacular end. After the combustion of helium, their mass during compression turns out to be sufficient to heat the core and shell to the temperatures necessary to launch the next nucleosynthesis reactions - carbon, then silicon, magnesium - and so on, as the nuclear masses grow. Moreover, with the start of each new reaction in the core of the star, the previous one continues in its shell. In fact, everything chemical elements up to the iron that makes up the Universe were formed precisely as a result of nucleosynthesis in the depths of dying stars of this type. But iron is the limit; it cannot serve as fuel for nuclear fusion or decay reactions at any temperature or pressure, since both its decay and the addition of additional nucleons to it require an influx of external energy. As a result, a massive star gradually accumulates an iron core inside itself, which cannot serve as fuel for any further nuclear reactions.
Once the temperature and pressure inside the nucleus reach a certain level, electrons begin to interact with the protons of the iron nuclei, resulting in the formation of neutrons. And in a very short period of time - some theorists believe that this takes a matter of seconds - the electrons free throughout the previous evolution of the star literally dissolve in the protons of the iron nuclei, the entire substance of the star’s core turns into a solid bunch of neutrons and begins to rapidly compress in gravitational collapse , since the counteracting pressure of the degenerate electron gas drops to zero. The outer shell of the star, from under which all support is knocked out, collapses towards the center. The energy of the collision of the collapsed outer shell with the neutron core is so high that it rebounds with enormous speed and scatters in all directions from the core - and the star literally explodes in a blinding flash supernova stars. In a matter of seconds, a supernova explosion can release more energy into space than all the stars in the galaxy put together during the same time.
After a supernova explosion and the expansion of the shell of stars with a mass of about 10-30 solar masses, the ongoing gravitational collapse leads to the formation of a neutron star, the matter of which is compressed until it begins to make itself felt pressure of degenerate neutrons - in other words, now neutrons (just as electrons did earlier) begin to resist further compression, requiring to myself living space. This usually occurs when the star reaches a size of about 15 km in diameter. The result is a rapidly rotating neutron star, emitting electromagnetic pulses at the frequency of its rotation; such stars are called pulsars. Finally, if the star's core mass exceeds 30 solar masses, nothing can stop its further gravitational collapse, and a supernova explosion results in
The birth of stars and entire galaxies occurs permanently, as well as their death. The disappearance of one star compensates for the appearance of another, so it seems to us that the same luminaries are constantly in the sky.
Stars owe their birth to the process of compression of the interstellar cloud, which is affected by a strong drop in gas pressure. Depending on the mass of the compressed gas, the number of stars being born changes: if it is small, then one star is born, if it is large, then the formation of an entire cluster is possible.
Stages of the emergence of a star
Here it is necessary to distinguish two main stages - the fast compression of the protostar and the slow one. In the first case distinctive feature is gravity: the matter of the protostar performs almost free fall to its center. At this stage, the temperature of the gas remains unchanged, its duration is about 100 thousand years, and during this time the size of the protostar decreases very significantly.
And if at the first stage the excess heat was constantly leaving, then the protostar becomes denser. Heat removal no longer occurs at such a high rate; the gas continues to compress and heat up quickly. The slow compression of the protostar lasts even longer - more than ten million years. Upon reaching an ultra-high temperature (more than a million degrees), thermonuclear reactions take their toll, leading to the cessation of compression. After which a new star is formed from the protostar.
Life cycle of a star
Stars are like living organisms: they are born, reach their peak of development, and then die. Major changes begin when the central part of the star runs out of hydrogen. It begins to burn out already in the shell, gradually increasing its size, and the star can turn into a red giant or even a supergiant.
All stars have completely different life cycles, it all depends on their mass. Those that weigh more live longer and eventually explode. Our sun is not a massive star, so celestial bodies of this type face a different end: they gradually fade away and become a dense structure called a white dwarf.
Red giant
Stars that have used up their hydrogen supply can acquire colossal sizes. Such luminaries are called red giants. Their distinguishing feature, in addition to their size, is their extended atmosphere and very low temperature surfaces. Research has shown that not all stars go through this stage of development. Only those stars with significant mass become red giants.
The most striking representatives are Arcturus and Antare, the visible layers of which have a relatively low temperature, and the discharged shell has a considerable extent. Inside the bodies, a process of ignition of helium occurs, characterized by the absence sharp fluctuations luminosity
White dwarf
Small stars in size and mass turn into white dwarfs. Their density is extremely high (about a million times higher than the density of water), which is why the substance of the star passes into a state called “degenerate gas.” No thermonuclear reactions are observed inside the white dwarf, and only the fact of cooling gives it light. The size of the star in this state is extremely small. For example, many white dwarfs are similar in size to Earth.
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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✪ Surdin V.G. Stellar Evolution Part 1
✪ S. A. Lamzin - “Stellar Evolution”
Subtitles
Thermonuclear fusion in the interior of stars
Young stars
The process of star formation can be described in a unified way, but the subsequent stages of 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 stars low mass(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 less than 0.0767 solar masses, this does not happen: the energy released during nuclear reactions is never enough to balance 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 Tauri, 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 for the hydrogen fuel supply in such stars to be depleted, modern theories are based on computer modeling 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 star average size(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 Mira-like stars, depending on their exact characteristics. The ejected gas is relatively rich in heavy elements produced in the interior of the star, such as oxygen and carbon. The gas forms an expanding shell and cools as it moves away from the star, allowing the formation of dust particles and molecules. With strong infrared radiation from the source star, ideal conditions for the activation of cosmic masers are formed in such shells.
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 its 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 jets of neutrinos carry matter away from the dying star [ ] into interstellar space. Subsequently, as it cools and moves through space, this supernova material can collide with other 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 incredible 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,
Stars, like people, can be newborn, young, old. Every moment some stars die and others are formed. Usually the youngest of them are similar to the Sun. They are at the stage of formation and are actually protostars. Astronomers call them T-Taurus stars, after their prototype. In terms of their properties - for example, luminosity - protostars are variable, since their existence has not yet entered a stable phase. Many of them have large amounts of matter around them. Powerful wind currents emanate from T-type stars.
Protostars: the beginning of their life cycle
If matter falls onto the surface of a protostar, it quickly burns and turns into heat. As a consequence, the temperature of protostars is constantly increasing. When it rises so high that nuclear reactions are triggered in the center of the star, the protostar acquires the status of an ordinary one. With the start of nuclear reactions, the star has a constant source of energy that supports its life for a long time. How long a star's life cycle in the Universe will be depends on its original size. However, it is believed that stars the diameter of the Sun have enough energy to exist comfortably for about 10 billion years. Despite this, it also happens that even more massive stars live only a few million years. This is due to the fact that they burn their fuel much faster.
Normal sized stars
Each of the stars is a clump of hot gas. In their depths, the process of generating nuclear energy constantly occurs. However, not all stars are like the Sun. One of the main differences is color. Stars are not only yellow, but also bluish and reddish.
Brightness and Luminosity
They also differ in characteristics such as shine and brightness. How bright a star observed from the Earth's surface will be depends not only on its luminosity, but also on its distance from our planet. Given their distance from Earth, stars can have completely different brightnesses. This indicator ranges from one ten-thousandth of the brilliance of the Sun to a brightness comparable to more than a million Suns.
Most stars are at the lower end of this spectrum, being dim. In many ways, the Sun is an average, typical star. However, compared to others, it has much greater brightness. A large number of dim stars can be observed even with the naked eye. The reason stars vary in brightness is due to their mass. Color, shine and change in brightness over time are determined by the amount of substance.
Attempts to explain the life cycle of stars
People have long tried to trace the life of stars, but the first attempts of scientists were rather timid. The first advance was the application of Lane's law to the Helmholtz-Kelvin hypothesis of gravitational contraction. This brought a new understanding to astronomy: theoretically, the temperature of a star should increase (its indicator is inversely proportional to the radius of the star) until an increase in density slows down the compression processes. Then the energy consumption will be higher than its income. At this moment, the star will begin to rapidly cool down.
Hypotheses about the life of stars
One of the original hypotheses about the life cycle of a star was proposed by astronomer Norman Lockyer. He believed that stars arise from meteoric matter. Moreover, the provisions of his hypothesis were based not only on theoretical conclusions available in astronomy, but also on data from spectral analysis of stars. Lockyer was convinced that the chemical elements that take part in the evolution of celestial bodies consist of elementary particles- “protoelements”. Unlike modern neutrons, protons and electrons, they do not have a general, but an individual character. For example, according to Lockyer, hydrogen decays into what is called “protohydrogen”; iron becomes “proto-iron”. Other astronomers also tried to describe the life cycle of a star, for example, James Hopwood, Yakov Zeldovich, Fred Hoyle.
Giant stars and dwarf stars
Stars large sizes are the hottest and brightest. They are usually white or bluish in appearance. Despite the fact that they are gigantic in size, the fuel inside them burns so quickly that they are deprived of it in just a few million years.
Stars small sizes, in contrast to giant ones, are usually not so bright. They are red in color and live long enough - for billions of years. But among the bright stars in the sky there are also red and orange ones. An example is the star Aldebaran - the so-called “eye of the bull”, located in the constellation Taurus; and also in the constellation Scorpio. Why are these cool stars able to compete in brightness with hot stars like Sirius?
This is due to the fact that they once expanded very much, and their diameter began to exceed huge red stars (supergiants). The huge area allows these stars to emit an order of magnitude more energy than the Sun. This is despite the fact that their temperature is much lower. For example, the diameter of Betelgeuse, located in the constellation Orion, is several hundred times larger than the diameter of the Sun. And the diameter of ordinary red stars is usually not even a tenth the size of the Sun. Such stars are called dwarfs. These types of star life cycles can go through every celestial body- the same star at different stages of its life can be both a red giant and a dwarf.
As a rule, luminaries like the Sun support their existence due to the hydrogen found inside. It turns into helium inside the star's nuclear core. The sun has a huge amount of fuel, but even it is not infinite - over the past five billion years, half of the supply has been used up.
Lifetime of stars. Life cycle of stars
Once the supply of hydrogen inside a star is depleted, major changes occur. The remaining hydrogen begins to burn not inside its core, but on the surface. At the same time, the lifespan of a star is increasingly shortened. During this period, the cycle of stars, at least most of them, enters the red giant stage. The size of the star becomes larger, and its temperature, on the contrary, decreases. This is how most red giants and supergiants appear. This process is part of general sequence changes occurring in stars, which scientists call stellar evolution. The life cycle of a star includes all its stages: ultimately, all stars age and die, and the duration of their existence is directly determined by the amount of fuel. Big stars end their lives with a huge, spectacular explosion. More modest ones, on the contrary, die, gradually shrinking to the size of white dwarfs. Then they just fade away.
How long does the average star live? Life cycle a star can last from less than 1.5 million years to 1 billion years or more. All this, as has been said, depends on its composition and size. Stars like the Sun live between 10 and 16 billion years. Very bright stars, like Sirius, have relatively short lives - only a few hundred million years. The star life cycle diagram includes the following stages. This is a molecular cloud - gravitational collapse of the cloud - the birth of a supernova - the evolution of a protostar - the end of the protostellar phase. Then follow the stages: the beginning of the young star stage - mid-life - maturity - red giant stage - planetary nebula- white dwarf stage. The last two phases are characteristic of small stars.
The nature of planetary nebulae
So, we briefly looked at the life cycle of a star. But what is Transforming from a huge red giant to a white dwarf, sometimes stars shed their outer layers, and then the core of the star becomes exposed. The gas shell begins to glow under the influence of the energy emitted by the star. This stage got its name due to the fact that luminous gas bubbles in this shell often look like disks around planets. But in reality they have nothing to do with planets. The life cycle of stars for children may not include all the scientific details. One can only describe the main phases of the evolution of celestial bodies.
Star clusters
Astronomers love to explore. There is a hypothesis that all luminaries are born in groups, and not individually. Since stars belonging to the same cluster have similar properties, the differences between them are true and not due to the distance to the Earth. Whatever changes occur to these stars, they originate at the same time and under equal conditions. Especially a lot of knowledge can be obtained by studying the dependence of their properties on mass. After all, the age of the stars in the clusters and their distance from the Earth are approximately equal, so they differ only in this indicator. Clusters will be of interest not only to professional astronomers - every amateur will be happy to make beautiful photo, admire them exclusively beautiful view in the planetarium.
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 goes 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 turning black dwarf.
