Theory about stars. Life cycle of a star
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. Change over time physical characteristics and the chemical composition of stars, 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 color-luminosity diagrams. 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 end product of stellar evolution is 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.
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 great 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 will be a thousand times older than it is now—a new type of object will appear in the universe: a black dwarf, a product of the cooling of a white dwarf.
There are no black dwarfs in space yet. Even the oldest cooling stars to date have lost a maximum of 0.2% of their energy; for a white dwarf with a temperature of 20,000 K, this means cooling to 19,960 K.
For the little ones
Science knows even less about what happens when the smallest stars, such as our nearest neighbor, the red dwarf Proxima Centauri, cool down than about supernovae and black dwarfs. Thermonuclear fusion in their cores proceeds slowly, and they remain on the main sequence longer than others - according to some calculations, up to 10 12 years, and after that, presumably, they will continue to live as white dwarfs, that is, they will shine for another 10 14 - 10 15 years before transformation into a black dwarf.
INTRODUCTION
CHAPTER 1. Evolution of stars
CHAPTER 2.Thermonuclear fusion in the interior of stars and the birth of stars
CHAPTER 3. Mid-life cycle of a star
CHAPTER 4. Later years and death of stars
CONCLUSION
Literature
INTRODUCTION
Modern scientific sources indicate that the universe consists of 98% stars, which “in turn” are the main element of the galaxy. Information sources give different definitions to this concept, here are some of them:
Star - celestial body, in which they are going, going or will go thermo nuclear reactions. Stars are massive luminous balls of gas (plasma). Formed from a gas-dust environment (hydrogen and helium) as a result of gravitational compression. The temperature of matter in the interior of stars is measured in millions of kelvins, and on their surface - in thousands of kelvins. The energy of the vast majority of stars is released as a result of thermonuclear reactions converting hydrogen into helium, occurring at high temperatures in the internal regions. Stars are often called the main bodies of the Universe, since they contain the bulk of luminous matter in nature.
Stars are huge objects spherical, consisting of helium and hydrogen, as well as other gases. The energy of a star is contained in its core, where helium interacts with hydrogen every second.
Like everything organic in our universe, stars arise, develop, change and disappear - this process takes billions of years and is called the process of “Star Evolution”.
CHAPTER 1. Evolution of stars
Evolution of stars- the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years while it emits light and heat.
A star begins its life as a cold, rarefied cloud of interstellar gas (a rarefied gaseous medium that fills all the space between stars), compressing under the influence of its own gravity and gradually taking the shape of a ball. During compression, gravitational energy (universal fundamental interaction between everyone material bodies) turns into heat, and the temperature of the object increases. When the temperature in the center reaches 15-20 million K, thermonuclear reactions begin and compression stops. The object becomes a full-fledged star. The first stage of a star's life is similar to the solar one - it is dominated by reactions of the hydrogen cycle. It remains in this state for most of its life, being on the main sequence of the Hertzsprung-Russell diagram (Fig. 1) (showing the relationship between absolute magnitude, luminosity, spectral type and surface temperature of the star, 1910), until the fuel reserves in its core. When all the hydrogen in the center of the star is converted into helium, a helium core is formed, and thermonuclear burning of hydrogen continues at its periphery. During this period, the structure of the star begins to change. Its luminosity increases, its outer layers expand, and its surface temperature decreases - the star becomes a red giant, which form a branch on the Hertzsprung-Russell diagram. The star spends significantly less time on this branch than on the main sequence. When the accumulated mass of the helium core becomes significant, it cannot support its own weight and begins to shrink; if the star is massive enough, the increasing temperature can cause further thermonuclear transformation of helium into heavier elements (helium into carbon, carbon into oxygen, oxygen into silicon, and finally silicon into iron).
Rice. 1. Hertzsprung-Russell diagram
Evolution of a class G star using the example of the Sun
CHAPTER 2. Thermonuclear fusion in the interior of stars
By 1939, it was established that the source of stellar energy was thermonuclear fusion occurring in the bowels of stars. Most stars radiate because in their core four protons combine through a series of intermediate steps into a single alpha particle. This transformation can occur in two main ways, called the proton-proton, or p-p, cycle, and the carbon-nitrogen, or CN, cycle. In low-mass stars, energy release is mainly provided by the first cycle, in heavy stars - by the second. The supply of nuclear fuel in a star is limited and is constantly spent on radiation. Process thermonuclear fusion, which releases energy and changes the composition of the star’s matter, in combination with gravity, which tends to compress the star and also releases energy, as well as radiation from the surface, which carries away the released energy, are the main driving forces of stellar evolution.
The Birth of Stars
The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the "empty" space in a galaxy actually contains between 0.1 and 1 molecule per cm³. The molecular cloud has a density of about a million molecules per cm³. The mass of such a cloud exceeds the mass of the Sun by 100,000-10,000,000 times due to its size: from 50 to 300 light years in diameter.
While the cloud rotates freely around the center of its home galaxy, nothing happens. However, due to the inhomogeneity of the gravitational field, disturbances may arise in it, leading to local concentrations of mass. Such disturbances cause gravitational collapse of the cloud. One of the scenarios leading to this is the collision of two clouds. Another event causing collapse could be the passage of a cloud through the dense arm of a spiral galaxy. Also a critical factor could be the explosion of a nearby supernova, the shock wave of which will collide with the molecular cloud at enormous speed. It is also possible that galaxies collide, which could cause a burst of star formation as the gas clouds in each galaxy are compressed by the collision. In general, any inhomogeneities in the forces acting on the mass of the cloud can initiate the process of star formation.
Due to the inhomogeneities that have arisen, the pressure of the molecular gas can no longer prevent further compression, and the gas begins to gather around the center of the future star under the influence of gravitational attraction forces. Half of the released gravitational energy goes to heating the cloud, and half goes to light radiation. In clouds, pressure and density increase towards the center, and the collapse of the central part occurs faster than the periphery. As compression progresses, the mean free path of photons decreases, and the cloud becomes less and less transparent to its own radiation. This leads to more rapid growth temperature and an even faster increase in pressure. As a result, the pressure gradient balances the gravitational force, and a hydrostatic core is formed, with a mass of about 1% of the mass of the cloud. This moment is invisible. The further evolution of the protostar is the accretion of matter that continues to fall onto the “surface” of the core, which due to this grows in size. The mass of freely moving matter in the cloud is exhausted and the star becomes visible in the optical range. This moment is considered the end of the protostellar phase and the beginning of the young star phase.
Like any bodies in nature, stars also cannot remain unchanged. They are born, develop and finally “die”. The evolution of stars takes billions of years, but there is debate about the time of their formation. Previously, astronomers believed that the process of their “birth” from stardust took millions of years, but not so long ago photographs of the sky region from the Great Orion Nebula were obtained. Over the course of several years, a small
Photographs from 1947 showed a small group of star-like objects in this location. By 1954, some of them had already become oblong, and five years later these objects broke up into separate ones. Thus, for the first time, the process of star birth took place literally before the eyes of astronomers.
Let's look in detail at the structure and evolution of stars, where their endless, by human standards, life begins and ends.
Traditionally, scientists assume that stars are formed as a result of the condensation of clouds of gas and dust. Under the influence of gravitational forces, an opaque gas ball, dense in structure, is formed from the resulting clouds. Its internal pressure cannot balance the gravitational forces compressing it. Gradually, the ball contracts so much that the temperature of the stellar interior rises, and the pressure of the hot gas inside the ball balances the external forces. After this, the compression stops. The duration of this process depends on the mass of the star and usually ranges from two to several hundred million years.
The structure of stars suggests very high temperature in their depths, which contributes to continuous thermonuclear processes (the hydrogen that forms them turns into helium). It is these processes that cause intense radiation from stars. The time during which they consume the available supply of hydrogen is determined by their mass. The duration of radiation also depends on this.
When hydrogen reserves are depleted, the evolution of stars approaches the formation stage. This happens as follows. After the release of energy ceases, gravitational forces begin to compress the core. At the same time, the star increases significantly in size. The luminosity also increases as the process continues, but only in a thin layer at the core boundary.
This process is accompanied by an increase in the temperature of the contracting helium core and the transformation of helium nuclei into carbon nuclei.
It is predicted that our Sun could become a red giant in eight billion years. Its radius will increase several tens of times, and its luminosity will increase hundreds of times compared to current levels.
The lifespan of a star, as already noted, depends on its mass. Objects with a mass that is less than the Sun “use up” their reserves very economically, so they can shine for tens of billions of years.
The evolution of stars ends with the formation. This happens to those of them whose mass is close to the mass of the Sun, i.e. does not exceed 1.2 of it.
Giant stars, as a rule, quickly deplete their supply of nuclear fuel. This is accompanied by a significant loss of mass, in particular due to the shedding of outer shells. As a result, all that remains is a gradually cooling central part, in which nuclear reactions have completely stopped. Over time, such stars stop emitting and become invisible.
But sometimes the normal evolution and structure of stars is disrupted. Most often this concerns massive objects that have exhausted all types of thermonuclear fuel. Then they can be converted into neutrons, or And the more scientists learn about these objects, the more new questions arise.
Occupies a point in the upper right corner: it has 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.
Duration initial stage 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☉ thousands of times more.
Young low mass stars
At the beginning of 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 converting 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 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 reaching the main sequence, the evolution of a high-mass star (>1.5 M☉) is determined by the combustion conditions of nuclear fuel in the bowels 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 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 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 compress, and the rate of energy release remains constant. At the same time, the star expands and moves 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 density of matter and temperature reach values of 10 9 kg/m 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 (Fig. 82, II).
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=>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 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 (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
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.
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 its evolution, a large-mass star has a very complex structure. Each layer has its own chemical composition, nuclear reactions occur in several layered sources, and an iron core is formed in the center (Fig. 85).
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 pressure of 10 9 kg/m 3. Material from the site
At this moment, two important processes begin, occurring in the nucleus 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 sharp increase temperatures in them. Hydrogen, helium, and 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, 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 flares
