Evolution and burning energy of stars thermonuclear fusion. Lifetime of stars
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 enter into a thermonuclear fusion reaction ( 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 is located at active stage The burning of hydrogen in the process of active nucleosynthesis has been around for about 5 billion years, and the reserves of hydrogen in the core for its continuation should be enough for our star 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 the helium itself - a kind of “ash” of the fading primary nucleosynthesis reaction - enters into a new thermonuclear fusion reaction: from three helium nuclei one carbon nucleus is formed. 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 moving freely 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 following nucleosynthesis reactions - carbon, then silicon, magnesium - and so on, as they grow nuclear masses. Moreover, with the beginning 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 bounces off at tremendous 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 with its rotation frequency; 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
Formed by condensation of the interstellar medium. Through observations, it was possible to determine that stars arose in different times and still arise to this day.
The main problem in the evolution of stars is the question of the origin of their energy, thanks to which they glow and emit huge amount energy. Previously, many theories were put forward that were designed to identify the sources of energy of stars. It was believed that a continuous source of stellar energy was continuous compression. This source is certainly good, but cannot maintain appropriate radiation for a long time. In the middle of the 20th century, the answer to this question was found. The radiation source is thermo nuclear reactions synthesis. As a result of these reactions, hydrogen turns into helium, and the released energy passes through the bowels of the star, is transformed and emitted into outer space (it is worth noting that the higher the temperature, the faster these reactions occur; this is why hot massive stars disappear faster main sequence).
Now imagine the emergence of a star...
A cloud of interstellar gas and dust medium began to condense. From this cloud a rather dense ball of gas is formed. The pressure inside the ball is not yet able to balance the forces of attraction, so it will shrink (perhaps at this time clumps with less mass will form around the star, which will eventually turn into planets). When compressed, the temperature rises. Thus, the star gradually sets on the main sequence. Then the gas pressure inside the star balances the gravity and the protostar turns into a star.
The early stage of the star's evolution is very small and the star at this time is immersed in a nebula, so the protostar is very difficult to detect.
The conversion of hydrogen into helium occurs only in the central regions of the star. In the outer layers, the hydrogen content remains practically unchanged. Since the amount of hydrogen is limited, sooner or later it burns out. The release of energy in the center of the star stops and the core of the star begins to shrink and the shell begins to swell. Further, if the star is less than 1.2 solar masses, it sheds its outer layer (formation of a planetary nebula).
After the envelope separates from the star, its inner, very hot layers are exposed, and meanwhile the envelope moves further and further away. After several tens of thousands of years, the shell will disintegrate and only a very hot and dense star will remain; gradually cooling, it will turn into a white dwarf. Gradually cooling, they turn into invisible black dwarfs. Black dwarfs are very dense and cool stars, slightly larger than the Earth, but with a mass comparable to the mass of the sun. The cooling process of white dwarfs lasts several hundred million years.
If the mass of a star is from 1.2 to 2.5 solar, then such a star will explode. This explosion is called supernova explosion. The flaring star increases its luminosity hundreds of millions of times in a few seconds. Such outbreaks occur extremely rarely. In our Galaxy, a supernova explosion occurs approximately once every hundred years. After such an outbreak, a nebula remains, which has a lot of radio emission and also scatters very quickly, and a so-called neutron star (more on this a little later). In addition to enormous radio emission, such a nebula will also be a source x-ray radiation, but this radiation is absorbed by the earth’s atmosphere, so it can only be observed from space.
There are several hypotheses about the cause of star explosions (supernovae), but there is no generally accepted theory yet. There is an assumption that this is due to too rapid a decline inner layers stars to the center. The star is rapidly shrinking to a catastrophic rate small size about 10 km, and its density in this state is 10 17 kg/m 3, which is close to the density of the atomic nucleus. This star consists of neutrons (at the same time, electrons are pressed into protons), which is why it is called "NEUTRON". Its initial temperature is about a billion Kelvin, but in the future it will quickly cool down.
This star, due to its small size and rapid cooling for a long time was considered impossible to observe. But after some time, pulsars were discovered. These pulsars turned out to be neutron stars. They are named so because of the short-term emission of radio pulses. Those. the star seems to “blink.” This discovery was made completely by accident and not so long ago, namely in 1967. These periodic impulses are due to the fact that during very rapid rotation, the cone of the magnetic axis constantly flashes past our gaze, which forms an angle with the axis of rotation.
A pulsar can only be detected for us under the conditions of orientation of the magnetic axis, and this is approximately 5% of them total number. Some pulsars are not located in radio nebulae, since nebulae dissipate relatively quickly. After a hundred thousand years, these nebulae cease to be visible, and the age of pulsars is tens of millions of years.
If the mass of a star exceeds 2.5 solar, then at the end of its existence it will seem to collapse in on itself and be crushed by its own weight. In a matter of seconds it will turn into a dot. This phenomenon was called “gravitational collapse”, and this object was also called a “black hole”.
From all that has been said above, it is clear that the final stage of the evolution of a star depends on its mass, but it is also necessary to take into account the inevitable loss of this very mass and rotation.
> Life cycle of a star
Description life and death of stars: stages of development with photos, molecular clouds, protostar, T Tauri, main sequence, red giant, white dwarf.
Everything in this world is evolving. Any cycle begins with birth, growth and ends with death. Of course, stars have these cycles in a special way. Let us at least remember that their time frames are larger and are measured in millions and billions of years. In addition, their death carries certain consequences. What does it look like life cycle stars?
The first life cycle of a star: Molecular clouds
Let's start with the birth of a star. Imagine a huge cloud of cold molecular gas that can quietly exist in the Universe without any changes. But suddenly a supernova explodes not far from it or it collides with another cloud. Due to such a push, the destruction process is activated. It is divided into small parts, each of which is retracted into itself. As you already understand, all these groups are preparing to become stars. Gravity heats up the temperature, and the stored momentum maintains the rotation process. The lower diagram clearly demonstrates the cycle of stars (life, stages of development, transformation options and death celestial body with photo).
Second life cycle of a star: Protostar
The material condenses more densely, heats up and is repelled by gravitational collapse. Such an object is called a protostar, around which a disk of material forms. The part is attracted to the object, increasing its mass. The remaining debris will group and create a planetary system. Further development of the star all depends on mass.
Third life cycle of a star: T Taurus
When material hits a star, a huge amount of energy is released. The new stellar stage was named after the prototype - T Tauri. This variable star, located 600 light years away (near).
It can reach great brightness because the material breaks down and releases energy. But in the central part there is not enough temperature to maintain nuclear fusion. This phase lasts 100 million years.
Fourth life cycle of a star:Main sequence
IN certain moment the temperature of the celestial body rises to the required level, activating nuclear fusion. All stars go through this. Hydrogen transforms into helium, releasing enormous heat and energy.
The energy is released as gamma rays, but due to the slow motion of the star, it falls with the same wavelength. Light is pushed out and comes into conflict with gravity. We can assume that an ideal balance is created here.
How long will she be in the main sequence? You need to start from the mass of the star. Red dwarfs (half the mass of the sun) can burn through their fuel supply for hundreds of billions (trillions) of years. Average stars (like ) live 10-15 billion. But the largest ones are billions or millions of years old. See what the evolution and death of stars looks like various classes on the diagram.
Fifth life cycle of a star: Red giant
During the melting process, hydrogen runs out and helium accumulates. When there is no hydrogen left at all, all nuclear reactions stop, and the star begins to shrink due to gravity. The hydrogen shell around the core heats up and ignites, causing the object to grow 1,000 to 10,000 times larger. At a certain moment, our Sun will repeat this fate, increasing to the Earth’s orbit.
Temperature and pressure reach a maximum and helium fuses into carbon. At this point the star shrinks and ceases to be a red giant. With greater massiveness, the object will burn other heavy elements.
Sixth life cycle of a star: White dwarf
A solar-mass star doesn't have enough gravitational pressure to fuse the carbon. Therefore, death occurs with the end of helium. The outer layers are ejected and a white dwarf appears. It starts out hot, but after hundreds of billions of years it cools down.
Let us briefly consider the main stages of stellar evolution.
Change in physical characteristics, internal structure and the chemical composition of the star over time.
Fragmentation of matter. .
It is assumed that stars are formed during gravitational compression of fragments of a gas and dust cloud. So, so-called globules can be places of star formation.
A globule is a dense opaque molecular-dust (gas-dust) interstellar cloud, which is observed against the background of luminous clouds of gas and dust in the form of a dark round formation. Consists predominantly of molecular hydrogen (H 2) and helium ( He ) with an admixture of molecules of other gases and solid interstellar dust grains. Gas temperature in the globule (mainly the temperature of molecular hydrogen) T≈ 10 ÷ 50K, average density n~ 10 5 particles/cm 3, which is several orders of magnitude greater than in the densest conventional gas and dust clouds, diameter D~ 0.1 ÷ 1. Mass of globules M≤ 10 2 × M ⊙ . In some globules, young type T Taurus.
The cloud is compressed by its own gravity due to gravitational instability, which can arise either spontaneously or as a result of the interaction of the cloud with a shock wave from a supersonic stellar wind flow from another nearby source of star formation. There are other possible causes of gravitational instability.
Theoretical studies show that under the conditions that exist in ordinary molecular clouds (T≈ 10 ÷ 30K and n ~ 10 2 particles/cm 3), the initial one can occur in cloud volumes with mass M≥ 10 3 × M ⊙ . In such a contracting cloud, further disintegration into less massive fragments is possible, each of which will also compress under the influence of its own gravity. Observations show that in the Galaxy, during the process of star formation, not one, but a group of stars with different masses, for example, an open star cluster, is born.
When compressed in the central regions of the cloud, the density increases, resulting in a moment when the substance of this part of the cloud becomes opaque to its own radiation. In the depths of the cloud, a stable dense condensation appears, which astronomers call oh.
Fragmentation of matter is the disintegration of a molecular dust cloud into smaller parts, the further part of which leads to the appearance.
– an astronomical object that is in the stage, from which after some time (for the solar mass this time T~ 10 8 years) normal is formed.
With the further fall of matter from the gas shell onto the core (accretion), the mass of the latter, and therefore the temperature, increases so much that the gas and radiant pressure are compared with the forces. Kernel compression stops. The formation is surrounded by a shell of gas and dust, opaque to optical radiation, allowing only infrared and longer wavelength radiation to pass through. Such an object (-cocoon) is observed as a powerful source of radio and infrared radiation.
With a further increase in the mass and temperature of the core, light pressure stops accretion, and the remnants of the shell are scattered in outer space. A young woman appears physical characteristics which depend on its mass and initial chemical composition.
The main source of energy for a nascent star is apparently the energy released during gravitational compression. This assumption follows from the virial theorem: in a stationary system, the sum of potential energy E p all members of the system and double kinetic energy 2 E to of these terms is equal to zero:
E p + 2 E k = 0. (39)
The theorem is valid for systems of particles moving in limited area space under the influence of forces, the magnitude of which is inversely proportional to the square of the distance between particles. It follows that thermal (kinetic) energy is equal to half of gravitational (potential) energy. When a star contracts, the total energy of the star decreases, while the gravitational energy decreases: half of the change in gravitational energy leaves the star through radiation, and due to the second half it increases thermal energy stars.
Young low mass stars(up to three solar masses) that are approaching the main sequence are completely convective; the convection process covers all areas of the star. These are essentially protostars, in the center of which nuclear reactions are just beginning, and all the radiation occurs mainly due to. It has not yet been established that the stars are decreasing at constant effective temperature. On the Hertzsprung-Russell diagram, such stars form an almost vertical track called the Hayashi track. As compression slows, the young approaches the main sequence.
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 growth of the central temperature 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 the internal pressure and. Such “understars” emit more energy than is produced during nuclear reactions, and are classified as so-called; their fate is constant compression until the pressure of the degenerate gas stops it, and then gradual cooling with the cessation of all nuclear reactions that have begun.
Young stars of intermediate mass (from 2 to 8 solar masses) evolve qualitatively in exactly the same way as their smaller sisters, except that they do not have convective zones until the main sequence.
Stars with a mass greater than 8 solar massesalready have the characteristics of normal stars, since they have gone through all the intermediate stages and were able to achieve such a rate of nuclear reactions that they compensate for the energy lost to radiation while the core mass accumulates. The outflow of mass from these stars is so great that it not only stops the collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, thaws them away. Thus, the mass of the resulting star is noticeably less than the mass of the protostellar cloud.
Main sequence
The temperature of the star increases until in the central regions it reaches values sufficient to enable thermonuclear reactions, which then become the main source of energy for the star. For massive stars ( M > 1 ÷ 2 × M ⊙ ) is the “combustion” of hydrogen in the carbon cycle; For stars with a mass equal to or less than the mass of the Sun, energy is released in the proton-proton reaction. enters the equilibrium stage and takes its place on the main sequence of the Hertzsprung-Russell diagram: a large-mass star has a very high core temperature ( T ≥ 3 × 10 7 K ), energy production is very intense, - on the main sequence it occupies a place above the Sun in the region of early ( O … A , (F )); a star of small mass has a relatively low core temperature ( T ≤ 1.5 × 10 7 K ), energy production is not so intense, - on the main sequence it occupies a place next to or below the Sun in the region of late (( F), G, K, M).
It spends up to 90% of the time allotted by nature for its existence on the main sequence. The time a star spends at the main sequence stage also depends on its mass. Yes, with mass M ≈ 10 ÷ 20 × M ⊙ O or B is in the main sequence stage for about 10 7 years, while the red dwarf K 5 with mass M ≈ 0.5 × M ⊙ is in the main sequence stage for about 10 11 years, that is, a time comparable to the age of the Galaxy. Massive hot stars quickly move into the next stages of evolution, cool dwarfs are in the main sequence stage throughout the existence of the Galaxy. It can be assumed that red dwarfs are the main type of population of the Galaxy.
Red giant (supergiant).
The rapid burning of hydrogen in the central regions of massive stars leads to the appearance of a helium core. When the mass fraction of hydrogen is several percent in the core, the carbon reaction of converting hydrogen into helium almost completely stops. The core contracts, causing its temperature to increase. As a result of heating caused by the gravitational compression of the helium core, hydrogen “ignites” and energy release begins in a thin layer located between the core and the extended shell of the star. The shell expands, the radius of the star increases, the effective temperature decreases and increases. “leaves” the main sequence and moves to the next stage of evolution - to the stage of a red giant or, if the mass of the star M > 10 × M ⊙ , into the red supergiant stage.
With increasing temperature and density, helium begins to “burn” in the core. At T ~ 2 × 10 8 K and r ~ 10 3 ¸ 10 4 g/cm 3 a thermonuclear reaction begins, which is called a ternary reaction a -process: of three a -particles (helium nuclei 4 He ) one stable carbon 12 C nucleus is formed. At the mass of the star's core M< 1,4 × M ⊙ тройной a -the process leads to an explosive energy release - a helium flare, which for a particular star can be repeated several times.
In the central regions of massive stars in the giant or supergiant stage, an increase in temperature leads to the sequential formation of carbon, carbon-oxygen and oxygen nuclei. After carbon burns out, reactions occur that result in the formation of heavier chemical elements, possibly iron nuclei. Further evolution of a massive star can lead to the ejection of the shell, the outburst of a star as a nova or, with the subsequent formation of objects that are the final stage of the evolution of stars: a white dwarf, a neutron star or a black hole.
The final stage of evolution is the stage of evolution of all normal stars after these stars have exhausted their thermonuclear fuel; cessation of thermonuclear reactions as a source of star energy; the transition of a star, depending on its mass, to the stage of a white dwarf, or black hole.
White dwarfs - last stage evolution of all normal stars with mass M< 3 ÷ 5 × M ⊙ after these have exhausted their thermonuclear fuel. Having passed the stage of a red giant (or subgiant), it sheds its shell and exposes the core, which, as it cools, becomes a white dwarf. Small radius (R b.k ~ 10 -2 × R ⊙ ) and white or white-blue color (T b.k ~ 10 4 K) determined the name of this class of astronomical objects. The mass of a white dwarf is always less than 1.4×M⊙ - it has been proven that white dwarfs with large masses cannot exist. With a mass comparable to the mass of the Sun and sizes comparable to the sizes of large planets solar system, white dwarfs have a huge average density: ρ b.k ~ 10 6 g/cm 3 , that is, a weight with a volume of 1 cm 3 of white dwarf matter weighs a ton! Acceleration free fall on surface g b.k ~ 10 8 cm/s 2 (compare with acceleration on the Earth’s surface - g ≈980 cm/s 2). With such a gravitational load on the inner regions of the star, the equilibrium state of the white dwarf is maintained by the pressure of the degenerate gas (mainly degenerate electron gas, since the contribution of the ion component is small). Let us recall that a gas in which there is no Maxwellian velocity distribution of particles is called degenerate. In such a gas, at certain values of temperature and density, the number of particles (electrons) having any speed in the range from v = 0 to v = v max will be the same. v max is determined by the density and temperature of the gas. With a white dwarf mass M b.k > 1.4 × M ⊙ maximum speed electrons in the gas is comparable to the speed of light, the degenerate gas becomes relativistic and its pressure is no longer able to withstand gravitational compression. The radius of the dwarf tends to zero - it “collapses” into a point.
The thin, hot atmospheres of white dwarfs consist either of hydrogen, with virtually no other elements detectable in the atmosphere; or from helium, while the hydrogen in the atmosphere is hundreds of thousands of times less than in the atmospheres of normal stars. According to the type of spectrum, white dwarfs belong to spectral classes O, B, A, F. To “distinguish” white dwarfs from normal stars, the letter D is placed in front of the designation (DOVII, DBVII, etc. D is the first letter in English word Degenerate - degenerate). The source of radiation from a white dwarf is the reserve of thermal energy that the white dwarf received as the core of the parent star. Many white dwarfs inherited from their parents a strong magnetic field, the intensity of which H ~ 10 8 E. It is believed that the number of white dwarfs is about 10% of the total number of stars in the Galaxy.
In Fig. 15 shows a photograph of Sirius - brightest star sky (α Canis Major; m v = -1 m .46; class A1V). The disk visible in the image is a consequence of photographic irradiation and diffraction of light on the telescope lens, that is, the disk of the star itself is not resolved in the photograph. The rays coming from the photographic disk of Sirius are traces of distortion of the wave front of the light flux on the elements of the telescope optics. Sirius is located at a distance of 2.64 from the Sun, the light from Sirius takes 8.6 years to reach the Earth - thus, it is one of the closest stars to the Sun. Sirius is 2.2 times more massive than the Sun; its M v = +1 m .43, that is, our neighbor emits 23 times more energy than the Sun.
Figure 15.The uniqueness of the photograph lies in the fact that, together with the image of Sirius, it was possible to obtain an image of its satellite - the satellite “glows” with a bright dot to the left of Sirius. Sirius - telescopically: Sirius itself is designated by the letter A, and its satellite by the letter B. The apparent magnitude of Sirius is B m v = +8 m .43, that is, it is almost 10,000 times weaker than Sirius A. The mass of Sirius B is almost exactly equal to the mass of the Sun, the radius is about 0.01 of the radius of the Sun, the surface temperature is about 12000K, but Sirius B emits 400 times less than the Sun . Sirius B is a typical white dwarf. Moreover, this is the first white dwarf, discovered, by the way, by Alfven Clarke in 1862 during visual observation through a telescope.
Sirius A and Sirius B orbit around the same with a period of 50 years; the distance between components A and B is only 20 AU.
According to the apt remark of V.M.Lipunov, “they “ripe” inside massive stars (with a mass of more than 10×M⊙ )". The cores of stars evolving into a neutron star have 1.4× M ⊙ ≤ M ≤ 3 × M ⊙ ; after the sources of thermonuclear reactions dry up and the parent flare throws off a significant part of the matter, these nuclei will become independent objects of the stellar world, with very specific characteristics. The compression of the core of the parent star stops at a density comparable to the nuclear density (ρ n. h ~ 10 14 ÷ 10 15 g/cm 3). With such mass and density, the radius of the birth is only 10 and consists of three layers. Outer layer(or outer cortex) formed crystal lattice from atomic nuclei iron ( Fe ) with a possible small admixture of atomic nuclei of other metals; The thickness of the outer crust is only about 600 m with a radius of 10 km. Beneath the outer crust is another inner hard crust made up of iron atoms ( Fe ), but these atoms are over-enriched with neutrons. The thickness of this bark≈ 2 km. The inner crust borders on the liquid neutron core, the physical processes in which are determined by the remarkable properties of the neutron liquid - superfluidity and, in the presence of free electrons and protons, superconductivity. It is possible that in the very center the substance may contain mesons and hyperons.
They rotate quickly around an axis - from one to hundreds of revolutions per second. Such rotation in the presence of a magnetic field ( H ~ 10 13 ÷ 10 15 Oe) often leads to the observed effect of pulsation of star radiation in different ranges electromagnetic waves. We saw one of these pulsars inside the Crab Nebula.
Total number the rotation speed is no longer sufficient for particle ejection, so it cannot be a radio pulsar. However, it is still great, and captured magnetic field the surrounding neutron star cannot fall, that is, accretion of matter does not occur.
Accrector (X-ray pulsar). The rotation speed decreases to such an extent that there is now nothing stopping the matter from falling onto such a neutron star. The plasma, falling, moves along the magnetic field lines and hits a solid surface in the region of the poles, heating up to tens of millions of degrees. Matter heated to such high temperatures glows in the X-ray range. The region in which the falling matter interacts with the surface of the star is very small - only about 100 meters. Due to the rotation of the star, this hot spot periodically disappears from view, which the observer perceives as pulsations. Such objects are called X-ray pulsars.
Georotator. The rotation speed of such neutron stars is low and does not prevent accretion. But the size of the magnetosphere is such that the plasma is stopped by the magnetic field before it is captured by gravity.
If it is a component of a close binary system, then matter is “pumped” from the normal star (the second component) to the neutron star. The mass may exceed critical (M > 3×M⊙ ), then the gravitational stability of the star is violated, nothing can resist gravitational compression, and “goes” under its gravitational radius
r g = 2 × G × M/c 2 , (40)
turning into a “black hole”. In the given formula for r g: M is the mass of the star, c is the speed of light, G is the gravitational constant.
A black hole is an object whose gravitational field is so strong that neither a particle, nor a photon, nor any material body can't reach the second one escape velocity and escape into outer space.
A black hole is a singular object in the sense that the nature of its flow physical processes inside it is not yet accessible to theoretical description. The existence of black holes follows from theoretical considerations; in reality, they can be located in the central regions of globular clusters, quasars, giant galaxies, including in the center of our galaxy.
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.
