The first minutes of the existence of the Universe. Origin of chemical elements. Origin of chemical elements Formation of chemical elements during the evolution of stars

Origin of chemical elements

The task of the theory of P.h.e. (nucleosynthesis) phenomenon. construction of an evolutionary picture of the formation of all the chemical diversity observed in nature. elements. The key to understanding the process of nuclear evolution of matter from the original hot plasma of elementary particles to the modern one. states are relative and their isotopes in the matter of the observable part of the Universe.

Modern approach to explaining the basic observed chemical patterns. and the isotopic composition of the matter of the Universe is as follows.

It seems most likely that isotopes of all significant elements are formed in stars. Helium was probably already contained in the protostellar matter from which the first stars of galaxies were formed, and thermonuclear fusion reactions in the early stages of the expansion of the hot Universe are responsible for its formation (see). Quite satisfactory agreement between the observed abundance of helium and the value predicted within the framework of , yavl. a strong argument in favor of such an assumption. The same mechanism is most likely responsible for the formation of the observed amounts of H2, He3 isotopes, as well as a certain fraction of Li7.

The rate of formation of elements in the Galaxy was significantly higher in the past than at the time of the formation of the Solar System (4.6 billion years ago), and mainly. enrichment of galactic matter with elements A>4 occurred 10-15 billion years ago (see). This suggests that among the first generations of stars, massive stars predominated, quickly ending their evolution by ejecting a significant fraction (or all) of the material processed and enriched with heavy elements into interstellar space, where it was part of the initial material for the formation of the next generations of stars.

The problem of the formation of light elements - Li, Be and B - has not yet been completely solved. These elements are easily destroyed in thermonuclear reactions, and therefore their efficient production is possible only in nonequilibrium processes. It is believed that they are formed by ch. arr. during the interaction of galactic particles with interstellar gas matter in spallation reactions (see). However, the difficulties that arise in explaining the unusual isotopic composition of Li and B (a pronounced predominance of odd isotopes) most likely indicate the presence of an additional effective source of their formation. The most likely candidate for this role is Yavl. explosions, because The passage through the ejected shell of a powerful flow of neutrino radiation from the collapsing core of the star, as well as a strong shock wave, leads to the formation of noticeable quantities of odd isotopes of light elements in the substance of the shell.

Most chemical isotopes elements, starting with carbon and up to the elements of the “iron peak” region (Fe, Ni, etc.), are formed under high temperature conditions in thermonuclear fusion reactions, and the initial stage of this sequence of nuclear transformations is the processes 4 He + 4 He + 4 He 12 C + and 4 He + 12 C 16 O + , leading to an effective increase in the amount of 12 C and 16 O at hydrostatically equilibrium stages. The most favorable conditions for the formation of Ne and all the heavier elements of this group are apparently realized during the explosive combustion of C, O and Si at the final, nonequilibrium stage of the evolution of massive stars.

The most common isotopes of elements heavier than iron were apparently formed in the interiors of massive stars as a result of successive neutron capture reactions. A number of characteristic features of the course of the abundance curve of these heavy nuclei indicate that the process of their construction should proceed quite efficiently as at a relatively long equilibrium stage of stellar evolution under conditions of low neutron flux intensities ( s-process), and at the moment of explosion of a star at a high intensity of neutron flux ( r-process).

The formation of rare (with a relatively low neutron content) isotopes of heavy elements, which could not be formed in the process of sequential addition of neutrons (hence the term), is possible only at the last, catastrophic stage of the evolution of massive stars, either under the influence of a flux of neutrino radiation from the collapsing core of the star, or in k.-l. other nonequilibrium processes.

The listed mechanisms of formation of each of the basic. chemical isotope groups elements turn out to be quite effective in physical. conditions, which can be realized in known types of astrophysics. objects and make it possible to explain, at least in general terms, the main patterns of the observed prevalence of chemicals. elements. In this sense, we can say that the main contours of the picture P.h.e. have already been outlined, while the construction of a consistent and self-consistent theory of P.h.e. It is practically just beginning and requires the solution of many more problems of cosmology, the theory of the structure and evolution of galaxies and stars, the physics of the atomic nucleus and elementary particles.

Lit.:
Frank-Kamenetsky D.A., Nuclear Astrophysics, M., 1967; Theiler R.J., Origin of chemical elements, trans. from English, M., 1975; Nuclear astrophysics, trans. from English, M. (in print)

(G.V. Domogatsky)

The process of formation of chemical elements in the Universe is inextricably linked with the evolution of the Universe. We have already become acquainted with the processes occurring near the “Big Bang”; we know some details of the processes that took place in the “primary soup” of elementary particles. The first atoms of chemical elements, located at the beginning of D.I. Mendeleev’s table (hydrogen, deuterium, helium), began to form in the Universe even before the emergence of first-generation stars. It was in the stars, their depths, heated up again (after the Big Bang, the temperature of the Universe began to rapidly fall) to billions of degrees, that the nuclei of the chemical elements following helium were produced. Considering the importance of stars as sources and generators of chemical elements, let us consider some stages of stellar evolution. Without understanding the mechanisms of star formation and the evolution of stars, it is impossible to imagine the process of formation of heavy elements, without which, ultimately, life would not have arisen. Without stars, hydrogen-helium plasma would exist forever in the Universe, in which the organization of life is obviously impossible (at the modern level of understanding of this phenomenon).

Earlier we noted three observational facts or tests of modern cosmology, extending over hundreds of parsecs, now we will point out the fourth - the prevalence of light chemical elements in space. It must be emphasized that the formation of light elements in the first three minutes and their prevalence in the modern Universe was first calculated in 1946 by an international trio of outstanding scientists: the American Alpher, the German Hans Bethe and the Russian Georgiy Gamow. Since then, atomic and nuclear physicists have repeatedly calculated the formation of light elements in the early Universe and their abundance today. It can be argued that the standard model of nucleosynthesis is well supported by observations.

Evolution of stars. The mechanism of formation and evolution of the main objects of the Universe - stars, has been studied most xoponio. Here, scientists were helped by the opportunity to observe a huge number of stars at various stages of development - from birth to death - including many so-called “stellar associations” - groups of stars born almost simultaneously. The comparative “simplicity” of the star’s structure, which quite successfully lends itself to theoretical description and computer modeling, also helped.

Stars are formed from clouds of gas that, under certain circumstances, break up into separate “clumps” that are further compressed by their own gravity. The compression of the gas under the influence of its own gravity is prevented by the increasing pressure. During adiabatic compression, the temperature must also increase - gravitational binding energy is released in the form of heat. While the cloud is rarefied, all the heat easily leaves with radiation, but in the dense core of the condensation, the removal of heat is difficult, and it quickly heats up. The corresponding increase in pressure slows down the compression of the core, and it continues to occur only due to the gas continuing to fall onto the nascent star. As the mass increases, the pressure and temperature in the center increases, until finally the latter reaches a value of 10 million Kelvin. At this moment, nuclear reactions begin in the center of the star, converting hydrogen into helium, which maintain the stationary state of the newly formed star for millions, billions or tens of billions of years, depending on the mass of the star.

The star turns into a huge thermonuclear reactor, in which, in general, the same reaction proceeds steadily and stably, which man has so far learned to carry out only in an uncontrolled version - in a hydrogen bomb. The heat released during the reaction stabilizes the star, maintaining internal pressure and preventing it from further compression. A small random increase in the reaction slightly “inflates” the star, and a corresponding decrease in density again leads to a weakening of the reaction and stabilization of the process. The star “burns” with almost constant brightness.

The temperature and radiation power of a star depends on its mass, and it depends nonlinearly. Roughly speaking, when the mass of a star increases by 10 times, its radiation power increases by 100 times. Therefore, more massive, hotter stars use up their fuel reserves much faster than less massive ones and have relatively short lives. The lower limit of the mass of a star, at which it is still possible to achieve temperatures in the center sufficient for the onset of thermonuclear reactions, is approximately 0.06 solar. The upper limit is about 70 solar masses. Accordingly, the faintest stars shine several hundred times weaker than the Sun and can shine this way for hundreds of billions of years, much longer than the existence of our Universe. Massive, hot stars can be a million times brighter than the Sun and only live for a few million years. The time of stable existence of the Sun is approximately 10 billion years, and of this period it has lived so far for half.

The stability of a star is disrupted when a significant portion of the hydrogen in its core burns out. A helium core devoid of hydrogen is formed, and the combustion of hydrogen continues in a thin layer on its surface. In this case, the core contracts, in the center its pressure and temperature increases, while at the same time the upper layers of the star, located above the hydrogen burning layer, on the contrary, expand. The diameter of the star increases, and the average density decreases. Due to the increase in the area of ​​the emitting surface, its total luminosity also slowly increases, although the temperature of the star's surface decreases. The star turns into a red giant. At some point in time, the temperature and pressure inside the helium core are sufficient to begin the following reactions of synthesis of heavier elements - carbon and oxygen from helium, and at the next stage even heavier ones. In the interior of a star, many elements of the Periodic Table can be formed from hydrogen and helium, but only up to the elements of the iron group, which have the highest binding energy per particle. Heavier elements are formed in other rarer processes, namely during the explosions of supernovae and partially novae, and therefore they are scarce in nature.

Let us note an interesting, paradoxical, at first glance, circumstance. While hydrogen is burning near the center of the star, the temperature there cannot rise to the threshold of the helium reaction. To do this, it is necessary that the combustion stops and the core of the star begins to cool! The cooling core of the star contracts, which increases the strength of the gravitational field and releases gravitational energy, which heats the matter. At higher field strengths, a higher temperature is needed so that the pressure can resist compression, and gravitational energy is sufficient to provide this temperature. We have a similar paradox when lowering a spacecraft: in order to transfer it to a lower orbit, it must be slowed down, but at the same time it turns out to be closer to Earth, where the force of gravity is greater, and its speed will increase. Cooling increases the temperature, and braking increases the speed! Nature is full of such seeming paradoxes, and one cannot always trust “common sense.”

After the start of helium combustion, energy consumption proceeds at a very rapid pace, since the energy yield of all reactions with heavy elements is much lower than during the hydrogen combustion reaction and, in addition, the overall luminosity of the star at these stages increases significantly. If hydrogen burns for billions of years, then helium burns for millions, and all other elements for no more than thousands of years. When all nuclear reactions die out in the bowels of a star, nothing can prevent its gravitational compression, and it occurs catastrophically quickly (as they say, it collapses). The upper layers fall towards the center with free fall acceleration (its magnitude is many orders of magnitude greater than the earth's fall acceleration due to the incomparable mass difference), releasing enormous gravitational energy. The substance is compressed. Part of it, passing into a new state of high density, forms a remnant star, and part (usually a large one) is thrown into space in the form of a reflected shock wave with enormous speed. A supernova explosion occurs. (In addition to gravitational energy, the thermonuclear burnout of part of the hydrogen remaining in the outer layers of the star also contributes to the kinetic energy of the shock wave, when the falling gas is compressed near the stellar core - an explosion of a grandiose “hydrogen bomb” occurs).

At what stage of the star’s evolution the compression will stop and what the supernova remnant will be, all these options depend on its mass. If this mass is less than 1.4 solar, it will be a white dwarf, a star with a density of 10 9 kg/m 3, slowly cooling without internal sources of energy. It is kept from further compression by the pressure of the degenerate electron gas. With a larger mass (up to about 2.5 solar), a neutron star is formed (their existence was predicted by the great Soviet physicist, Nobel laureate Lev Landau) with a density approximately equal to the density of the atomic nucleus. Neutron stars were discovered as so-called pulsars. With an even greater initial mass of the star, a black hole is formed - an uncontrollably contracting object that no object, not even light, can escape. It is during supernova explosions that elements heavier than iron are formed, which require extremely dense streams of high-energy particles in order for multi-particle collisions to be sufficiently likely. Everything material in this world is the descendants of supernovae, including people, since the atoms of which we are composed once arose during supernova explosions.

Thus, stars are not only a powerful source of high-quality energy, the dissipation of which contributes to the emergence of complex structures, including life, but also reactors in which the entire periodic table is produced - the necessary material for these structures. The explosion of a star ending its life throws into space a huge variety of elements heavier than hydrogen and helium, which mix with the galactic gas. During the life of the Universe, many stars have ended their lives. All stars like the Sun and more massive ones that arose from the primordial gas have already passed their life path. So now the Sun and similar stars are stars of the second generation (and maybe even the third), significantly enriched in heavy elements. Without such enrichment, it is unlikely that terrestrial planets and life could have arisen near them.

Here is information about the prevalence of some chemical elements in the Universe:

As we can see from this table, the predominant chemical elements at present are hydrogen and helium (almost 75% and 25% each). The relatively small content of heavy elements, however, turned out to be sufficient for the formation of life (at least on one of the islands of the Universe near an “ordinary” star, the Sun - a yellow dwarf). In addition to what we have already indicated earlier, we must remember that in open outer space there are cosmic rays, which are essentially streams of elementary particles, primarily electrons and protons of different energies. In some regions of interstellar space there are local areas of increased concentration of interstellar matter, called interstellar clouds. Unlike the plasma composition of a star, the matter of interstellar clouds already contains (as evidenced by numerous astronomical observations) molecules and molecular ions. For example, interstellar clouds of molecular hydrogen H2 have been discovered; compounds such as hydroxyl ion OH, CO molecules, water molecules, etc. are often present in the absorption spectra. Now the number of chemical compounds discovered in interstellar clouds is over one hundred. Under the influence of external radiation and without it, various chemical reactions occur in clouds, often those that cannot be carried out on Earth due to special conditions in the interstellar medium. Probably, about 5 billion years ago, when our solar system was formed, the primary material in the formation of planets were the same simple molecules that we now observe in other interstellar clouds. In other words, the process of chemical evolution that began in the interstellar cloud then continued on the planets. Although quite complex organic molecules have now been discovered in some interstellar clouds, chemical evolution probably led to the appearance of “living” matter (that is, cells with mechanisms of self-organization and heredity) only on planets. It is very difficult to imagine the organization of life in the volume of interstellar clouds.

Planetary chemical evolution.

Let's consider the process of chemical evolution on Earth. The primary atmosphere of the Earth contained mainly the simplest hydrogen compounds H 2, H 2 O, NH 3, CH 4. In addition, the atmosphere was rich in inert gases, primarily helium and neon. Currently, the abundance of noble gases on Earth is negligible, which means that at one time they were discordant into interplanetary space. Our modern atmosphere is of secondary origin. At first, the chemical composition of the atmosphere differed little from the original one. After the formation of the hydrosphere, ammonia NH 3, dissolved in water, practically disappeared from the atmosphere, atomic and molecular hydrogen evaporated into interplanetary space, the atmosphere was saturated predominantly with nitrogen N. Saturation of the atmosphere with oxygen occurred gradually, first due to the dissociation of water molecules by ultraviolet radiation from the Sun, and then mainly way, thanks to plant photosynthesis.

It is possible that some amount of organic matter was brought to Earth by the fall of meteorites and perhaps even comets. For example, comets contain compounds such as N, NH 3, CH 4, etc. It is known that the age of the earth's crust is approximately 4.5 billion years. There is also geological and geochemical evidence indicating that already 3.5 billion years ago the earth's atmosphere was rich in oxygen. Thus, the primary atmosphere of the Earth existed for no more than 1 billion years, and life probably arose even earlier.

Currently, significant experimental material has been accumulated illustrating how such simple substances as water, methane, ammonia, carbon monoxide, ammonium and phosphate compounds are transformed into highly organized structures that are the building blocks of the cell. American scientists Kelvin, Miller and Urey conducted a series of experiments that showed how amino acids could have arisen in the primordial atmosphere. Scientists have created a mixture of gases - methane CH 4, molecular hydrogen H 2, ammonia NH 3 and water vapor H 2 O, simulating the composition of the Earth's primary atmosphere. Electrical discharges were passed through this mixture; as a result, glycine, alanine and other amino acids were discovered in the initial mixture of gases. Probably, the Sun had a significant influence on chemical reactions in the primary atmosphere of the Earth with its ultraviolet radiation, which did not linger in the atmosphere due to the absence of ozone.

Not only electrical discharges and ultraviolet radiation from the Sun, but also volcanic heat, shock waves, and the radioactive decay of potassium K (the share of potassium decay energy about 3 billion years ago on Earth was second only to the energy of ultraviolet radiation from the Sun) had an important impact on chemical evolution. For example, gases released from primary volcanoes (O 2, CO, N 2, H 2 O, H 2, S, H 2 S, CH 4, SO 2), when exposed to various types of energy, react to form a variety of small organic compounds, type: hydrogen cyanide HCN, formic acid HCO 2 H, acetic acid H 3 CO 2 H, glycine H 2 NCH 2 CO 2 H, etc. Subsequently, again under the influence of various types of energy, small organic compounds react to form more complex organic compounds: amino acids

Thus, on Earth there were conditions for the formation of complex organic compounds necessary to create a cell.

At present, there is still no single logically consistent picture of how life arose from the primordial “superdrop of matter” called the Universe after the Big Bang. But scientists already imagine many elements of this picture and believe that this is how everything really happened. One element of this unified picture of evolution is chemical evolution. Perhaps, chemical evolution is one of the reasoned elements of a unified picture of evolution, if only because it allows experimental modeling of chemical processes (which, for example, cannot be done in relation to conditions similar to those near the “big bang”). Chemical evolution can be traced down to the elementary building blocks of living matter: amino acids, nucleic acids.

For many centuries, man has been studying various natural phenomena, discovering its laws one after another. However, even now there are many scientific problems that people have long dreamed of solving. One of these complex and interesting problems is the origin of the chemical elements that make up all the bodies around us. Step by step, man learned the nature of chemical elements, the structure of their atoms, as well as the prevalence of elements on Earth and other cosmic bodies.

The study of the laws of nuclear reactions allows us to create a theory of the origin of chemical elements and their prevalence in nature. According to nuclear physics and astrophysics, the synthesis and transformation of chemical elements occur during the development of stars. The formation of atomic nuclei is carried out either due to thermonuclear reactions, or through reactions of absorption of neutrons by nuclei. It is now generally accepted that various nuclear reactions take place in stars at all stages of their development. The evolution of stars is caused by two opposing factors: gravitational compression, leading to a reduction in the volume of the star, and nuclear reactions, accompanied by the release of huge amounts of energy.

As modern data from nuclear physics and astrophysics show, the synthesis and transformation of elements occurs at all stages of the evolution of stars as a natural process of their development. Thus, the modern theory of the origin of chemical elements is based on the assumption that they are synthesized in a variety of nuclear processes at all stages of stellar evolution. Each state of a star and its age correspond to certain nuclear processes of synthesis of elements and the corresponding chemical composition. The younger the star, the more light elements it contains. The heaviest elements are synthesized only during the process of explosion - the dying of a star. In stellar corpses and other cosmic bodies of lower mass and temperature, substance transformation reactions continue to occur. Under these conditions, nuclear decay reactions and various processes of differentiation and migration occur.

Studying the abundance of chemical elements sheds light on the origin of the solar system and allows us to understand the origin of chemical elements. Thus, in nature there is an eternal birth, transformation and decay of atomic nuclei. The current opinion about a one-time act of the origin of chemical elements is, at least, incorrect. In reality, atoms are eternally (and constantly) born, eternally (and constantly) die, and their set in nature remains unchanged. “In nature there is no priority to creation or destruction - one arises, the other is destroyed.”

In general, based on modern concepts, most chemical elements, except for a few of the lightest, arose in the Universe mainly during secondary or stellar nucleosynthesis (elements up to iron - as a result of thermonuclear fusion, heavier elements - during the sequential capture of neutrons by atomic nuclei and subsequent beta decay, as well as in a number of other nuclear reactions). The lightest elements (hydrogen and helium - almost completely, lithium, beryllium and boron - partially) were formed in the first three minutes after the Big Bang (primary nucleosynthesis). One of the main sources of especially heavy elements in the Universe should be, according to calculations, mergers of neutron stars, with the release of significant quantities of these elements, which subsequently participate in the formation of new stars and their planets.

NEW DATA

Russian scientists have found evidence of how heavy elements appear in the Universe, from which planets and, ultimately, people were formed. An article about this was published in one of the most prestigious scientific journals – Nature. Until now, it was believed that heavy elements such as iron and silicon were born in the explosion of so-called supernovae. This theory has a lot of indirect evidence, but there was no direct evidence. In particular, astrophysicists were able to register the decay of the theoretically predicted isotopes of radioactive cobalt-56 and iron-56 in the remnant of one of the supernovae. However, this is clearly not enough to confirm the theory. Maybe it all ended with cobalt and iron. But how did other elements appear?

The theory indicated the direction of further search - an isotope of titanium (titanium-44). It is he who should be born after the decay of cobalt and iron. It is clear that astrophysicists around the world are targeting titanium. But without success. It was difficult to grasp, and doubts already appeared as to whether the theory was correct? Verna! This conclusion follows from the work of Russian physicists from the Space Research Institute of the Russian Academy of Sciences and Chris Winkler, an employee of the European Center for Space Research and Technology. Using the INTEGRAL international orbital gamma-ray observatory, they managed to detect in X-rays the radiation from the radioactive decay of titanium-44. Which was the first direct evidence of the formation of titanium at the time of the explosion of this unique supernova.

But scientists did not stop there. They were able to estimate the mass of the born titanium - about 100 Earth masses. What's next? The theory predicts that titanium decays into scandium, which decays into calcium. If scientists manage to record this entire chain, this will be a decisive argument that the theory of the formation of heavy elements during supernova explosions is correct.

Chemical evolution or prebiotic evolution- the stage preceding the emergence of life, during which organic, prebiotic substances arose from inorganic molecules under the influence of external energetic and selection factors and due to the deployment of self-organization processes characteristic of all relatively complex systems, which, undoubtedly, are all carbon-containing molecules.

These terms also denote the theory of the emergence and development of those molecules that are of fundamental importance for the emergence and development of living matter.

Everything that is known about the chemistry of matter allows us to limit the problem of chemical evolution within the framework of the so-called “water-carbon chauvinism”, which postulates that life in our Universe is presented in the only possible version: as a “mode of existence of protein bodies”, realized thanks to a unique combination of polymerization properties of carbon and depolarizing properties of the liquid-phase aqueous environment, as jointly necessary and/or sufficient(?) conditions for the emergence and development of all forms of life known to us. This implies that, at least within one formed biosphere, there can be only one heredity code common to all living beings of a given biota, but the question remains open whether other biospheres exist outside the Earth and whether other variants of the genetic apparatus are possible.

It is also unknown when and where chemical evolution began. Any timing is possible after the end of the second cycle of star formation, which occurred after the condensation of the products of explosions of primary supernovae, supplying heavy elements (with an atomic mass of more than 26) into interstellar space. The second generation of stars, already with planetary systems enriched in heavy elements that are necessary for the implementation of chemical evolution, appeared 0.5-1.2 billion years after the Big Bang. If certain quite probable conditions are met, almost any environment can be suitable for launching chemical evolution: the depths of the oceans, the interiors of planets, their surfaces, protoplanetary formations and even clouds of interstellar gas, which is confirmed by the widespread detection in space by astrophysics methods of many types of organic substances - aldehydes, alcohols, sugars and even the amino acid glycine, which together can serve as the starting material for chemical evolution, which has as its final result the emergence of life.

14.1 Stages of synthesis of elements

To explain the prevalence of various chemical elements and their isotopes in nature, Gamow proposed the Hot Universe model in 1948. According to this model, all chemical elements were formed at the moment of the Big Bang. However, this claim was later refuted. It has been proven that only light elements could be formed at the time of the Big Bang, and heavier elements arose in the processes of nucleosynthesis. These provisions are formulated in the Big Bang model (see paragraph 15).
According to the Big Bang model, the formation of chemical elements began with the initial nuclear fusion of light elements (H, D, 3 He, 4 He, 7 Li) 100 seconds after the Big Bang at a temperature of the Universe of 10 9 K.
The experimental basis of the model is the expansion of the Universe observed on the basis of redshift, the initial synthesis of elements and cosmic background radiation.
The great advantage of the Big Bang model is the prediction of the abundance of D, He and Li, which differ from each other by many orders of magnitude.
Experimental data on the abundance of elements in our Galaxy showed that there are 92% hydrogen atoms, 8% helium atoms, and 1 atom in 1000 of heavier nuclei, which is consistent with the predictions of the Big Bang model.

14.2 Nuclear fusion - synthesis of light elements (H, D, 3 He, 4 He, 7 Li) in the early Universe.

• The abundance of 4 He or its relative share in the mass of the Universe is Y = 0.23 ±0.02. At least half of the helium produced by the Big Bang is contained in intergalactic space.
• The original deuterium exists only inside the stars and quickly turns into 3 He.
  From the observational data, the following restrictions on the abundance of deuterium and He relative to hydrogen are obtained:

10 -5 ≤ D/H ≤ 2·10 -4 and
1.2·10 -5 ≤ 3 He/H ≤ 1.5·10 -4 ,

and the observed D/H ratio is only a fraction ƒ of the original value: D/H = ƒ(D/H) initial. Since deuterium quickly converts to 3 He, the following estimate for abundance is obtained:

[(D + 3 He)/H] initial ≤ 10 -4.

• The abundance of 7 Li is difficult to measure, but data from studies of stellar atmospheres and the dependence of the abundance of 7 Li on the effective temperature are used. It turns out that, starting from a temperature of 5.5·10 3 K, the amount of 7 Li remains constant. The best estimate of the average abundance of 7 Li is:

7 Li/H = (1.6±0.1)·10 -10 .

• The abundance of heavier elements such as 9 Be, 10 B and 11 B is lower by several orders of magnitude. Thus, the prevalence of 9 Be/H< 2.5·10 -12 .

14.3 Nuclei synthesis in Main Sequence stars at T< 108 K

The synthesis of helium in Main Sequence stars in the pp and CN cycles occurs at a temperature T ~ 10 7 ÷7·10 7 K. Hydrogen is processed into helium. Nuclei of light elements appear: 2 H, 3 He, 7 Li, 7 Be, 8 Be, but there are few of them due to the fact that they subsequently enter into nuclear reactions, and the 8 Be nucleus decays almost instantly due to its short lifetime (~10 -16 s)

8 Be → 4 He + 4 He.

The synthesis process seemed to have to stop, But nature has found a workaround.
When T > 7 10 7 K, helium "burns", turning into carbon nuclei. A triple helium reaction occurs - “Helium flash” - 3α → 12 C, but its cross section is very small and the process of formation of 12 C occurs in two stages.
A fusion reaction of 8 Be and 4 He nuclei occurs with the formation of a carbon nucleus 12 C* in an excited state, which is possible due to the presence of a level of 7.68 MeV in the carbon nucleus, i.e. reaction occurs:

8 Be + 4 He → 12 C* → 12 C + γ.

The existence of the 12 C nuclear energy level (7.68 MeV) helps to bypass the short lifetime of 8 Be. Due to the presence of this level in the 12 C nucleus, Breit-Wigner resonance. The 12 C nucleus goes to an excited level with energy ΔW = ΔМ + ε,
where εM = (M 8Be − M 4He) − M 12C = 7.4 MeV, and ε is compensated by kinetic energy.
This reaction was predicted by astrophysicist Hoyle and then reproduced in the laboratory. Then the reactions begin:

12 C + 4 He → 16 0 + γ
16 0 + 4 He → 20 Ne + γ and so on until A ~ 20.

The required level of 12 C core made it possible to pass through the bottleneck in the thermonuclear fusion of elements.
The 16 O nucleus does not have such energy levels and the reaction to form 16 O proceeds very slowly

12 C + 4 He → 16 0 + γ.

These features of the reactions led to the most important consequences: thanks to them, the number of nuclei 12 C and 16 0 was equal, which created favorable conditions for the formation of organic molecules, i.e. life.
A change in the level of 12 C by 5% would lead to a catastrophe - further synthesis of elements would cease. But since this did not happen, nuclei with A in the range are formed

A = 25÷32

This leads to the values ​​of A

All Fe, Co, Cr nuclei are formed due to thermonuclear fusion.

It is possible to calculate the abundance of nuclei in the Universe based on the existence of these processes.
Information about the abundance of elements in nature is obtained from spectral analysis of the Sun and Stars, as well as cosmic rays. In Fig. Figure 99 shows the intensity of nuclei at different values ​​of A.

Rice. 99: The abundance of elements in the Universe.

Hydrogen H is the most common element in the Universe. Lithium Li, beryllium Be and boron B are 4 orders of magnitude smaller than neighboring nuclei and 8 orders of magnitude smaller than H and He.
Li, Be, B are good fuels; they burn quickly already at T ~ 10 7 K.
It is more difficult to explain why they still exist - most likely due to the process of fragmentation of heavier nuclei at the protostar stage.
There are many more Li, Be, and B nuclei in cosmic rays, which is also a consequence of the processes of fragmentation of heavier nuclei during their interaction with the interstellar medium.
12 C÷ 16 O is the result of the Helium Flash and the existence of a resonant level in 12 C and the absence of one in 16 O, the nucleus of which is also doubly magical.
12 C - semi-magic core.
Thus, the maximum abundance of iron nuclei is 56 Fe, and then there is a sharp decline.

For A > 60, synthesis is energetically unfavorable.

14.5 Formation of nuclei heavier than iron
The fraction of nuclei with A > 90 is small - 10 -10 from hydrogen nuclei. The processes of nuclear formation are associated with side reactions occurring in stars. There are two known such processes:
s (slow) – slow process,
g (rapid) – fast process. Both of these processes are associated with neutron capture

13 C + 4 He → 16 0 + n – helium combustion,
12 C + 12 C → 23 Mg + n – carbon flare,
16 O + 16 O → 31 S + n – oxygen flash,
21 Ne + 4 He → 24 Mg + n – reaction with α-particles.

As a result, a neutron background accumulates and s- and r-processes—neutron capture—can occur. When neutrons are captured, neutron-rich nuclei are formed, and then β decay occurs. It turns them into heavier nuclei.

Origin of Chemical Elements in the Universe

Creation of chemical elements on Earth

Everybody knows periodic table of chemical elements - table Mendeleev . There are quite a lot of elements there and physicists are continuously working to create more and more heavy transuranium elements . There is a lot of interesting things in nuclear physics related to the stability of these nuclei. There are all sorts of islands of stability and people working on the corresponding accelerators are trying to create chemical elements with very high atomic numbers. But all these elements They don't live very long. That is, you can create several kernels of this element , have time to research something, prove that you really synthesized it and discovered this element . Get the right to give it a name, maybe get a Nobel Prize. But in the nature of these chemical elements It seems not, but in fact they can arise in some processes. But they disintegrate in absolutely insignificant quantities and in a short time. Therefore, in Universe , basically we see elements starting with uranium and lighter.

Evolution of the Universe

But Universe ours is evolving. And in general, as soon as you come to the idea of ​​some kind of global change, you inevitably come to the idea that everything you see around, in one sense or another, becomes perishable. And if, in the sense of people, animals and things, we have somehow come to terms with this, then taking the next step sometimes seems strange. For example, is water always water or is iron always iron?! The answer is no, because it evolves. Universe in general and once upon a time, naturally, there was no earth, for example, the earth and all its component parts were scattered throughout some nebula from which the solar system was formed. You need to go even further back and it turns out that once upon a time there was not only Mendeleev and his periodic table, but there were no elements included in it. Since our Universe was born, passing through a very hot, very dense state. And when it’s hot and dense, all complex structures are destroyed. And so, in very early history Universe there were no stable substances or even elementary particles familiar to us.

Origin of light chemical elements in the Universe

Formation of the chemical element hydrogen

As The universe was expanding , cooled down and became less dense, some particles appeared. Roughly speaking, we can assign energy to each particle mass using the formula E=mc 2 . For each energy we can associate a temperature and when the temperature drops below this critical energy, the particle can become stable and can exist.
Respectively The universe is expanding , cools down and naturally appears first from the periodic table hydrogen . Because it's just a proton. That is, protons appeared, and we can say that hydrogen . In this sense Universe on 100% consists of hydrogen, plus dark matter, plus dark energy, plus a lot of radiation. But from ordinary matter there is only hydrogen . Appear protons , begin to appear neutrons . Neutrons a little heavier protons and this leads to the fact that neutrons appears a little less. So that there are some temporary factors in the head, we are talking about the first fractions of a second of life Universe .

"First three minutes"
Appeared protons And neutrons , seems to be hot and tight. And with proton And neutron thermonuclear reactions can begin, as in the depths of stars. But in fact, it is still too hot and dense. Therefore, you need to wait a little and somewhere from the first seconds of life Universe until the first minutes. There is a famous book by Weinberg called "First three minutes" and it is dedicated to this stage in life Universe .

Origin of the chemical element helium

In the first minutes, thermonuclear reactions begin to occur, because all Universe similar to the interior of a star and thermonuclear reactions can occur. begin to form hydrogen isotopes – deuterium and correspondingly tritium . Heavier ones begin to form chemical elements – helium . But it’s difficult to move further, because stable nuclei with the number of particles 5 And 8 No. And it turns out to be such a complicated plug.
Imagine that you have a room strewn with Lego pieces and you need to run around and assemble structures. But the details scatter or the room expands, that is, somehow everything moves. It’s difficult for you to collect parts, and in addition, for example, you put two together, then you put two more together. But it’s impossible to stick the fifth one in. And therefore, in these first minutes of life Universe , basically, only manages to form helium , a little lithium , a little deuterium remains. It simply burns up in these reactions, turns into the same helium .
So basically Universe turns out to consist of hydrogen And helium , after the first minutes of his life. Plus a very small number of slightly heavier elements. And as it were, this is where the initial stage of the formation of the periodic table ended. And there is a pause until the first stars appear. The stars again turn out to be hot and dense. Conditions are being created for continuation thermonuclear fusion . And stars spend most of their lives engaged in synthesis helium from hydrogen . That is, it is still a game with the first two elements. Therefore, due to the existence of stars, hydrogen is getting smaller helium getting bigger. But it is important to understand that for the most part, the substance in Universe is not in the stars. Mostly ordinary matter scattered throughout Universe in clouds of hot gas, in galaxy clusters, in filaments between clusters. And this gas may never turn into stars, that is, in this sense, Universe will still remain mainly consisting of hydrogen And helium . If we are talking about an ordinary substance, but against this background, at the percentage level, the amount of light chemical elements decreases, and the amount of heavy elements increases.

Stellar nucleosynthesis

And so after the initial era nucleosynthesis , the era of stardom is coming nucleosynthesis , which continues to this day. In the star, in the beginning hydrogen turns into helium . If conditions allow, and the conditions are temperature and density, then the following reactions will take place. The further we move along the periodic table, the more difficult it is to start these reactions, the more extreme conditions are needed. Conditions are created in a star by themselves. The star presses on itself, its gravitational energy is balanced with its internal energy associated with gas pressure and study. Accordingly, the heavier the star, the more it compresses itself and receives a higher temperature and density in the center. And there the next ones can go atomic reactions .

Chemical evolution of stars and galaxies

In the Sun after synthesis helium , the next reaction will start and will form carbon And oxygen . The reactions will not go further and the Sun will turn into oxygen-carbon white dwarf . But at the same time, the outer layers of the Sun, already enriched by the fusion reaction, will be thrown off. The sun will turn into a planetary nebula, the outer layers will fly apart. And for the most part, the ejected matter, after it mixes with the matter of the interstellar medium, can become part of the next generation of stars. So stars have this kind of evolution. There is chemical evolution galaxies , each subsequent star that forms, on average, contains more and more heavy elements. Therefore, the very first stars that formed from pure hydrogen And helium , they, for example, could not have rocky planets. Because there was nothing to make them from. It was necessary for the evolutionary cycle of the first stars to go through, and what is important here is that massive stars evolve the fastest.

The origin of heavy chemical elements in the Universe

Origin of the chemical element iron

The sun and its total lifetime is almost 12 billion years. And massive stars live several times millions years. They bring reactions to gland , and at the end of their life they explode. During an explosion, except for the innermost core, all matter is thrown out and therefore a large amount is thrown out, naturally, and hydrogen , which remained unprocessed in the outer layers. But it is important that a large amount is thrown away oxygen , silicon , magnesium , that is already enough heavy chemical elements , slightly short of reaching gland and those related to him, nickel And cobalt . Very highlighted elements. Maybe I remember this picture from my school days: number chemical element and the release of energy during fusion or decomposition reactions, and there such a maximum is obtained. AND iron, nickel, cobalt are at the very top. This means that the decay heavy chemical elements profitable up to gland , synthesis from the lungs is also beneficial to iron. Further energy needs to be spent. Accordingly, we move from the side of hydrogen, from the side of light elements, and the thermonuclear fusion reaction in stars can reach iron. They must come with the release of energy.
When a massive star explodes, iron , basically, is not thrown away. It remains in the central core and turns into neutron star or black hole . But they are thrown away chemical elements heavier than iron . Iron is released in other explosions. White dwarfs can explode, what remains, for example, from the Sun. The white dwarf itself is a very stable object. But it has a limiting mass when it loses this stability. The thermonuclear combustion reaction begins carbon .

Supernova explosion
And if it is an ordinary star, it is a very stable object. You heated it a little in the center, it will react to it, it will expand. The temperature in the center will drop, and everything will regulate itself. No matter how much it is heated or cooled. And here white dwarf can't do that. You triggered the reaction, it wants to expand, but cannot. Therefore, the thermonuclear reaction quickly covers the entire white dwarf and it completely explodes. It turns out Type 1A Supernova explosion and this is a very good very important Supernova. They allowed it to open. But the most important thing is that during this explosion the dwarf is completely destroyed and a lot is synthesized there gland . All glands oh around, all the nails, nuts, axes and all the iron are inside us, you can prick your finger and look at it or taste it. So that's all iron came from white dwarfs.

Origin of heavy chemical elements

But there are even heavier elements. Where are they synthesized? For a long time it was believed that the main site of synthesis is more heavy elements , This Supernova explosions associated with massive stars. During an explosion, that is, when there is a lot of extra energy, when all sorts of extra things fly neutrons , it is possible to carry out reactions that are energetically unfavorable. It’s just that the conditions have developed this way and in this scattering substance, reactions can take place that synthesize enough heavy chemical elements . And they really are coming. Many chemical elements , heavier than iron, are formed in exactly this way.
In addition, even stars that do not explode, at a certain stage of their evolution, when they turned into red giants can synthesize heavy elements . Thermonuclear reactions take place in them, as a result of which a few free neutrons are formed. Neutron , in this sense, is a very good particle, since it has no charge, it can easily penetrate the atomic nucleus. And having penetrated the nucleus, the neutron can then turn into proton . And accordingly the element will jump to the next cell in periodic table . This process is quite slow. It is called s-process , from the word slow. But it is quite effective and many chemical elements are synthesized in red giants in this way. And in Supernovas it goes r-process , that is, fast. By the way, everything really happens in a very short time.
Recently it turned out that there is another good place for the r-process, unrelated to supernova explosion . There is another very interesting phenomenon - the merger of two neutron stars. Stars love to be born in pairs, and massive stars are mostly born in pairs. 80-90% massive stars are born in binary systems. As a result of evolution, doubles can be destroyed, but some reach the end. And if we had in our system 2 massive stars, we can get a system of two neutron stars. After this, they will approach each other due to the emission of gravitational waves and eventually merge.
Imagine you take an object of size 20 km with a mass of one and a half solar masses, and almost with speed of light , drop it onto another similar object. Even according to a simple formula, kinetic energy equals (mv 2)/2 . If as m let's say you substitute 2 mass of the Sun, as v put a third speed of light , you can count and get absolutely fantastic energy . It will also be released in the form of gravitational waves, apparently in the installation LIGO They are already seeing such events, but we don’t know about it yet. But at the same time, since real objects collide, an explosion actually occurs. A lot of energy is released in gamma range , V x-ray range. In general, in all ranges and part of this energy goes to synthesis of chemical elements .

Origin of the chemical element gold

Origin of the chemical element gold
And modern calculations, they are finally confirmed by observations, show that, for example, gold is born precisely in such reactions. Such an exotic process as the merger of two neutron stars is truly exotic. Even in a system as large as ours Galaxy , happens about once every 20-30 thousand years. It seems quite rare, however, it is enough to synthesize something. Well, or vice versa, we can say that it happens so rarely, and therefore gold so rare and expensive. And in general it is clear that many chemical elements turn out to be quite rare, although they are often more important to us. There are all sorts of rare earth metals that are used in your smartphones, and modern people would rather live without gold than without a smartphone. All these elements are not enough, because they are born in some rare astrophysical processes. And for the most part, all these processes, one way or another, are associated with stars, with their more or less quiet evolution, but with later stages, explosions of massive stars, with explosions white dwarfs or conditions neutron stars .