We are used to living in a changing world. Cell phone models, governments, climate change. Even the Universe is constantly expanding. However, both new gadgets and prime ministers are made of the same elements, which we remember from the table on the wall of the chemistry office, but we rarely think about how they came to be. In the early stages of evolution, the Universe did not have most of those elements that make up you and me, and in the very first moments of its existence - none of them.
Our universe was born very hot and immediately began to expand and cool. High density and temperature make it impossible for any complex formations to exist. Therefore, in a very young Universe there are not only atoms familiar to us, not only their nuclei, but even the simplest nucleus, hydrogen, that is, a single proton, cannot exist for a long time. The substance of the Universe is a boiling "soup" of elementary particles and quanta of radiation, which continuously transform into each other according to the famous formula of the theory of relativity E \u003d mc 2.
For a proton to feel "calm", the Universe must cool down to a temperature when the particle energy becomes less than the proton's mass. Only from this moment it makes sense to talk about the "chemical composition", and at first it is more than simple: it is pure hydrogen. In addition to protons, electrons and neutrons are also present in dense matter, the content is determined by the equilibrium conditions: when protons and electrons collide, neutrons are born, which then spontaneously decay into protons and electrons, the collision of a neutron and a positron (electron antiparticle) gives a proton. Also in these reactions neutrinos are emitted, but they are not important for us now.
Then an episode begins in the history of the Universe in which conditions resemble the current state of matter in the interiors of stars and hydrogen can be converted into heavier elements. Primary nucleosynthesis begins - the formation of heavy elements from lighter ones. But this does not last long - only a few minutes. The density and temperature of a substance rapidly decrease, which leads to a sharp slowdown in nuclear reactions. Therefore, only helium and an insignificant amount of deuterium, lithium and beryllium have time to appear.It all starts with the simplest reaction: a proton combines with a neutron to form the nucleus of deuterium - heavy hydrogen. Having received deuterium, nature continues to play construction sets as long as density and temperature permit. If deuterium interacts with a proton, you get helium-3 - a light helium isotope containing two protons and one neutron, and if with a neutron - tritium, a superheavy isotope of hydrogen (one proton, two neutrons). As you can see, particles always enter into nuclear reactions in pairs. The thing is that processes that require the simultaneous interaction of several particles are extremely unlikely, just as it is unlikely to accidentally meet two former classmates in the metro, who, without saying a word, ended up in one place. It is easy to guess that at the next stage, helium-3 adds another neutron (or tritium - a proton), and the nucleus of helium-4 is formed, consisting of two protons and two neutrons, one of the most stable in the Universe.
This nucleus is emitted in many reactions and even received a special name from physicists - an alpha particle. In many cases, the helium nucleus is considered as a particle, forgetting for a while about the complex internal structure. It would seem that helium-4 can continue to add protons and neutrons, but that was not the case! Two serious obstacles stand in the way of further complication: in nature there are no stable nuclei with masses of 5 and 8 units, that is, consisting of five and eight nucleons (protons and neutrons). In any combination of five protons and neutrons, one of the particles turns out to be superfluous and is thrown out of the nucleus, which stubbornly wants to remain an alpha particle. And even if you try to combine six nucleons at once according to one of the schemes "helium-3 + tritium", "helium-3 + helium-3", "helium-4 + deuterium", all the same, as a rule, helium-4 is formed, and an extra pair of nucleons is rejected.
You can jump over this barrier only if helium-4 merges with the nucleus of tritium and helium-3. Then lithium-7 or beryllium-7 are born, respectively. But these reactions are reluctant, since the electric charge of helium nuclei is twice that of hydrogen. Equally charged particles are repelled, and in order to merge them with each other, a higher collision energy is needed, that is, a higher temperature. Meanwhile, the rapid expansion in the first minutes after the Big Bang is accompanied by a drop in temperature and density of matter - the Universe ceases to be a "star to itself". As a result, very little lithium and beryllium are formed. The fusion process does not go further - there are practically no hunters to "storm" the second barrier (instability of a nucleus of 8 nucleons). And without this, you cannot get to carbon - the most important for the existence of the life of the atom.
The Universe has only a few minutes to play the constructor of protons and neutrons. When the game is over, three quarters of the mass falls on ordinary hydrogen, and a quarter on helium-4 (therefore astronomers call all other elements heavy, or even “metals”). There is still a very small amount of deuterium, helium-3 and lithium (tritium and beryllium-7 are unstable and soon decay). By determining their content, one can obtain very important information about the first minutes of the life of the Universe, but no alchemist will make not only a homunculus from such materials, but also a stone (not even philosophical, but the most ordinary one). But we do exist! And the Earth is there. This means that there must be some kind of crucible in nature, in which carbon, and oxygen, and silicon are formed. You just need to wait a little - some tens of millions of years ...
Star crucible
After long "dark ages" the first stars are lit in the Universe. In their depths, at a temperature of about 10 million degrees and a density several times higher than that of the densest metal on Earth, the conditions for playing the alchemical constructor again arise - stellar nucleosynthesis begins. At first, this game is very similar to the game that was played immediately after the birth of the universe. And yet there are some differences. At first, there are almost no free neutrons in stellar matter (in a free state, they live only about 15 minutes), and therefore deuterium is formed when two protons collide. One of them, in the process of fusion, turns into a neutron, emitting a positron - the positively charged antiparticle of an electron - to get rid of the excess charge. In the absence of neutrons, tritium is not formed from deuterium. Deuterium quickly combines with another proton and turns into helium-3. A direct transition from it to helium-4 by capturing a neutron, as in the early Universe, is impossible, but there are a number of workarounds.
Two helium-3 nuclei can collide to form an extremely unstable beryllium-6 nucleus (4 protons + 2 neutrons), which instantly breaks down into helium-4 and a pair of protons. Another option is more complicated: in the reactions of helium-3 and helium-4, beryllium and lithium nuclei with an atomic weight of 7 are born. However, adding one more proton, they become unstable (remember - all nuclei of 8 nucleons are extremely unstable) and immediately fall apart into two nuclei helium-4. In general, all roads lead to Rome.The result of any of these processes is the transformation of four protons into one helium-4 nucleus. It is important that the mass of the helium-4 nucleus is slightly (by about 0.7%) less than the mass of four protons. Where does the excess mass disappear? In accordance with the same formula E \u003d mc 2, it turns into energy. It is due to this, as physicists say, the mass defect and stars shine. And, importantly, a stellar thermonuclear reactor is able to regulate itself: if too much energy is released, the star expands a little, matter cools and the reaction rate, which is very dependent on temperature, decreases. If there is little energy, then the opposite process takes place. As a result, the star stably maintains the temperature at a level corresponding to a fairly low rate of reactions. Therefore, the stars (at least some of them) live long enough to have enough time for biological evolution and the emergence of such highly organized creatures as you and me.
Eventually, the star's hydrogen reserves are depleted. We must move on, and we remember that this is not easy, because there are no stable nuclei with masses of 5 and 8. But nature finds a way out. Remembering the meeting of classmates in the subway, we can say that although it is extremely unlikely that three of them will accidentally collide at once, but if two met and ride together for some time, then the chances that a third will be added on the way to them increase. Something similar happens with the nuclear combustion of helium. In the beginning, two alpha particles merge to form an unstable beryllium-8 nucleus. Its life is extremely short, 3.10 -16 s (this is less than one millionth of one billionth of a second), but with a sufficiently high density and temperature, even this tiny interval is enough for sometimes another alpha particle to react with beryllium. And voila! - carbon-12 in person!
Then already carbon can capture alpha particles, giving oxygen. Thus, the two basic elements necessary for life to emerge are born in the stars. The transformation of carbon into oxygen is so efficient that the latter in the Universe is even slightly more carbon. If the parameters of nuclear particles were slightly different, then almost all of the carbon would "burn out" into oxygen, which would make life in the form that we know extremely rare or even impossible. Perhaps, in some other universes, particles are arranged somewhat differently and there is little carbon there, but then there are no observers there (at least, those like us).
Nuclei, elements and isotopes
Protons and neutrons (collectively they are called nucleons) are not, in the strict sense of the word, elementary particles. They consist of three quarks, tightly bound by strong nuclear forces. It is impossible to break a nucleon into separate quarks: the energy required for this is sufficient for the production of new quarks, which, having combined with the fragments of the original nucleon, again form composite particles. The strong interaction is not completely closed inside the nucleons, but also acts at a small distance from them. If two nucleons, say a proton and a neutron, come close to each other, nuclear forces will bind them together and a composite atomic nucleus will appear - in this case, deuterium (heavy hydrogen). By combining together different numbers of protons and neutrons, you can get all the variety of nuclei, but not all of them will be stable. A nucleus in which there are too many protons or neutrons falls apart before it even has time to form properly. Physicists know more than three thousand combinations of protons and neutrons that can hold out together for at least some time. There are nuclei that live only for a short fraction of a second, others for tens of years, and there are those that can wait billions of years in the wings. And only a few hundred nuclei are considered stable - their decay has never been observed. Chemists are usually not as meticulous as physicists, and they do not distinguish between any two nuclei, but only different elements, that is, nuclei with different numbers of protons. Actually, chemists generally do not look into the nucleus, but only study the behavior of electrons surrounding it in a calm environment. Their number is exactly equal to the number of protons, which makes the atoms electrically neutral. A total of 118 elements are known today, but only 92 of them have been discovered in the natural environment, the rest are obtained artificially in nuclear reactors and accelerators. Most of the elements are represented by nuclei with different numbers of neutrons. These variations are called isotopes. For some elements, up to forty isotopes are known; when mentioned, they are distinguished by indicating the number of nucleons in the nucleus. For example, uranium-235 and uranium-238 are two isotopes of the 92nd element uranium with 143 and 146 neutrons, respectively. Most of the isotopes of each element (and some of them all) are unstable and subject to radioactive decay. This makes isotopic composition an important source of information about the history of matter. For example, the age of organic remains, rocks, meteorites and even some stars is determined by the ratio of radioactive isotopes and their decay products. However, the ratio of stable isotopes can also tell a lot. For example, the Earth's climate in the distant past is determined by the isotopes of oxygen-16 and -18 in Antarctic ice deposits: water molecules with a heavy isotope of oxygen evaporate less readily from the ocean surface, and there are more of them in warm climates. For any such isotopic studies, it is essential that the studied sample does not exchange matter with the environment from the moment of its appearance.
Adult Games
Single stars are two times lighter than our Sun, they stop at the stage of helium synthesis. Heavier stars produce carbon and oxygen, and only the largest stars, in excess of 10 solar masses, can continue playing with elements at the end of their life. After depletion of helium reserves, their inner regions shrink, heat up, and carbon begins to "burn" in them. Two carbon nuclei combine to form neon and an alpha particle. Or sodium and proton. Or magnesium and neutron. The appeared protons and neutrons also do not go to waste. They go into action, converting carbon into nitrogen, oxygen and, further, by capturing alpha particles into neon, silicon, magnesium and aluminum. Thus, we already have something to make of the earthly firmament afterwards.
After carbon, neon starts to "burn" out of turn, and it does it in the "wrong" way: instead of immediately merging with some other nucleus and increasing its mass, neon nuclei under the action of especially energetic gamma quanta decay into oxygen and alpha -particle. And then the resulting alpha particles, interacting with other neon nuclei, give magnesium. So, as a result, one oxygen and one magnesium appear on two neon nuclei.
After depletion of reserves of neon, the core of the star becomes oxygen-magnesium, it is compressed again, the temperature rises and the game continues. Now the oxygen nuclei, merging in pairs, turn into silicon or sulfur. In addition, some argon, calcium, chlorine and other elements appear.
Next in line is silicon. Directly two silicon nuclei cannot merge - because of the large charge, the electrical repulsion between them is too great. Therefore, many different reactions with the participation of alpha particles begin to take place. The term "silicon combustion" is rather arbitrary, since there are indeed many different reaction channels. At this stage, various elements arise, up to iron.
Iron (and nickel close to it) stands out from all elements in that it has the maximum binding energy. Nucleons cannot be packed more efficiently: both breaking the iron nucleus into pieces and making heavier nuclei out of it require energy. Therefore, at first it was not clear how the formation of elements in stars could go beyond iron, and the existence of heavy nuclei in the Universe, such as in gold or uranium, remained completely inexplicable. An approach to the explanation was found in the mid-1950s, when two mechanisms were proposed at once for the formation of elements heavier than iron in stars. Both are based on the ability of nuclei to capture neutrons.
Great slow kings
The first of these mechanisms is called slow neutron capture, or s-process (from the English slow - "slow"). It occurs at the end of the life of stars with a mass of 1 to 3 solar masses, when they reach the stage of a red giant. Moreover, this process takes place not in the dense hot core of the star, but in the layers lying above. In such relatively light stars, the giant stage has a long duration, measured in tens of millions of years, and this is enough for a significant transformation of matter.
The slowness of the s-process, reflected in the name, is due to the fact that it proceeds for a long time at a low neutron concentration. However, even a small amount of neutrons must be taken from somewhere - there can be no supply of these particles. In giant stars, several types of reactions take place in which neutrons are released. For example, carbon-13, capturing an alpha particle, turns into oxygen-16, and a neutron is emitted. Free neutrons, since they are not interfered with by the Coulomb repulsion, easily penetrate into the nuclei of atoms and increase their mass. True, if there are too many neutrons, the nucleus will lose stability and fall apart. But since there are few free neutrons in red giants, the nucleus has time to assimilate the alien relatively painlessly, emitting an electron if necessary. In this case, one of the neutrons in the nucleus becomes a proton, and the charge of the nucleus increases by one, which corresponds to the transformation of one element into another - the next one in the periodic table. In this way, very heavy elements such as lead and barium can be obtained. Or technetium. At one time, the discovery of this heavy and fairly rapidly decaying element in the atmospheres of red giants was even interpreted by some scientists as evidence in favor of the existence of extraterrestrial civilizations! In fact, it is simply carried out from the depths to the surface due to the mixing of the substance.
When the life of such a red giant comes to an end, its core turns into a dense white dwarf, and the shell is scattered in the surrounding space due to the stellar wind or the formation of a planetary nebula. Thus, the interstellar medium is replenished with the heavy elements accumulated during the lifetime of the star, and gradually the chemical composition of the Galaxy evolves due to stellar nucleosynthesis. By the time the solar system was formed, this process had been going on for 8 billion years, and about 1% of interstellar matter had time to turn into heavy elements, of which, in particular, our planet is composed.
Catalysts for stellar life
In massive stars, the conversion of hydrogen into helium proceeds differently than in dwarf stars like the Sun. At a temperature of about 20 million degrees, the so-called carbon-nitrogen-oxygen (CNO) cycle works. Carbon plays the role of a nuclear catalyst in it, and itself is not consumed in reactions. For the reactions to be effective, very little of it is needed, but nevertheless the CNO cycle is possible only in stars of modern chemical composition, whose matter has already been enriched with carbon during the life of previous generations of stars. Carbon-12 captures a proton and turns into nitrogen-13, and that, emitting a positron, into carbon-13. Further, capturing two protons in a row, it becomes first nitrogen-14 and then oxygen-15. He again ejects a positron and turns into nitrogen-15, which, colliding with the fourth proton, decays into an alpha particle (that is, a helium nucleus) and carbon-12. As a result, we return to the original carbon nucleus, but along the way we convert 4 protons into a helium nucleus. True, occasionally (in one of 880 cases) at the last stage of the cycle, nitrogen-15 can merge with a proton into a stable oxygen-16 nucleus. This leads to a slow consumption of the catalyst-carbon.
Order of the Phoenix
Almost all the atoms of your body have been in the bowels of the stars at one time. Many of them have experienced catastrophic supernova explosions, and, moreover, some were formed precisely at the time of such explosions. We, like a phoenix, were born from the ashes, but from the ashes of the stars. Supernova explosions are very important already because they are an effective way to eject elements produced in a star into space. If the result of the explosion, as is most often the case, becomes a neutron star, only the relatively small core of the red giant, consisting mainly of iron and nickel, turns into it. For example, with an initial mass of a star of 20 solar, no more than 7% of the matter will turn into a neutron star, everything else is swept out by an explosion into space and is available for the formation of new stars.
However, the role of supernovae is not exhausted by the maintenance of this cosmic circulation of matter. New elements can form in them right during the explosion. For about 10 seconds, a newborn neutron star has time to be an "alchemist". Before the explosion, the structure of the massive star is like an onion. The core is surrounded by several shells of increasingly lighter elements. At the very moment when the core begins to catastrophically shrink, turning into a neutron star or a black hole, a wave of explosive nuclear combustion runs through the layers above from the center outward. As a result, the chemical composition of the substance is strongly shifted towards heavy elements.
It is believed that stars with masses of 12 to 25 solar masses are most efficiently enriched with heavy elements. Their iron core is surrounded by a powerful silicon-oxygen shell, which, after discharge, gives elements from sodium to germanium (including iron). In more massive stars, too much matter, composed of heavy elements, falls into the black hole, and only light enough escapes outward. Smaller stars, with masses of 8-12 solar masses, do not have such a shell, and therefore little iron group elements are formed in them. But ... much heavier elements appear.
Fresh neutrino wind
When the monstrous forces of gravity squeeze the star's core, tired of resisting, the nuclei of the atoms literally press against each other. The electrons rushing between them, being trapped, are pressed into the nuclei and merge with the protons, turning them into neutrons. At the same time, neutrinos are released - subtle particles that usually easily penetrate the entire thickness of the star and go into space. However, at the moment of formation of a neutron star, there are so many of them that they can no longer be neglected.
The so-called neutrino wind appears. Just as the pressure of light in massive stars leads to the outflow of matter in the form of a stellar wind, neutrinos carry protons and neutrons away. Even if initially there weren't too many neutrons, they appear as a result of reactions between protons and neutrinos. An excess of neutrons is formed in the substance, which can penetrate into nuclei, forming heavier and heavier isotopes. Due to the huge flux of neutrons, the nuclei literally overflow with them, which is why they become extremely unstable and begin to get rid of excess neutronization very quickly - the neutrons in them turn into protons. But as soon as this happens, as new waves of neutrons again bring the nuclei "to the limit."
This whole orgy, lasting only a few seconds, was called the r-process (from the English rapid - "fast"). It results in the nuclei of all masses up to the heaviest. For example, to identify the consequences of the r-process, traces of such a rare element as europium are often looked for, since it is most likely generated only by this mechanism. In the r-process, for example, platinum and actinides are formed - heavy radioactive elements, which include, in particular, uranium. The relative abundances of the isotopes of the latter, as well as of thorium, are often used to estimate the age of stars.Also, in the wind of a newborn neutron star, reactions can take place with the participation of charged particles - protons and helium nuclei - carried away by the flow of neutrinos. This is how zirconium, silver, iodine, molybdenum, palladium and many other elements are formed. The theory of all these processes is very complex, since it is necessary to simultaneously take into account many effects, among which all are not completely clear. Moreover, we are talking here not only about astrophysical effects, but also about uncertainties within the framework of nuclear physics - far from all the parameters of reactions occurring at this stage are precisely defined.
Scientists also continue to debate whether this scenario can claim to be complete: whether it is able to explain the production of heavy elements in the proportions we observe. Therefore, research in this area is in full swing, and, perhaps, we still have interesting discoveries. For example, scenarios are discussed in which matter trapped in the superstrong magnetic fields of newborn magnetars (magnetized neutron stars) allows the production of heavy elements in the r-process. Testing such ideas requires sophisticated 3D supercomputer calculations that have yet to be done.
Homunculus
And finally, after billions of years in the giant retort of the Universe, conditions have developed for a homunculus to appear. Life as we know it could not have arisen within the first billion years after the Big Bang - then there simply were not enough many of the necessary elements.
Every particle of our body went through the cosmic crucible. Some of the hydrogen atoms could have remained unchanged since the "first three minutes", but the bulk of its constituent elements appeared in stars at the stage of stable thermonuclear combustion. Many nuclei were formed during supernova explosions. Others were thrown out by stars in the form of a lace of planetary nebulae. Perhaps a tiny fraction of the nuclei is associated with collisions of cosmic rays with the matter of interstellar gas, when the most interesting "chipping reactions" take place, in which a fast particle knocks out the nuclei of light elements. A whole "laboratory" of the cosmic alchemist was needed for the appearance of man.
The composition of matter in the Universe continues to slowly change even today: through the efforts of trillions of stars, the proportion of elements heavier than helium is gradually increasing. Observations show that stars with a higher metallicity, that is, the content of elements heavier than helium, has a higher probability of planetary systems. This means that the chemical evolution of the Universe so far favors the emergence of intelligent beings made of "stellar matter". And yet it is worth remembering that only a small part of the matter in the Universe undergoes such processing. In general, hydrogen will remain its most abundant element, simply because not all matter will be able to get into stars (for example, intergalactic gas does not have such a prospect). If you remember that this substance also makes up five percent of the strength against the background of a colossal mass of dark matter and dark energy, then you will feel how incredibly lucky that lump of substance was in this, which could look around and appreciate the greatness of the surrounding universe.
Scientists managed to obtain and register a lithium-helium molecule LiHe. It is one of the most fragile molecules known. And its size is more than ten times the size of water molecules.
As you know, neutral atoms and molecules can form more or less stable bonds with each other in three ways. First, through covalent bonds, where two atoms share one or more common electron pairs. Covalent bonds are the strongest of the three. The characteristic energy of their rupture is usually equal to several electron-volts.
Significantly weaker than covalent hydrogen bonds. This is the attraction that occurs between a bonded hydrogen atom and an electronegative atom of another molecule (usually this atom is oxygen or nitrogen, less often fluorine). Despite the fact that the energy of hydrogen bonds is hundreds of times less than that of covalent bonds, it is they that largely determine the physical properties of water, and also play an important role in the organic world.
Finally, the weakest is the so-called van der Waals interaction. It is sometimes also called dispersed. It arises as a result of a dipole-dipole interaction of two atoms or molecules. In this case, dipoles can be both initially inherent in molecules (for example, water has a dipole moment), or induced as a result of interaction.
The characteristic energy of the van der Waals bond is in units of kelvin (the electron volt mentioned above corresponds to approximately 10,000 kelvin). The weakest of the van der Waals ones is the relationship between the two induced dipoles. If there are two non-polar atoms, then as a result of thermal motion, each of them has a certain randomly oscillating dipole moment (the electron shell seems to tremble slightly relative to the nucleus). These moments interacting with each other, as a result, predominantly have such orientations that two atoms begin to attract.
The most inert of all atoms is helium. It does not enter into covalent bonds with any other atom. Moreover, the value of its polarizability is very small, that is, it is difficult for it to form dispersed bonds. There is, however, one important circumstance. The electrons in the helium atom are so strongly bound by the nucleus that, without fear of the appearance of repulsive forces, it can be brought very close to other atoms - up to a distance of the order of the radius of this atom. Disperse forces grow very rapidly with decreasing distance between atoms - inversely proportional to the sixth power of the distance!
Hence the idea was born: if you bring two helium atoms closer to each other, then a fragile van der Waals bond will still arise between them. This, indeed, was realized in the mid-1990s, although it required significant efforts. The energy of such a bond is only 1 mK, and the He2 molecule was detected in small quantities in supercooled jets of helium.
Moreover, the properties of the He2 molecule are in many respects unique and unusual. So, for example, its size is ... about 5 nm! For comparison, the size of a water molecule is about 0.1 nm. In this case, the minimum potential energy of the helium molecule falls on a much smaller distance - about 0.2 nm - however, most of the time - about 80% - the helium atoms in the molecule spend in the tunneling mode, that is, in the region where they are located within the framework of classical mechanics could not.
The next largest atom after helium is lithium, therefore, after obtaining a helium molecule, it became natural to study the possibility of fixing the bond between helium and lithium. And now, finally, scientists managed to do this. The lithium-helium molecule LiHe has a higher binding energy than helium-helium - 34 ± 36 mK, and the distance between atoms, on the contrary, is smaller - about 2.9 nm. However, even in this molecule, the atoms are most of the time in classically forbidden states under the energy barrier. Interestingly, the potential well for the LiHe molecule is so small that it can exist only in one vibrational energy state, which is, indeed, a doublet split due to the spin of the 7Li atom. Its rotation constant is so large (about 40 mK) that excitation of the rotational spectrum leads to the destruction of the molecule.
Brett Esry / Kansas State University
So far, the results obtained are interesting only from a fundamental point of view. However, they are already of interest to related fields of science. Thus, helium clusters of many particles can become a tool for studying the effects of retardation in the Casimir vacuum. The study of the helium-helium interaction is also important for quantum chemistry, which could test its models on this system. And, of course, there is no doubt that scientists will come up with other interesting and important applications for such extravagant objects as He2 and LiHe molecules.
"The two most common elements in the universe are hydrogen and stupidity." - Harlan Ellison. After hydrogen and helium, there are many surprises on the periodic table. Among the most amazing facts is the fact that every material that we have ever touched, which we saw, with which we interacted, consists of the same two things: atomic nuclei, positively charged, and electrons, negatively charged. The way these atoms interact with each other - how they push, bind, attract and repel, creating new stable molecules, ions, electronic energy states - actually determines the picturesqueness of the world around us.
Even if it is the quantum and electromagnetic properties of these atoms and their constituents that allow our Universe, it is important to understand that it did not begin with all these elements at all. Quite the opposite, she started out almost without them.
You see, it takes a lot of atoms to achieve a variety of bond structures and build the complex molecules that underlie everything we know. Not in quantitative terms, but in various terms, that is, so that there are atoms with a different number of protons in their atomic nuclei: this is what makes the elements different.
Our bodies need elements such as carbon, nitrogen, oxygen, phosphorus, calcium, and iron. The crust of our Earth needs elements such as silicon and many other heavy elements, while the core of the Earth - in order to generate heat - needs elements, probably from the entire periodic table, which are found in nature: thorium, radium, uranium and even plutonium.
But back to the early stages of the Universe - before the appearance of man, life, our solar system, to the very first solid planets and even the first stars - when all we had was a hot, ionized sea of \u200b\u200bprotons, neutrons and electrons. There were no elements, no atoms, and no atomic nuclei: the universe was too hot for all this. And only when the Universe expanded and cooled down, at least some kind of stability appeared.
Some time has passed. The first nuclei fused together and never parted, producing hydrogen and its isotopes, helium and its isotopes, and tiny, barely discernible volumes of lithium and beryllium, the latter subsequently decaying radioactively into lithium. From this began the Universe: in terms of the number of nuclei - 92% hydrogen, 8% helium and approximately 0.00000001% lithium. By weight - 75-76% hydrogen, 24-25% helium and 0.00000007% lithium. In the beginning there were two words: hydrogen and helium, this is, one might say, everything.
Hundreds of thousands of years later, the universe cooled down enough for neutral atoms to form, and tens of millions of years later, gravitational collapse allowed the first stars to take place. At the same time, the phenomenon of nuclear fusion not only filled the Universe with light, but also allowed the formation of heavy elements.
By the time the first star was born, somewhere between 50 and 100 million years after the Big Bang, abundant amounts of hydrogen began to coalesce into helium. But more importantly, the most massive stars (8 times more massive than our Sun) burned their fuel very quickly, burning out in just a couple of years. As soon as the cores of such stars ran out of hydrogen, the helium core shrank and began to merge three atomic nuclei into carbon. It took just a trillion of these heavy stars in the early universe (which formed many more stars in the first few hundred million years) for lithium to be defeated.
So you're probably thinking that carbon has become element number three these days? One might think about it, since stars synthesize elements in layers, like an onion. Helium is synthesized into carbon, carbon into oxygen (later and at higher temperatures), oxygen into silicon and sulfur, and silicon into iron. At the end of the chain, iron cannot merge into anything else, so the core explodes and the star goes supernova.
These supernovae, the stages that led to them, and the consequences enriched the Universe with the contents of the outer layers of the star, hydrogen, helium, carbon, oxygen, silicon and all heavy elements that were formed in the course of other processes:
- slow neutron capture (s-process), sequentially arranging the elements;
- fusion of helium nuclei with heavy elements (to form neon, magnesium, argon, calcium, and so on);
- rapid neutron capture (r-process) with the formation of elements up to uranium and beyond.
But we had more than one generation of stars: we had many of them, and the generation that exists today is built primarily not on virgin hydrogen and helium, but also on the remnants of previous generations. This is important, because without it we would never have solid planets, only gas giants of hydrogen and helium, exclusively.
Over billions of years, the process of star formation and death has been repeated, with more and more enriched elements. Instead of simply merging hydrogen into helium, massive stars merge hydrogen in the C-N-O cycle, over time equalizing the volumes of carbon and oxygen (and slightly less nitrogen).
Also, when stars go through helium fusion to form carbon, it is fairly easy to grab an extra helium atom to form oxygen (and even add another helium to oxygen to form neon), and even our Sun will do so during the red giant phase.
But there is one killer step in stellar forges that removes carbon from the cosmic equation: when a star becomes massive enough to initiate carbon fusion - such is the need for a Type II supernova - the process that converts gas to oxygen goes to the brink, creating much more. oxygen than carbon by the time the star is ready to explode.
When we look at supernova remnants and planetary nebulae - remnants of very massive stars and sun-like stars, respectively - we find that oxygen outnumbers carbon both massively and quantitatively in each case. We also found that none of the other elements are heavier or come close.
So, hydrogen # 1, helium # 2 - there are a lot of these elements in the Universe. But of the remaining elements, oxygen holds a confident # 3, followed by carbon # 4, neon # 5, nitrogen # 6, magnesium # 7, silicon # 8, iron # 9 and the medium completes the top ten.
What does the future hold for us?
After a fairly long period of time, which is thousands (or millions) of times the current age of the Universe, stars will continue to form, either spewing fuel into intergalactic space, or burning it whenever possible. In the process, helium can finally bypass hydrogen in abundance, or hydrogen will remain on the first line if it is sufficiently isolated from fusion reactions. Over a long distance, matter that is not ejected from our galaxy can merge over and over again, so that carbon and oxygen will bypass even helium. Perhaps elements # 3 and # 4 will offset the first two.
The universe is changing. Oxygen is the third most abundant element in the modern Universe, and in the very, very distant future, it may rise above hydrogen. Every time you breathe in air and feel the satisfaction of this process, remember: stars are the only reason for oxygen to exist.
You may have heard the phrase "you are made from stardust" - and this is true. Many of the particles that make up your body and the world around you were formed within stars billions of years ago. But there are some materials that were formed at the very beginning, after the birth of the universe.
Some astronomers believe they appeared just minutes after the Big Bang. The most abundant elements in the universe are hydrogen and helium, and very small amounts of a chemical such as lithium.
Astronomers can determine with little accuracy how much lithium was in the young universe. To do this, you need to explore the oldest stars. But the results obtained do not coincide - in old stars there was 3 times less lithium than expected to be found! The reason for this mystery is still unknown.
Let's take a closer look ...
Strictly speaking, given the current level of our observations, there should be no error: there is very little lithium. The situation clearly hints at some kind of new physics, an unknown process that took place immediately after the Big Bang.
The most recent study on this topic touched on the regions least changed after the Big Bang - the atmospheres of old stars located on the periphery of the Milky Way. Since they are isolated from the core, where lithium can be produced, the likelihood of late contamination affecting the results should be extremely low. In their atmospheres, lithium-7 is found only about a third of the level predicted by simulations. Causes? One suggested explanation: he drowned. Lithium from the atmosphere of the stars simply began to sink in the matter of the stars, gradually reaching their depths. Therefore, it is not visible in their atmospheres.
Christopher Hawk from the University of Notre Dame (Indiana, USA) and his colleagues undertook to check the results based on data on the Small Magellanic Cloud, a satellite galaxy of the Milky Way. And to save the data from the effect of "lithium immersion" and other influences of local stellar processes, the researchers analyzed the contents of interstellar gas in this dwarf galaxy, suggesting that he should be proud of his lithium: he simply has nothing to drown in here.
Using observations from the ESO's Very Large Telescope, astronomers found just as much lithium there as the Big Bang model predicted, as reported in the journal Nature. But this, alas, did not help much in resolving the issue. The fact is that lithium is constantly formed in the Universe in the course of natural processes, and supernovae explosions evenly carry it across the Metagalaxy, like all other elements accumulated in the depths. The new results, according to Christopher Hawk, only exacerbated the lithium mystery: "One can only talk about a solution to this problem if there has been no change in the amount of lithium available since the Big Bang." And then only on the scale of the Small Magellanic Cloud!
Most importantly, it is very difficult to imagine that for 12-13 billion years of thermonuclear fusion, which created the very heavy elements that make life on Earth possible, lithium for some reason was not produced. At least, our current understanding of thermonuclear nucleosynthesis does not allow us to put forward such a hypothesis.
Even worse, new work by Miguel Pato of the Technical University of Munich (Germany) and Fabio Iocco of the Stockholm University (Sweden) showed that not only supermassive black holes in galactic nuclei, but also the most common (and more numerous) BH of stellar origin should generate lithium in their accretion disks, and very intensively.
Now it turns out that practically every microquasar (simply the BH system - accretion disk) must create lithium. But theoretically there should be much more of them than SMChD, notes Miguel Pato.
In a word, there is no clarity on this issue yet. Christopher Hawk, for example, suggests that immediately after the Big Bang, some exotic from a physical point of view reactions could have taken place in the Universe, in which dark matter particles participated, and they suppressed the formation of lithium. This could explain the fact that there was more lithium in the Small Magellanic Cloud than in our Galaxy: dwarf galaxies, to which the MMO belongs, should have been less active in attracting dark matter in the early Universe. This means that these hypothetical reactions had less effect on the concentration of lithium in them. Mr. Hawk intends to test this idea with a deeper study of the Small Magellanic Cloud ...
Until now, we could only search for lithium in the stars closest to us in our Galaxy. And now a group of astronomers was able to determine the level of lithium in a star cluster outside our Galaxy.
The Messier 54 star cluster has a secret - it does not belong to the Milky Way, and is part of a satellite galaxy - a dwarf elliptical galaxy in Sagittarius. This arrangement of the cluster allowed scientists to check whether the lithium content in stars outside the Milky Way is just as low.
In the vicinity of the Milky Way there are more than 150 globular star clusters, which are composed of hundreds of thousands of ancient stars. One of these clusters, along with others in the constellation Sagittarius, was discovered at the end of the 18th century by the French scientist “comet hunter” Charles Messier, and bears his name Messier 54.
For more than two centuries, scientists mistakenly believed that M54 was the same cluster as everyone else in the Milky Way, but in 1994 it was discovered that this star cluster belongs to another galaxy - the elliptical dwarf galaxy in Sagittarius. The object was also found to be 90,000 light-years from Earth, more than three times the distance between the Sun and the center of the galaxy.
Astronomers are currently observing M54 with the VLT Survey telescope, trying to solve one of the most mysterious questions of modern astronomy, concerning the presence of lithium in stars.
In this image you can see not only the cluster itself, but also a very dense foreground, consisting of the stars of the Milky Way. Photo by ESO.
Previously, astronomers were able to determine the lithium content only in the stars of the Milky Way. However, a research team led by Alessio Mucciarelli of the University of Bologna has now used the VLT Survey to measure the lithium abundance in the extragalactic star cluster M54. The study showed that the amount of lithium in old M54 stars does not differ from the stars in the Milky Way. Therefore, wherever lithium disappears, the Milky Way has nothing to do with it.
metallic lithium
Lithium is the lightest metal, 5 times lighter than aluminum. Lithium got its name from the fact that it was found in "stones" (Greek λίθος - stone). The name was suggested by Berzelius. It is one of three elements (in addition to hydrogen and helium) that formed during the era of primordial nucleosynthesis after the Big Bang, even before the birth of stars. Since then, its concentration in the Universe has remained practically unchanged.
Lithium can rightfully be called the most important element of modern civilization and technological development. In the past and the century before last, indicators of the production of the most important acids and metals, water and energy carriers were the criteria for the development of the industrial and economic power of states. In the 21st century, lithium has entered the list of such indicators for a long time. Today, lithium is of exceptional economic and strategic importance in the developed industrial countries.
By studying the new star Nova Delphini 2013 (V339 Del), astronomers were able to detect the chemical precursor of lithium, thus making the first direct observations of the formation of the third element in the periodic table - which were previously assumed only theoretically.
"Until now, scientists have not had direct confirmation of observations of the formation of lithium in new stars, but after conducting our research, we can say that such processes are taking place," - said the main author of the new scientific paper Akito Taitsu from the National Observatory of Japan.
Explosions of new stars occur when, in a close binary star system, matter flows from one of its constituent stars to the surface of a companion star - a white dwarf. An uncontrolled thermonuclear reaction causes a sharp surge in the star's luminosity, which, in turn, leads to the formation of elements heavier than hydrogen and helium, which are present in significant quantities inside most stars in the Universe.
One of the chemical elements formed as a result of such an explosion is the widespread isotope of lithium, Li-7. While most of the heavy elements form in stellar cores and in supernova explosions, Li-7 is too fragile to withstand the high temperatures found in most stellar cores.
Some of the lithium present in the universe was formed by the Big Bang. In addition, some amounts of lithium could have been formed as a result of the interaction of cosmic rays with stars and interstellar matter. However, these processes do not explain the too large amounts of lithium present in the universe today.
In the 1950s. Scientists have suggested that lithium in the Universe can be formed from the isotope of beryllium Be-7, which forms near the surface of stars and can be transported into space, where the effect of high temperatures on the material is reduced, and the newly formed lithium remains in a stable state. Until now, however, observations from Earth of lithium formed near the surface of a star have been a rather difficult task.
Taitsu and his team used the Subaru telescope in Hawaii for their observations. During the observation period, the team clearly recorded how the Be-7 nuclide, which has a half-life of 53 days, was converted to Li-7.
MOSCOW, 6 Feb - RIA Novosti. Russian and foreign chemists declare the possibility of the existence of two stable compounds of the most "xenophobic" element - helium, and experimentally confirmed the existence of one of them - sodium helide, according to an article published in the journal Nature Chemistry.
"This study demonstrates how completely unexpected phenomena can be detected using the most modern theoretical and experimental methods. Our work once again illustrates how little we know today about the effect of extreme conditions on chemistry, and the role of such phenomena on processes inside planets. still to be explained, "says Artem Oganov, professor at Skoltech and Moscow Phystech in Dolgoprudny.
Secrets of noble gases
The primary matter of the Universe, which arose several hundred million years after the Big Bang, consisted of only three elements - hydrogen, helium and trace amounts of lithium. Helium is still the third most widespread element of the universe, but it is very scarce on Earth, and the reserves of helium on the planet are constantly decreasing due to the fact that it escapes into space.
A distinctive feature of helium and other elements of the eighth group of the periodic table, which scientists call "noble gases", is that they are extremely reluctant - in the case of xenon and other heavy elements - or, in principle, like neon, are not able to enter into chemical reactions. There are only a few dozen compounds of xenon and krypton with fluorine, oxygen and other strong oxidants, zero compounds of neon and one compound of helium, discovered experimentally in 1925.
This compound, the union of a proton and helium, is not a real chemical compound in the strict sense of the word - in this case, helium does not participate in the formation of chemical bonds, although it does affect the behavior of hydrogen atoms devoid of an electron. As chemists previously suggested, the "molecules" of this substance should have met in the interstellar medium, but over the past 90 years, astronomers have not found them. A possible reason for this is that this ion is extremely unstable and breaks down on contact with almost any other molecule.
Artem Oganov and his team wondered if helium compounds could exist under exotic conditions, which terrestrial chemists rarely think about - at ultrahigh pressures and temperatures. Oganov and his colleagues have been studying such "exotic" chemistry for a long time and even developed a special algorithm to search for substances that exist under such conditions. With his help, they discovered that in the bowels of the gas giants and some other planets there can be exotic orthocarbonic acid, "impossible" versions of common table salt, and a number of other compounds that "violate" the laws of classical chemistry.
Using the same system, USPEX, Russian and foreign scientists discovered that at ultrahigh pressures exceeding atmospheric pressures by 150 thousand and a million times, there are two stable helium compounds at once - sodium gelide and sodium oxygelide. The first compound consists of two sodium atoms and one helium atom, and the second is made up of oxygen, helium, and two sodium atoms.
Ultra-high pressure made salt "break" the rules of chemistryAmerican-Russian and European chemists have turned common table salt into a chemically "impossible" compound, the molecules of which are organized into exotic structures of different numbers of sodium and chlorine atoms.Atom on a diamond anvil
Both pressures can be easily obtained using modern diamond anvils, which Oganov's colleagues did under the leadership of another Russian - Alexander Goncharov from the Geophysical Laboratory in Washington. As shown by his experiments, sodium gel forms at a pressure of about 1.1 million atmospheres and remains stable up to at least 10 million atmospheres.
Interestingly, sodium gel is similar in structure and properties to fluorine salts, the "neighbor" of helium on the periodic table. Each helium atom in this "salt" is surrounded by eight sodium atoms, just like calcium fluoride or any other hydrofluoric acid salt works. The electrons in Na2He are "attracted" to atoms so strongly that this compound, unlike sodium, is an insulator. Scientists call such structures ionic crystals, since electrons take the role and place of negatively charged ions in them.
"The compound we have discovered is very unusual: although helium atoms are not directly involved in chemical bonding, their presence fundamentally changes the chemical interactions between sodium atoms, contributing to the strong localization of valence electrons, which makes the resulting material an insulator," explains Xiao Dong of the university Nankan in Tianjin (China).
Another compound, Na2HeO, turned out to be stable in the pressure range from 0.15 to 1.1 million atmospheres. The substance is also an ionic crystal and has a structure similar to Na2He, only the role of negatively charged ions in them is played not by electrons, but by oxygen atoms.
Interestingly, all other alkali metals, which have a higher reactivity, are much less willing to form compounds with helium at pressures no more than 10 million times higher than atmospheric.
Oganov and his colleagues attribute this to the fact that the orbits along which electrons move in the atoms of potassium, rubidium and cesium change noticeably with increasing pressure, which, for reasons not yet understood, does not occur with sodium. Scientists believe that sodium gel and other similar substances can be found in the cores of some planets, white dwarfs and other stars.