Review article on nucleosynthesis in stars, stellar evolution and supernovae

    Remains of a supernova in the constellation Taurus, which broke out in 1054 AD and was recorded by Chinese astronomers.

    We owe all the variety of chemical elements in nature to stars. Indeed, at the very beginning of the existence of the Universe, primary nuclear synthesis gave the Universe only hydrogen and helium.

    After hundreds of thousands of years, the first stars were lit, inside which the synthesis of nuclei of heavier elements began. After all, what is a star? A star is a balance between the energy released during nucleosynthesis in its core and the gravitational force compressing the star. Ultimately, gravity always wins - it's just a matter of time.

    How does interstellar alchemy work?

    The primary resource for fusion is hydrogen nuclei, of which more than 90% are stars. As a result of the thermonuclear fusion reaction of four protons, the helium nucleus is ultimately formed, with the release of a number of various elementary particles. In the final state, the total mass of the formed particles is less than the mass of the four initial protons, which means that free energy is released during the reaction. Because of this, the inner core of the newborn star quickly warms up to ultra-high temperatures, and its excess energy begins to splash out towards its less hot surface. At the same time, the pressure in the center of the star is also increasing (the Mendeleev-Clapeyron equation). Thus, by “burning” hydrogen in the process of a thermonuclear reaction, the star does not allow gravitational forces to compress themselves to an ultradense state, contrasting the gravitational collapse with continuously renewed internal thermal pressure, resulting in a stable energy balance. This period of the star’s life is called the main sequence (on the Hertzsprung-Russell diagram) and is the longest. In particular, the Sun has been at the active stage of hydrogen burning in the process of active nucleosynthesis for about 5 billion years, and the reserves of hydrogen in the core for its continuation should be enough for another 5.5 billion years.

    The Hertzsprung-Russell diagram

    It must be said that the defining property of a star is, of course, its mass. Most stars range from 0.1 to 100 solar masses. We, as patriots, naturally measure the mass of stars in the solar masses.

    The main phases of stars vary in properties and duration depending on mass, but the beginning of the end is the same for everyone.

    With the depletion of hydrogen reserves in the bowels of the star, the forces of gravitational compression, patiently waiting for this hour from the very moment of the birth of the star, begin to gain the upper hand - and under their influence the star begins to shrink and become denser. This process leads to a twofold effect: the temperature in the layers surrounding the star’s core rises to the level at which the hydrogen contained therein undergoes a thermonuclear fusion reaction with the formation of helium. At the same time, the temperature in the core itself, which now consists of almost one helium, rises so much that the helium itself - a kind of "ash" of the damped primary nucleosynthesis reaction - enters into a new thermonuclear fusion reaction: one carbon nucleus is formed from three helium nuclei. This process of the secondary reaction of fusion, the fuel for which are the products of the primary reaction,

    During the secondary combustion of helium, so much energy is released in the star’s core that the star begins to literally swell. In particular, the shell of the Sun at this stage will expand beyond the orbit of Venus. In this case, the total radiation energy of the star remains at about the same level as during the main phase of its life, but since this energy is now emitted through a much larger surface area, the outer layer of the star cools down to the red part of the spectrum. A star turns into a red giant.

    For stars of the class of the Sun, after the depletion of fuel that feeds the secondary nucleosynthesis reaction, the stage of gravitational collapse again sets in, this time the final one. The temperature inside the nucleus is no longer able to rise to the level necessary to start the next thermonuclear fusion reaction. Therefore, the star contracts until the forces of gravitational attraction are balanced by the pressure of the degenerate electron gas. Electrons, up to this point not playing a prominent role in the evolution of a star, at a certain stage of compression, due to the high pressure and temperature inside the nucleus, almost all leave their nuclear orbitals. Being in such a high-energy state, they themselves are resisting gravitational compression. The star’s condition stabilizes, and it turns into a white dwarf,

    Stars are more massive than the Sun, waiting for a much more spectacular end. After the combustion of helium, their mass during compression turns out to be sufficient to heat the core and shell to the temperatures necessary to start the following nucleosynthesis reactions - carbon, then silicon, magnesium - and so on, as the nuclear mass grows. Moreover, at the beginning of each new reaction in the star’s core, the previous one continues in its shell. Thus, the star begins to resemble a bulb with different synthesis reactions in certain layers. In fact, all the chemical elements up to the iron that make up the Universe were formed precisely as a result of nucleosynthesis in the bowels of dying stars of this type. But iron is the limit; it cannot serve as fuel for nuclear fusion or decay reactions at any temperature and pressure, because as for its decay, so to add additional nucleons to it, an influx of external energy is required. As a result, a massive star gradually accumulates an iron core within itself, unable to serve as fuel for any further nuclear reactions.

    As soon as the temperature and pressure inside the nucleus reaches a certain level, the electrons begin to press into the protons of the iron nuclei, as a result of which neutrons are formed. And in a very short period of time - some theorists believe that it takes a matter of seconds, the electrons literally dissolve in the protons of the iron nuclei, and all the material of the star’s nucleus turns into a continuous bunch of neutrons and begins to rapidly compress in the gravitational collapse, since the degenerate electron pressure that counteracted it gas drops to zero. The outer shell of the star, from which any support is knocked out, collapses to the center. The collision energy of a collapsed outer shell with a neutron nucleus is so high that it bounces with great speed and scatters in all directions from the core - and the star literally explodes in a blinding flash of a supernova. In a matter of seconds, during a supernova burst, more energy can be released into space than all the stars of the galaxy taken together during the same time release.

    After a supernova burst and shell expansion in stars with masses of the order of 10-30 solar masses, the ongoing gravitational collapse leads to the formation of a neutron star, the substance of which is compressed until it begins to make itself felt the pressure of degenerate neutrons - in other words, now neutrons (like to how electrons did it before) begin to resist further compression.

    Finally, if the mass of the star’s core exceeds 30 solar masses, nothing can stop its further gravitational collapse, and a black hole will form as a result of the supernova burst.

    Why are supernovae so important?

    Recently, thanks to observational data, the hypothesis has been confirmed that thermonuclear fusion also occurs at the very moment of a supernova explosion - a shock wave passes through all layers of the star, instantly significantly increasing pressure, and starts a short-term synthesis of the heaviest elements of the periodic table.

    Moreover, supernovae are the main distributors of elements throughout the Universe, scattering them many hundreds of light years from their birthplace. And the radiation pressure on the surrounding gas and dust clouds starts the process of the birth of new stars.

    How do we find out about the chemical composition of objects such as stars?

    The fact is that the atoms of each chemical element have strictly defined resonant frequencies, as a result of which they emit or absorb light at these frequencies. This leads to the fact that in the spectroscope the spectra show lines (dark or light) in certain places characteristic of each substance. The intensity of the lines depends on the amount of substance and its condition.

    Optical spectroscopy originated in 1802, when dark lines were discovered in the spectrum of the sun. These lines were rediscovered and described by Fraunhofer in 1814. In the 60s of the XIX century, Kirchhoff gave an almost correct interpretation of these lines, believing that these are absorption lines due to the presence of various gases in the atmosphere of the Sun, and that a specific line is associated with each gas.

    Purposeful scientific spectroscopy began in 1853, when Angstrom compared the emission lines of gases with various chemical elements - this was the origin of a new method for obtaining information on the composition of substances - spectral analysis. Now it is one of the most powerful tools of modern science. This sensitive method is widely used in analytical chemistry, astrophysics, metallurgy, mechanical engineering, geological exploration, archeology and other branches of science.

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