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 registered by Chinese astronomers.}

All the variety of chemical elements existing in nature we owe to the stars. Indeed, at the very beginning of the existence of the Universe, primary nuclear fusion gave the Universe only hydrogen and helium.

After hundreds of thousands of years, the first stars were lit, within which the synthesis of nuclei of heavier elements began. 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. In the end, gravity always wins - it's only a matter of time.
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How does interstellar alchemy work?

The primary resource for thermonuclear fusion are the nuclei of hydrogen, of which more than 90% are stars. As a result of the fusion reaction of four protons, a helium nucleus is ultimately formed, with the release of a number of different elementary particles. In the final state, the total mass of the formed particles is less than the mass of the four original protons, which means that free energy is released during the reaction. Because of this, the inner core of the newborn star is quickly heated to ultra-high temperatures, and its excess energy begins to spill out towards its less hot surface. At the same time, the pressure in the center of the star also increases (the Mendeleev-Clapeyron equation). Thus, by burning hydrogen during a thermonuclear reaction, the star does not give gravity forces to compress itself to a superdense state, opposing the gravitational collapse to continuously renewable internal thermal pressure, as a result of which stable energy equilibrium occurs. This period of the life of a star is called the main sequence (on the Hertzsprung-Russell diagram) and is the longest. In particular, the Sun is in the active stage of hydrogen combustion in the process of active nucleosynthesis for about 5 billion years, and our hydrogen should be enough for another 5.5 billion years to store hydrogen in the core.


^{Hertzsprung-Russell Diagram}

I must say that the determining 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 solar masses.

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

With the depletion of hydrogen reserves in the depths of the star, the forces of gravitational compression, which patiently waited for this hour from the very moment of the birth of the star, begin to prevail - and under their influence the star begins to contract and condense. This process leads to a twofold effect: the temperature in the layers surrounding the core of the star rises to the level at which the hydrogen contained there reacts into thermonuclear fusion to form helium. At the same time, the temperature in the core itself, which is now practically from one helium, rises so much that helium itself, a kind of ashes of the decaying primary nucleosynthesis reaction, enters a new fusion reaction: one carbon nucleus is formed from three helium nuclei. This process of the secondary reaction of thermonuclear fusion, the fuel for which are the products of the primary reaction, is one of the key moments of the life cycle of stars.

During the secondary combustion of helium in the core of the star, so much energy is released that the star begins to literally swell. In particular, the solar envelope at this stage will expand beyond the limits of the orbit of Venus. At the same time, the total radiation energy of the star remains approximately at the same level as during the main phase of its life, but since this energy is now radiated through a much larger surface area, the outer layer of the star cools down to the red part of the spectrum. The star turns into a red giant.

For stars of the Sun class, after the depletion of fuel, which feeds the secondary nucleosynthesis reaction, the stage of gravitational collapse again begins - this time the final one. The temperature inside the nucleus is no longer able to rise to the level required for the start of the next thermonuclear fusion reaction. Therefore, the star is compressed until the forces of gravitational attraction are balanced by the pressure of the degenerate electron gas. Electrons, which until this moment did not play a prominent role in the evolution of a star, at a certain stage of compression, due to high pressure and temperature inside the nucleus, almost all leave their nuclear orbitals. Being in such a high-energy state, they themselves resist gravitational compression. The condition of the star stabilizes, and it turns into a white dwarf, which will radiate residual heat into space until it cools down completely.

Stars are more massive than the Sun, waiting for a much more spectacular end. After the combustion of helium, their mass during compression is sufficient to heat the core and the shell to the temperatures necessary to start the next nucleosynthesis reactions — carbon, then silicon, magnesium — and so on, as the nuclear masses grow. At the same time, at the beginning of each new reaction in the core of the star, the previous one continues in its shell. Thus, the star begins to resemble an onion with different fusion reactions in certain layers. In fact, all the chemical elements up to iron, of which the Universe is composed, were formed precisely as a result of nucleosynthesis in the depths of dying stars of this type. But iron is the limit; it cannot serve as a fuel for reactions of nuclear fusion or decay at any temperature or pressure, since both its decay and the addition of additional nucleons to it require an influx of external energy. As a result, the massive star gradually accumulates an iron core inside itself, which cannot serve as fuel for any further nuclear reactions.

As soon as the temperature and pressure inside the nucleus reach a certain level, the electrons begin to be pressed into the protons of the iron nuclei, resulting in the formation of neutrons. And in a very short period of time - some theorists believe that it takes a few seconds, electrons literally dissolve in the protons of the nuclei of iron, and all the matter of the star’s core turns into a continuous neutron bunch and begins to rapidly contract in a gravitational collapse, as the pressure of the degenerate electron gas drops to zero. The outer shell of the star, from under which any support is knocked out, collapses towards the center. The collision energy of a collapsed outer envelope with a neutron core is so high that it bounces off at great speed and flies away from the nucleus in all directions - and the star literally explodes in a dazzling flash of a supernova. In seconds, with a supernova explosion, more energy can be released into space than all the stars of the galaxy together emit during this time.

After a supernova explosion and scattering of stars with a mass of about 10–30 solar masses, the ongoing gravitational collapse leads to the formation of a neutron star, the substance of which shrinks until it starts to feel the pressure of degenerate neutrons — in other words, now neutrons how the electrons had previously done this) begins to oppose further compression.

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

Why are supernovae so important?

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

Moreover, supernovae are the main distributors of elements in the universe, scattering them many hundreds of light years from the place of their birth. And the radiation pressure on the surrounding gas and dust clouds triggers the process of the birth of new stars.

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

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

Optical spectroscopy originated in 1802, when dark lines were detected 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, considering that these are absorption lines due to the presence of various gases in the solar atmosphere, and that a certain 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 is how a new method of obtaining information on the composition of substances originated - 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.