NUCLEAR Fusion in the stars

All the stars are stars that release a large amount of energy that they emit in the form of radiation. Emissions from the Sun, the star closest to us, reach us in the form of light and heat. This fact is perceived naturally and humanity has been aware of it since the beginning of time. However, what is no longer so obvious is the type of reaction that results in the release of this enormous amount of energy. In this lesson from a PROFESSOR we tell you what the nuclear fusion in the stars.

Index

1. What is nuclear fusion and how is it produced?
2. How is energy generated in stars?
3. How does nuclear fusion occur in stars?

Nuclear fusion reactions, as their name suggests, are those nuclear reactions in which several nuclei of light atoms combine to create a new atom with a heavier nucleus product of the combination of protons from several different atomic nuclei. The mass of the nucleus resulting from this type of reaction may be slightly less than the sum of the mass of the two atoms that have participated in the reaction. The difference in mass is released in the form of reaction energy, according to the

Which involves the release of a large amount of energy. Therefore, nuclear fusion is a very efficient way of generating energy, much more than any chemical reaction such as the combustion of oil or wood. A nuclear fusion reaction generates approximately ten million times more energy than any chemical reaction.

However, these kinds of reactions they only occur under very specific conditions in which the repulsion energy between the positive charges of two light nuclei is exceeded by the nuclear force, which is always attractive and short-range. This condition only occurs when atomic nuclei move at high speed, which implies very high temperatures. At high temperatures the atoms are in the form of plasma. The plasma state is that physical state of matter, in which the nuclei are detached from the electrons.

Thus, for a fusion reaction to occur, the Lawson's criterion, which establishes the temperature conditions, the density of the plasma and the time in which these are maintained conditions (called the plasma confinement time), which are necessary to trigger a reaction of nuclear fusion.

The simplest definition of a starIt is the one that says that it is a star that emits its own light. Going deeper, we can say that it is a huge gas sphere in revolution, where the gas is attracted to the center of the sphere by gravitational forces, reaching high pressures and temperatures that they trigger nuclear reactions that release large amounts of energy outward in the form of electromagnetic radiation, light and hot.

Given the a star is a great nuclear reactor, its composition is not constant and evolves over time from its birth, when the star is ignited or "turned on"; until the star uses up all its fuel and "dies."

Throughout the different stages of a star's life, the composition and conditions in which its plasma is found change, and with them the nuclear fusion reactions that we find at its core.

In this lesson we will see in detail the most common nuclear fusion reaction in main sequence stars, such as our Sun.

Image: Prezi

In the stars the Lawson's criterion for nuclear fusion reactions to take place. In this case, the confinement of the plasma is given by the enormous gravitational force. Different fusion reactions require different temperature and density conditions for them to take place optimally.

Depending on the mass and age of the star, the fusion reactions that take place in its core can be of three different types: proton-proton fusion, helium fusion or the carbon cycle. As a summary, we will see the proton-proton reaction, which is the most frequent.

Proton-proton fusion: transformation of Hydrogen to Helium.

A main sequence star, it is made up of 70% Hydrogen, 28% Helium and 1.5% carbon, ozone, oxygen and neon and 0.5% iron and other elements. Therefore your main fuel is hydrogen, which is the simplest atom and whose nucleus is formed by a single proton (subatomic particle with mass and positive charge).

The proton-proton fusion reaction cycle is summarized in five steps:

1.- Fusion of two protons

Inside the star, two hydrogen atoms, that is, two protons, fuse to form a single nucleus.

2.- Deuterium formation

In this nucleus formed by two protons, one of them is transformed into a neutron (subatomic particle with mass, but without charge), giving rise to a nucleus of deuterium, a heavy isotope of hydrogen that has a nucleus made up of a proton and a neutron. This step of the cycle requires energy and in it a positron or antielectron is released (a particle with the same characteristics of the electron but with a positive charge) and an electron neutrino or lepton (subatomic particle that has a mass one million times less than that of the electron).

3.- Formation of Helium-3 nuclei

Deuterium, produced in the second reaction of the cycle, participates in a new nuclear fusion reaction, incorporating a proton. The fusion of the deuterium nucleus with another proton (hydrogen nucleus) gives rise to a Helium-3 nucleus (made up of two protons and one neutron).

4- Fusion of two helium nuclei

In this fourth reaction, two helium nuclei fuse together, to give rise to a single nucleus made up of two neutrons and four protons.

5.- Release of two protons

In the last reaction of the cycle, the nucleus formed in the previous reaction forms a Helium nucleus, also called an alpha particle, which is made up of two neutrons and two protons, through the energetic release of two protons.

The complete cycle of proton-proton fusion involves a generation of energy of 25 MeV (mega-electron volts).

Image: Planets

If you want to read more articles similar to Nuclear fusion in the stars: summary, we recommend that you enter our category of Astronomy.

• Borexino Collaboration * (2014). Neutrinos from the primary proton – proton fusion process in the Sun. Stuttgart: Macmillan Publishers Limited.
• Davide Castelvecchi. (2020). Neutrinos reveal the final secret of nuclear fusion on the Sun. Research and science. Barcelona: Scientific Press S.L.