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Fusion Reaction in Sun with Examples for JEE

What is Nuclear Fusion?

Last updated date: 26th Mar 2023
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A nuclear reaction is a process by which an atom's nucleus is altered by being split apart or joined with the nucleus of another atom. There are two types of nuclear reactions. The first one is nuclear fusion. The nuclear fusion reactions of light elements, which power the energy production of stars and the Sun, are among the most notable nuclear reactions. The second one is nuclear fission. The fission reaction, which occurs in nuclear reactors, is the most well-known man-controlled nuclear reaction. In this article, we mainly focus on the nuclear fusion process and what is thermonuclear fusion?


Nuclear Fusion Process

Fusion reactions occur in a state of matter known as plasma. Plasma is a hot, charged gas composed of positively charged ions and free-moving electrons with properties distinct from solids, liquids and gases. Fusion occurs when two low-mass isotopes, typically hydrogen isotopes, combine under extreme pressure and temperature. Under extreme pressure and temperature, atoms of Tritium and Deuterium (hydrogen isotopes Hydrogen-3 and Hydrogen-2, respectively) combine to produce a neutron and a helium isotope. Along with this, a massive amount of energy is released, which is several times that of fission. Nuclear fusion equation  $\mathrm{B}=\left(\mathrm{Zm} \mathrm{p}_{\mathrm{p}}+N \mathrm{~m}_{\mathrm{n}}-\mathrm{M}\right) \mathrm{c}^{2}$, where mp and mn are the proton and neutron masses and c is the speed of light.

Nuclear Fusion Reaction

Nuclear Fusion Reaction

Nuclear Fusion Examples :

$\begin{align} &{ }_{1} \mathrm{H}^{1}+{ }_{1} \mathrm{H}^{1} \rightarrow{ }_{1} \mathrm{H}^{2}+1 \mathrm{e}^{0}+\nu+0.42 \mathrm{MeV} \\ \\ &{ }_{1} \mathrm{H}^{2}+{ }_{1} \mathrm{H}^{2} \rightarrow{ }_{1} \mathrm{H}^{3}+{ }_{0} \mathrm{n}^{1}+3.27 \mathrm{MeV} \end{align}$

Nuclear Fusion in the Sun

The Sun is the biggest star in our solar system and is 4.5 billion years old. The mass of the Sun contains 99.98% of the solar system’s mass. The Sun has different layers with different properties; these layers are made up of material that is roughly 75% hydrogen and 25% helium by mass. So, how does the Sun continue to generate heat? How does the Sun create energy?

In 1921, physicist Jean Perrin proposed that nuclear reactions, or reactions between atomic nuclei, were the source of energy production. Hans Bethe, a German, proposed and developed this idea several years later, explicitly describing the nuclear reactions produced in the Sun's core. This physicist demonstrated that a star uses its nuclear reserves to compensate for its constant loss of energy for the majority of its life. 

Fusion reactions in the Sun's densest and hottest central regions transform four hydrogen nuclei (protons) into a helium nucleus, 4He, an element that is particularly stable and releases energy that compensates for that lost at the Sun's surface. This energy is emitted and transported in the form of photons and neutrinos (massless fundamental particles with no electric charge; very low mass particles with no charge). Since the mass of the produced nucleus is less than the sum of the masses of the initial nuclei, a fusion reaction releases energy. 

According to Einstein's famous equation, $E=\Delta mc^2$. This mass difference is converted into energy. These reactions cannot occur unless the temperature and pressure are sufficiently high for the two protons, which have been stripped of their electrons and are thus positively charged, to fuse.

Thermonuclear Fusion

Thermonuclear fusion is the process by which two atoms combine to form a larger atom, releasing a large amount of energy. Natural fusion occurs in stars, including the Sun, when intense pressure and heat fuse hydrogen atoms together, producing helium and energy.


This process is what gives the Sun its power and causes it to be so hot and bright. Thermonuclear fusion can be classified into two types: uncontrolled and controlled. Uncontrolled thermonuclear fusion occurs in thermonuclear weapons ("hydrogen bombs") and most stars. The second type is controlled fusion, which occurs in an environment that allows some or all of the energy released to be used constructively.



Nuclear fusion is the process by which two or lighter nuclei combine to form a heavier, more stable nucleus. The mass of the product nucleus is slightly less than the sum of the masses of the fusing lighter nuclei. According to Einstein's mass-energy relation $E = mc^2$, this difference in masses (m) results in the release of enormous amounts of energy. The primary requirement for carrying out nuclear fusion is to raise the temperature of the material to the point where particles have enough energy due to their thermal motions alone to pass through the Coulomb barrier. This is known as thermonuclear fusion.

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FAQs on Fusion Reaction in Sun with Examples for JEE

1. What are the properties of thermal neutrons?

Thermal neutrons are low-energy neutrons that move slowly. Their total energy is 1.40 eV. Thermal neutrons have a velocity of 2.2 km/s, which corresponds to the random motion of atoms and molecules in a gas at room temperature. This is why these neutrons are referred to as thermal neutrons. A thermal neutron with relatively low kinetic energy can penetrate a nucleus, whereas a proton or an alpha particle would require significantly more energy to do so. This is primarily because positively charged protons or alpha must overcome the repulsive forces of protons in the nucleus.

2. Is the nuclear fusion process managed or unmanaged?

The foundation of a fusion reactor is controlled thermonuclear fusion, which is the future source of unlimited and unpolluted energy. The particle density is very high. Only when the density (number) of interacting particles is very high can the deuteron-deuteron collision rate be very high. Deuterium will be completely ionised at the required high temperatures, forming neutral plasma.

A high plasma temperature of around 109 K is required for interacting particles to pass through the Coulomb barrier and fuse together. A long confinement time. The hot plasma must be kept at a sufficiently high density and temperature for an extended period of time in order for sufficient fuel to be fusioned. These conditions are insurmountable for any solid container.