Nuclear fusion in the Sun

Sun is well known for its extreme hotness. Nuclear fusion reaction is responsible for it. Even before we know how nuclear fusion happens in sun. Let us just understand what nuclear fusion reaction is?

Nuclear fusion

Nuclear fusion is nuclear reaction in which two or more nuclei combine to form a massive structure. It is an exothermic reaction. So when the collision of nuclei is happening there will be lot of reactional outcomes. Heat, light etc. are results of exothermic reactions. Generally the fusing nuclei have lower masses. So the energy released during this reaction is due to interaction between two opposite forces. Nuclear force and Coulomb force are the two opposite forces.

How this reaction happens in sun?

Firstly, Sun is made up of enormous amount of Hydrogen and helium. About 75% of Earth is made up of Hydrogen and the rest is Helium. Fusion happens between hydrogen nuclei to form Helium nucleus. This reaction happens at high temperature. This fusion of sun elements is known as proton-proton cycle. The temperature during this reaction is greater than 15 million degrees Kelvin. That’s really huge!

There are two main reactions in fusion i.e.

  • Proton-Proton cycle
  • CNO cycle


The combining of two hydrogen atom to generate Helium-4 and energy happens in following steps

The Helium-4 atom is lesser in mass compared to Hydrogen. Thus this difference in their mass is converted to energy. The energy so emitted can be in different forms of light like ultra violet, X-Rays, visible light, gamma rays, microwaves and radio waves. Sun emits some energized particles like protons and neutrions. For past 4.57 billion years the sun’s nuclear fusion of Hydrogen and Helium is taking place.




The carbon nitrogen oxygen (CNO) cycle accounts for a smaller ration. Though the proton-proton cycle is dominantly occurring, the end results of both are same.



Where does this reaction occur in sun?

This nuclear fusion reaction happens in the hottest part of sun i.e. its core. Sun has 6.995xKm total radius. And its core extends up to 1.391xKm.  Sun’s core is hot because of its phenomenally high density. 99% of solar energy is produced by proton- proton chain reaction. And the remaining energy is produced by CNO cycle.


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  1. cecil May 24, 2016
  2. ShonMAuzat October 4, 2016
  3. Drum October 29, 2016
  4. Juana November 19, 2016

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