Why does a small amount of mass become energy during fusion in the Sun?

In the core of the Sun, energy is generated through nuclear fusion, a process where hydrogen atoms are converted into helium.12

This release of energy occurs because the resulting helium nucleus is slightly less massive than the four individual hydrogen atoms that combined to form it. This difference in mass is known as the mass defect. When these particles bind together, the total mass of the resulting nucleus is less than the sum of the masses of the separate particles; this "missing" mass is converted into energy according to Albert Einstein's mass-energy equation, $E=m{c}^2$.23

In this formula:
* E represents the energy released.2
* m is the mass converted into energy.2
* c is the speed of light.2

Essentially, the binding energy of the nucleus—the energy required to hold the nucleons together—is released during this fusion process, allowing the Sun to radiate energy for billions of years.123