Nuclear fusion is when two small, light nuclei join together to make one heavy nucleus. Fusion reactions occur in stars where two hydrogen nuclei fuse together under high temperatures and pressure to form a nucleus of a helium isotope.
There are a number of different nuclear fusion reactions happening in the Sun. The simplest is when four hydrogen nuclei become one helium nuclei.
The combined mass of four hydrogen nuclei is 6.693 × 10-27 kilograms (kg). The mass of one helium nucleus is 6.645 × 10-27 kg. This means that there is a missing amount of mass equalling 0.048 × 10-27 kg.
The missing mass is converted to energy, which radiates away. This is seen happening in the Sun.
In all nuclear reactions a small amount of the mass changes to energy. This may not seem like a lot of energy but this energy is a result of the fusion of only four hydrogen nuclei.
A 250 millilitre (ml) glass of water will contain around 1.6 × 1025 hydrogen atoms. Complete fusion of all these hydrogen nuclei would release about 17,200,000,000,000 joules (J) of energy. It is estimated that the sun releases 3.8 × 1026 joules of energy every second.
However, the issue with fusion is that it requires the fusing of nuclei, which are positive particles. As two nuclei approach each other, they will repel because they have the same charge. The fusion of the nuclei has to happen quickly so that the repulsion of the charges does not have time to stop it from happening.
A way that particles can travel that quickly is by being in a hot gas or in plasma, like in the Sun. In fact, for fusion to occur, the temperature of the hot gas or plasma needs to be at least 150,000,000 degrees Celsius (°C).