Nuclear fusion is what powers the Sun and gives us the light and warmth to live, but what is it really?
Fusion is literally the process of join two things together to make them one, that is to fuse them. An everyday process that you may region use that is similar is welding where two separate bits of metal are fused together to make a single piece suck as the metal parts that make up your bicycle frame. Nuclear fusion is the process of fusing atoms together to create different atoms. Nuclear refers to the atomic scale.
However, it is not as easy as just putting two things next to each other, you need to apply some pressure and heat to get them to fuse. In the case of atoms you need to apply a lot of pressure and a lot of heat. Fortunately the Sun has pressure and heat in spades. Thanks to the immense mass of the Sun (See fact one here) the pressure at the core is really high, and with pressure comes heat, lots of it. The heat causes the atoms to move about faster and faster and the pressure squeezes them closer and closer together. When the atoms are moving fast enough and they get closer enough they will bump into each other and if they can overcome the repulsion of the electromagnetic force they will fuse together through the Strong Nuclear force.
Each time two atoms fuse together a tiny amount of energy is released in the form of a photon. Through the principle of conservation of energy, the sum total of the mass of the fused atom is very slightly less than the sum of mass of the two individual atoms prior to fusion. The very small loss in mass is converted to energy and released as the photon.
This process repeats and repeats (See fact two here) releasing a vast amount of energy and forming different chemical elements in the process. This is the process of nuclear fusion and it is what powers our Sun. Nuclear fusion is also what has given us all the chemical elements we see around us today, including the elements that we are made off. Awesome isn’t it?