Definition: the combination of atomic nuclei to form a larger nucleus and release energy.
How it works: Essentially, two light nuclei (nuclei with only a small number of protons and neutrons) such as hydrogen fuse together, producing a heavier element (nuclei with more protons and neutrons.
It is a process which releases energy.
This process is important → it powers every star in the universe, including our sun (please remember that the sun is a star, people seem to forget despite it being the thing that gives us life).
In stars, it is the gravitational force which starts fusion in a star → light elements are literally pulled together, squashing them so strongly that their nuclei fuse together. The gravity also keeps the reaction contained. Fusion is maintained by a combination of high temperature and high pressure.
The conditions for fusion to occur are extreme: millions of degrees celcius are required (about 100 million degrees celcius for hydrogen). We say it occurs in plasma conditions.
Fusion would be the perfect energy source, as the only waste products are heavier elements such as helium → very easy to dispose of. However, the conditions are very difficult to achieve on earth → it is not currently a viable option as an energy source. Scientists are working very hard to try and make it work on earth, but it isn’t yet. (So please don’t say in the exam that it is.)
Two isotopes of hydrogen are deuterium (one neutron in the nucleus) and tritium (two neutrons in the nucleus).
A typical nuclear fusion process in any star is:
deuterium + tritium → fusion → helium + neutron