What is nuclear fusion?

When two atomic nuclei are heated to a very high temperature and high pressure, for example inside a star, their nuclei can join to make a larger nucleus. Energy is released when this fusion takes place and the new nucleus is a different element to the original nuclei. For example, inside our sun two hydrogen nuclei can fuse to form one helium nucleus. This fusion can then happen with largerr nuclei to form larger elements like carbon.