Many of you reading this article will be aware of the problems the world faces over energy supply: can we rely on fossil fuels (oil and coal), or should we look again at nuclear power? By nuclear power we usually mean fission, the break up of heavy atoms (uranium) to lighter ones. There is another type of nuclear power, though, fusion, where energy is released by combining light atoms (hydrogen) into heavier  ones. This is the process that fuels the stars.