But why do stars shine at all? The intense pressure and temperature at the core of a star allow nuclear fusion reactions to take place. This is where atoms of hydrogen are fused into atoms of helium (through several stages). This reaction releases an enormous amount of energy in the form of gamma rays.

Every second, a star like our Sun converts 4 million tons of its material into heat and light through the process of nuclear fusion. Our Sun has provided an essentially constant amount of heat and light to Earth for about 4.5 billion years.

Nuclear Fusion reactions power the Sun and other stars. In a fusion reaction, two light nuclei merge to form a single heavier nucleus. The process releases energy because the total mass of the resulting single nucleus is less than the mass of the two original nuclei. The leftover mass becomes energy.

On Earth it is very difficult to start nuclear fusion reactions that release more energy than is needed to start the reaction. The reason is that fusion reactions only happen at high temperature and pressure, like in the Sun, because both nuclei have a positive charge, and positive repels positive.

Because fusion requires such extreme conditions, “if something goes wrong, then it stops. No heat lingers after the fact.” With fission, uranium is split apart, so the atoms are radioactive and generate heat, even when the fission ends. Despite its many benefits, however, fusion power is an arduous source to achieve.

No CO₂: Fusion doesn’t emit harmful toxins like carbon dioxide or other greenhouse gases into the atmosphere. Its major by-product is helium: an inert, non-toxic gas. No long-lived radioactive waste: Nuclear fusion reactors produce no high activity, long-lived nuclear waste.

Q: Why doesn’t nuclear fusion occur naturally on Earth? A: Nuclear fusion doesn’t occur naturally on Earth because it requires temperatures far higher than Earth temperatures.