Fusion in the sun

Many different fusion reactions take place in the core of the Sun. We think the most common conversion from hydrogen to helium, producing a high proportion of the Sun’s energy output, is in these three steps.

Two single protons (hydrogen nuclei) combine to form deuterium, an isotope of hydrogen with one proton and one neutron. At the same time some of the mass is converted into energy and a positron and a neutrino are produced.

Hydrogen fusion step 1

The deuterium nucleus (one proton and one neutron) combine with a hydrogen nucleus (a single proton) to form helium 3 emitting gamma radiation.

Hydrogen fusion step 2

Two deuterium nuclei combine to create helium and two protons releasing energy (binding energy) and there is an overall reduction in the mass.

Hydrogen fusion step 3

Nuclear fusion reactors

Research continues at a fast pace to find practical ways of harnessing nuclear fusion power but so far no commercial stations are planned. There are huge practical difficulties in producing and containing the huge temperatures and pressures needed for fusion to occur.

Two of the current projects are described here, first the Tokamak

The aim is to contain the hot plasma with the magnetic field without the plasma touching the sides of the container.

A PDF copy of these notes can be downloaded here: Tokamak reactor

The ICF nuclear fusion reactor.

Standing for inertial confinement fusion the reaction relies on speed of reaction rather than any pressurised container to hold the fuel.

Powerful laser beams are fired at a small capsule of fuel.

The heating is so massive that the outer layer of gas forms a plasma which explodes outwards.


The reaction to the outward explosion sends a compressive shockwave into the centre of the capsule causing the temperature rise to 100 million degrees and the gas to be compressed to a density as much as 20 times that of lead, resulting in a fusion reaction using a high proportion of the fuel.


Nuclear fusion video lesson