The proton-proton-cycle
High temperature means fast
Protons repel each other strongly. So they have to be going very fast to get close enough for the strong force to act.
Activity Find out the temperature needed for fusion of two protons. It acts a bit like firing a pinball. The further you drag the nucleus back along the temperature scale, the faster it's released.
In other words we need the temperature to be very high, at least 15 million degrees Celsius. This is the temperature at the centre of the Sun.
Even at this temperature, fusion is incredibly rare and can only take place through a process called 'quantum tunneling'.
When the two protons fuse one of them turns into a neutron via the weak interaction. We end up with a deuterium nucleus. The electric charge is taken away by a positron (the electron’s antiparticle). An electron neutrino is also formed.
There are a number of ways that the nucleus can continue to grow.
The proton-proton cycle
In a star the size of the Sun the deuterium nucleus captures another proton to become helium-3. Helium-3 is stable but eventually one helium-3 will encounter another with the right energy.
The two helium-3 nuclei fuse to form helium-4 and two protons are released. Helium-4 is a ‘normal’ helium nucleus.
This is called the proton-proton cycle because you need two extra protons for the process to happen but in the end two protons (doesn't matter which ones) are released.
More complex cycles exist in bigger stars to form elements heavier than helium.