not just described, Explained


Contact us

Misconceptions quiz

Subscriptions FAQ




Physics subscription prices

subscribe log in



Work and Money

PSHE subscription prices

subscribe log in

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.

Try Why Do Astronauts Float by Julian Hamm

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.

back to Lesson 12: Stability, Fusion and Fission