Which force is responsible for the nuclear fusion that powers the Sun?

In the Sun, energy is produced mainly by the proton-proton chain. The very first step requires one proton to transform into a neutron so nuclei can build up into deuterium, and that transformation depends on the weak force. Without the weak interaction, the Sun’s main fusion pathway would not work as it does. The strong force then binds the resulting nuclear particles together, while gravity provides the immense pressure and temperature needed for fusion to begin. This makes the Sun a dramatic example of multiple fundamental forces working together.