Step 1: Two protons (medium size pink balls at the left) meet to form a deuteron (large blue ball), emitting a positron and a neutrino (two small orange balls).
Step 2: The deuteron combines with a proton to form He3 (triplet of pink balls), emitting a photon (not shown in photograph).
Step 3: Two He3 nuclei combine to form He4 (big blue ball) plus two protons (medium size pink balls). The process then repeats. Note that steps 1 and 2 must occur twice before step 3 can occur.