Even so, it was unclear how proton–proton fusion might proceed, because the most obvious product, helium-2 (diproton), is unstable and almost instantly dissociates back into two protons.
Čak i tada, bilo je nejasno kako nuklearna fuzija po nizu proton – proton se odvija, budući da najvjerovatniji proizvod, helij-2, je nestabilan i gotovo istovremeno se vraća u par protona.