7
The Proton-Proton chain
•The first step is slow because it relies on two rare events happening simultaneously
–Two protons collide with enough energy to overcome the Coulomb barrier
–While they are close the weak force turns one proton into a neutron
•The resulting combination of a proton and a neutron IS a stable nucleus
–1H  +  1H  ®  2H  +  e+  +  n p+  +  p+  ®  (p+ n)  +  e+  +  n
–
•The next two steps go quickly because they rely only on the strong force
–2H  +  1H  ®  3H              (p+ n)     +   p+         ®  (p+ p+ n  )
–3H  +  3H  ®  4He  +  1H  +  1H             (p+ p+ n) + (p+ p+ n) ® (p+ p+ n n) + p+ + p+
•
•The net effect is       4  1H ®  4He
From Realm of the Universe, by Abell et al.