What is the rate-limiting step in the Proton-Proton chain reaction?

Answer

Two protons fusing to form a deuterium nucleus

The initial step where two protons fuse to create a deuterium nucleus, releasing a positron and a neutrino, is the slowest part of the P-P chain and thus controls the overall reaction rate.

What is the rate-limiting step in the Proton-Proton chain reaction?
hydrogenCoresequencestarFusion