Solar neutrinos are produced in the core of the Sun through various nuclear fusion reactions, each of which occurs at a particular rate and leads to its own spectrum of neutrino energies. Details of the more prominent of these reactions are described below.
The main contribution comes from the proton–proton chain. The reaction is:
or in words:
: two protons  deuteron + positron + electron neutrino.
Of all Solar neutrinos, approximately 91% are produced from this reaction. As shown in the figure titled "Solar neutrinos (proton–proton chain) in the standard solar model", the deuteron will fuse with another proton to create a He nucleus and a gamma ray. This reaction can be seen as:
The isotope He can be produced by using the He in the previous reaction which is seen below.
With both helium-3 and helium-4 now in the environment, one of each weight of helium nucleus can fuse to produce beryllium:
Beryllium-7 can follow two different paths from this stage: It could capture an electron and produce the more stable lithium-7 nucleus and an electron neutrino, or alternatively, it could capture one of the abundant protons, which would create boron-8. The first reaction via lithium-7 is:
This lithium-yielding reaction produces approximately 7% of the solar neutrinos. The resulting lithium-7 later combines with a proton to produce two nuclei of helium-4. The alternative reaction is proton capture, that produces boron-8, which then beta decays into beryllium-8 as shown below:
This alternative boron-yielding reaction produces about 0.02% of the solar neutrinos; although so few that they would conventionally be neglected, these rare solar neutrinos stand out because of their higher average energies. The asterisk (*) on the beryllium-8 nucleus indicates that it is in an excited, unstable state. The excited beryllium-8 nucleus then splits into two helium-4 nuclei: