The basic energy source in our sun is believed to be the pp cycle, in which four protons fuse to form
, ie, . The dominant initial reactions are
(1)
The first of these results in the low energy pp neutrinos. Their number is the firmest prediction of the solar model because it is closely tied to the overall solar luminosity. However, they are very hard to detect because of their low energy. Most of the
is from
(2)
However, approximately 15% is believed to be produced from the sequence
(3)
which yields
neutrinos at two discrete energies, one of which is somewhat above the pp spectrum. Finally, a rare side reaction,
(4)
is associated with about 0.02% of the produced
. This is insignificant energetically, but the resulting neutrino spectrum extends to much higher energy than the others, so they are easier to detect. The predicted spectrum from the pp cycle and the rare CNO cycle neutrinos are shown in Figure 1.
