This article is from the Fusion FAQ, by Robert F. Heeter firstname.lastname@example.org with numerous contributions by others.
Fortunately for life on earth, the sun is an aneutronic fusion
reactor, and we are not continually bombarded by fusion neutrons.
Unfortunately, the aneutronic process which the sun uses is
extremely slow and harder to do on earth than any of the reactions
mentioned above. The sun long ago burned up the "easy" deuterium
fuel, and is now mostly ordinary hydrogen. Now hydrogen has a
mass of one (it's a single proton) and helium has a mass of four
(two protons and two neutrons), so it's not hard to imagine sticking
four hydrogens together to make a helium. There are two major
problems here: the first is getting four hydrogens to collide
simultaneously, and the second is converting two of the four protons
The sun evades the first problem, and solves the second, by using a
catalyzed cycle: rather than fuse 4 protons directly, it fuses a
proton to an atom of carbon-12, creating nitrogen-13; the N-13 emits
a neutrino and a positron (an antielectron, that is an electon with
positive instead of negative charge) and becomes carbon-13.
(Effectively, the Carbon-12 converted the proton to a
neutron + positron + neutrino, kept the neutron, and became C-13).
The C-13 eventually fuses with another proton to become N-14.
N-14 then fuses with a proton to become oxygen-15. Oxygen-15 decays
to N-15 (emitting another positron), and N-15 plus another proton
yields carbon-12 plus a helium-4 nucleus, (aka an alpha particle).
Thus 4 protons are tacked one by one onto heavier elements, two of
the protons are converted to neutrons, and the result is production
of helium and two positrons. (The positrons will undergo
matter-antimatter annihilation with two electrons, and the result
of the whole process is formation of a helium, two neutrinos, and
a bunch of gamma rays. The gamma rays get absorbed in the solar
interior and heat it up, and eventually the energy from all this
fusion gets emitted as sunlight from the surface of the sun.)
The whole process is known as the carbon cycle; it's catalyzed
because you start with carbon and still have carbon at the end.
The presence of the carbon merely makes it possible to convert
protons to helium. The process is slow because it's difficult
to fuse protons with carbon and nitrogen, and the positron-emitting
nuclear decays are also slow processes, because they're moderated
by the weak nuclear force.