undergoes a form of radioactive decay, turning into a neutron and
emitting a positron (the antiparticle of an electron) and a neutrino.
Normally it is the neutron that decays, due to its extra mass and
associated instability, into a proton, electron, and neutrino. An
isolated proton being the lightest baryon, by contrast, is stable. But
when two protons encroach, they feel electrostatic repulsion; this
contributes to their total energy making it exceed that of a deuteron
(a proton and neutron bound together). As a result one of the
protons can turn into a neutron, which then binds to another
proton, increasing the stability. This decay of the proton leads to a
neutron, neutrino, and positron, the positively charged antiparticle
of an electron.
So the very first part of the solar fusion cycle produces antimatter!
The positron is almost immediately destroyed as it collides with an
electron in the plasma, producing two photons which are scattered
by the electrically charged plasma, eventually working their way to
the solar surface (this takes several thousand years), by which time
their energy is much reduced and they help form part of sunlight.
The neutrinos pour out from the centre unhindered and reach us
within a few minutes.
So what has become of the neutron and proton? They grip one
another tightly, courtesy of the strong nuclear force, and bind
together: this doublet is a nucleus of heavy hydrogen – the
deuteron. This deuteron finds itself in the midst of a vast number of
protons, which still form the bulk of the Sun. Very rapidly the
deuteron links with another proton to make a nucleus of helium:
helium-3. Two of these helium-3 can join and rearrange their pieces
to form a nucleus of helium-4 (the stable common form), releasing
two spare protons.
So the net result of all this is that four protons have produced a
single helium, two positrons, and two neutrinos. Protons are the
fuel, helium the ash, and the energy is released in the form of
gamma rays, positrons, and neutrinos.
108
Particle Physics