The Nucleus and the Strong Interaction 237
The fusion reaction is also responsible for the release of energy
in the stars and the Sun. The threshold energy for fusion in a star is
easily achieved because of the high temperatures of the star. A star
before it is ignited by fusion achieves the threshold energy for fusion
through gravitational collapse. Once enough interstellar material collects
(basically hydrogen gas), a gravitational collapse occurs in which
extremely high temperatures are reached. Eventually the kinetic energy
of the nuclei becomes so great that the hydrogen fuses into helium.
The process is actually quite complicated, proceeding through several
steps: First, two hydrogen nuclei fuse to form a deuteron H^1 + H^1 →
H^2 + e+ + γ. Then another proton combines with the deuteron to form
helium-3, H^1 + H^2 → He^3 + γ. Finally, two He^3 nuclei combine to form
He^4 and two protons, He^3 + He^3 → He^4 + 2 H^1.