First 231 words of the document:
Nuclear fusion is the process which two atomic nuclei combine to form a large nucleus
releasing energy. The energy it liberates is far greater (more than a million times) than one
from a typical chemical reaction. Such a large amount of energy is created since when two
nuclei fuse the sum of the mass of the product nuclei is actually less than the sum of the
masses of the initial nuclei. Einstein's equation - E=mc2, explains that the mass that is lost is
converted into energy carried by the fusion products.
Nuclear fusion does not happen easily. Nuclei all exert electrostatic repulsion between them
as they come together. Before the small nuclei will join together they have to be colliding so
fast that they overcome this force. This can only happen at extremely high temperatures so
that nuclei can have enough energy. This happens in stars. Fusion reactions have been going
on for billions of years in our universe. In fact, nuclear fusions are responsible for the energy
output of most stars, including our Sun.
How is energy created in stars?
The sequence of nuclear fusion reactions in a star is complex, but overall hydrogen nuclei
join to form helium nuclei. Here is one nuclear fusion reaction that takes place: