We need to find another, vastly more effective mechanism for energy generation. Let's
look at the structure of the Sun.
Transparency with structure of the Sun.
As we go deeper in the Sun, the density of the Sun goes up, and the temperature
goes up. Mathematical models of the Sun's interior indicate that in the innermost
core, densities reach nearly 200 grams/cm 
, and the temperature is about 15 million Kelvin.
At these temperatures, nuclear fusion reactions can occur, and at these densities they occur a lot. A good example of a nuclear fusion reaction (a change in atomic nuclei) is
We can write this in shorthand as 
, it is called the proton-proton cycle.
It is a nuclear fusion reaction because the nuclei of the atoms are involved.