Nuclear fusion takes place in the sun's core. The core is where the temperature of the sun is hot enough for nuclear fusion to take place.
In the sun's core, hydrogen is converted into helium during nuclear fusion. Once this process is complete, the energy from the core makes its way to the surface, where it cools down to around 10,000 F.
Because of the ongoing nuclear fusion going on in the center of the sun, the sun neither grows nor collapses under its own weight. Its size stays relatively constant as does the heat it produces. Once the sun has used up its entire hydrogen supply, it will begin expanding and will eventually die.