This is an interesting question. The Sun is mainly composed of an enormous amount of Hydrogen, which makes the Sun very massive. There is an inward pull force in the Sun which attracts all its matter, creating high pressures and temperatures inside it. This fact creates reactions called “nuclear fusion”. Nuclear fusion is a nuclear reaction in which two or more atomic nuclei collide at very high speed and join to form a new type of atomic nucleus.

So, these are reasons why the Sun is hot, but there is also another reason: the Sun is an excellent thermal insulator. This means that the core of the Sun is very much insulated because of the huge distance that the heat has to travel to reach the Sun’s surface, allowing a tremendous building up of temperature!