Why is the Sun so hot?

Why Is The Sun Hot?

The Sun is the hottest place in the Solar System. The surface of the Sun is a mere 5,800 

Kelvin, but down at the core of the Sun, the temperatures reach 15 million Kelvin.

The Sun is just a big plasma ball of hydrogen, held together by the mutual gravity of all its mass. This enormous mass pulls inward, trying to compress the Sun down. It’s the same reason why the Earth and the rest of the planets are spheres. 

As the pull of gravity compresses the gas inside the Sun together, it increases the temperature and pressure in the core which results in small, sudden bursts of heat and energy, called nano-flares, responsible for hot temperature of the sun's tenuous atmosphere.

The process where protons are merged together into atoms of helium is called nuclear fusion. It can only happen in hot temperatures, and under incredible pressures. 

But the process of fusion gives off more energy than it uses. So once it gets going, each fusion reaction gives off gamma radiation. It’s the radiation pressure of this light created in the core of the Sun that actually stops it from compressing any more.
Gravity is trying to squeeze it together into a little ball, but this creates the right conditions for fusion and increases its temperature to a greater extent.