Why sun is warm?

Answer 1

The short answer is nuclear fusion.

The longer answer is:

The large amounts of hydrogen, under immense pressure and temperature at the sun's core, are fused together to form Helium.

Hydrogen has 1 proton, so a 1 proton element (hydrogen) + another 1 proton element (hydrogen) = a 2 proton element (Helium).

Sometimes a 1 proton element (hydrogen) is fused with a 2 proton element (helium) to form a 3 proton element (lithium).

Larger stars with greater mass than our sun, can continue this process, making most of the elements, up to iron.

Typically, a super nova is required to create elements with more protons than iron.

The heat from this continuous process, in the forms of many types of electromagnetic radiation, typically results in warmth.