Why is the sun hot?

Heat is a form of energy produced by the sun, but how is it created or emitted? 
The sun is a star, and like Earth, is composed of several layers of matter. Unlike the outermost layer, or lithosphere, which is solid the inner layers of Earth are liquid. The gravitational force of our planet pulls all matter towards its center, creating an enormous amount of pressure. To get an idea of the effect this pressure has on matter, I will use the example of the metals iron and nickel. The increased pressure in the outer core causes these elements, normally solids on the lithosphere, to liquify.
The most abundant elements found on the Sun are hydrogen and helium. Hydrogen, which is the first element on the Periodic Table of Elements, contains one proton and one electron and is a flammable gas. Helium, the second element, contains two protons and two electrons and is more stable by comparison. Hydrogen's reactivity is due to the single electron in a shell that can hold two. Helium's two electrons makes its shell relatively stable.
The increased pressure in the inner layers of the Sun causes atoms to behave differently than we might experience on the lithosphere of Earth. Similar to Earth, the increase in pressure generates heat. Unlike on Earth, the hydrogen ignites. This reaction is called nuclear fusion; the results being that two hydrogen atoms combine to create one helium atom. The combination of two hydrogen atoms into one helium atom results in an excess of energy, which is then released in the form of light and the heat of solar wind.
http://volcano.oregonstate.edu/earths-layers-lesson-1

https://www.space.com/14735-sun-heat-source-explained.html