Sun is enormous source of heat and energy. Surface temperature of sun is 10000 degree Fahrenheit. It is equivalent to 380 billion billion megawatt power. In one second sun produce more energy than all the power produced in Earth.
Sun is producing this much energy for billions of years. Earlier astronomers' didn't know how sun can generate that much energy for such a long time. How Sun generates its energy was a mystery for the scientists.
In early 19th century scientists assume that sun is like any fire on earth and there is a source of fuel, perhaps coal, that is slowly burning away. But there is serious problem in this theory. If we ignite wood which is same size as sun then it would last only 5000 to 6000 years. It's a long time but not long enough to sustain life on earth.
There has to be other process unknown to us that was powering the sun. In 1920 scientists found the solution to the puzzle. That solution eventually leads to Hydrogen bomb which is the product of nuclear fusion.
Nuclear fusion occurs when two atoms smash into each other at high speed fuse together. For this to condition has to be just right. In order to get the two positive charge photon close enough they has to be extremely hot, dense and fast enough to get close and fuse. Core of the sun is perfect for fusion. It is the hottest place in our solar system. It's temperature is 27 million degrees Fahrenheit and it is extremely dense. It is ten times denser than led. But it is not solid. At extremely High temperature electron fall from atom and behave like a soup. Its behavior is different form a gas. It is called plasma.
Inside sun Hydrogen atom flow together with immense pressure to from Helium atom. In this process resulting atom's mass is slightly less than previously two atoms combined. Missing mass is emitted as energy. Each second inside sun 600 million tons Hydrogen are fused into 595 million ton helium. Missing 5 million ton lost in the fusion process is converted into energy which is equivalent to 1 billion Hydrogen bombs. Nuclear fusion is powering our SUN. It is the only fuel that can burn for billions of years.