the Sun is not the same solid as the Earth. To prove it very simply: the surface temperature of the Sun reaches 6000°C. At this temperature, any metal or stone turns into gas, so the Sun must be gas ball! The sun is actually made up of gases: 75% hydrogen and 25% helium.
In the past, scientists believed that the sun's light and heat are the result of combustion. But the surface of the Sun is hot already hundreds of millions of years, and so long nothing can not burn. Today scientists believe that the Sun generates heat as a result of processes similar to those that occur in a nuclear bomb. The sun converts matter into energy. This energy is released in the Sun in thermo-nuclear fusion of hydrogen into helium.
However, the Sun will be so hot not forever. As the hydrogen in the core of the Sun will burn out, its outer shell will begin to expand, and the core is compressed and heated. This process will begin, according to scientists ' estimates, after 4-5 billion years. After the end of the phase of expansion, the Sun will cool down and turn into a nebula.