The production of heat from the sun is quite small per unit of volume. In solids, volume increases by the cube, surface by the square. Also, the sun has had billions of years to heat up. Even though the sun's energy output is small compared to its huge size, it turns out energy on an enormous scale by Earth's standards, some 10 million megatons of energy per second. There is estimated to be enough hydrogen in the sun convertible to helium to allow the sun to shine at its present rate for 10 billion years to come.
Man sees only the glowing surface of the sun.
During solar eclipses when the main disc of the sun is blotted out, one can also see the solar atmosphere or corona. The corona is made up of ionized hydrogen, a hydrogen atom with its one electron knocked off. However, the corona also contains all of the elements common on Earth. 3,600,000 degrees F. just above the sun's surface. This corona is thought to extend in diffused form to the outer limits of the solar system.