Like other stars, our Sun is a large ball of hydrogen and helium gas. In the centre of the Sun – the core – hydrogen is burning in a nuclear fusion process, turning into helium. Very high temperatures are needed for nuclear fusion. The temperature inside the core of the Sun is about 14 million degrees Celsius. At these temperatures, all the hydrogen is ionized into protons which hit each other at speeds high enough to make them fuse into helium nuclei. Each time this happens, a tiny amount of mass becomes pure energy. The Sun is using up its mass in this way at the rate of 4 billion kg per second and has been doing so for about 4.5 billion years. Even though it is only about halfway through its life.
The energy is released in the core of the star, and must travel about 700,000 km to reach the surface and be radiated to us. This takes a long time. The Sun is not transparent. The radiation is being constantly absorbed and re – emitted. The radiation heats the gas of the Sun, causing it to expand, but at the same time the force of gravity makes the Sun tend to contract. There is a balance between these two forces, keeping the Sun stable.
When the radiation reaches the surface of the Sun it can be radiated frrely into space. Some of it reaches the Earth, and is absorbed by plants so they can photosynthesis. This is at the basis of the food chain on Earth.