The process that powers the Sun is called nuclear fusion. It works by releasing nuclear energy as other forms of energy. To get the amount of energy (E) released, take the mass lost (m) and multiply it by a huge number (the speed of light squared). E = mc2.
The centre of the Sun is extremely hot (around 15 million degrees) and very dense. It is so hot that the atoms lose all their electrons becoming bare nuclei. Usually, in cooler conditions these nuclei would not get close enough to fuse, or join (electric forces push them apart), but in the Sun, with temperatures that are incredibly high, the nuclei can bang into one another so hard that they can actually fuse together and release enormous of energy.
What's the proton-proton reaction?
That is the process that Bethe suggested powered the Sun.
With the very high temperature and density in the Sun's core, it is possible for 4 protons (hydrogen nuclei) to join together to form a new nucleus containing 2 protons and 2 neutrons. But 2 protons and 2 neutrons make a helium nucleus, so we have converted hydrogen into helium. The important point is that in this process a little bit of mass is lost and reappears as a lot of energy (E = mc2).
Scientists are working hard to try to recreate what's happening in the Sun so that we can have a virtually unlimited source of energy. If you fancy giving them a helping hand, get in touch!
If there is a huge hydrogen 'bomb' at the centre of the Sun why doesn't it blow apart? Because the sheer weight of all the Sun’s material around the core keeps the lid on it!
Here's a photo inside the Joint European Torus (JET), which was built to explore the possibility of providing energy from fusion reactions here on Earth. The machine has a diameter of about 15m and, during operations, the gas inside reaches a temperature of 300 million degrees.