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All things are made up of tiny buliding blocks called atoms. Atoms themselves are made up of smaller pieces. In the center, they have a nucleus which is made of protons (with positive electrical charge) and neutrons (with no charge). Two positive charges repel each other and try to move apart, so any nucleus with more than one proton in it must have neutrons to glue them together. Around the nucleus electrons (with negative electrical charge) circle. Negative charges and postive charges attract each other so the electrons must spin quickly around the nucleus, like a satellite around Earth, in order not to crash into it. Atoms are extremely small. It would take about 2 million hydrogen atoms in a line to cross this period. Different types of things (different elements) are composed of atoms with different numbers of protons. For example, Gold is made up of atoms each of which has 79 protons. Each Oxygen atom has 8 protons. Most of the Sun is made up of Hydrogen. It only has one proton and no neutrons in its nucleus.
In the core of the Sun the temperature and pressure are so high that nuclear fusion occurs. This means that the atomic nuclei start sticking together. Four Hydrogen nuclei (composed of one proton each) are glued together to make one Helium nucleus which has two protons and two neutrons. When this happens two protons are changed into two neutrons and huge amounts of energy is released. The name Helium comes from the Greek work "helios" which means Sun. It was discovered on the Sun during a solar eclipse almost thirty years before it was found on Earth.