In the core of a star the intense heat destroys the internal structure of an atom and consequently all atoms are broken down into their constituent parts. An atom is constructed of protons, electrons and neutrons. Neutrons have no electric charge and therefore do not interact much with the surrounding medium. As a result neutrons leave the core fairly quickly. The protons, which have positive electric charge, and the electrons, which have negative electric charge, remain in the core and drive the reactions which fuel the Sun. The charge neutral material of protons and electrons that makes up the core is called plasma.
The high temperature provides the protons and electrons with a large amount of thermal energy and as a result they move around quite quickly. This motion, combined with the high density of the plasma, causes the particles to continuously slam into one another creating nuclear reactions. It is the fusion, or slamming together, of particular combinations of particles that provides the energy source of the Sun.