The figure illustrates hydrogen fusion to form helium. Hydrogen (H) nuclei, which are just protons, collide to produce heavier nuclei. When protons collide one of them (at least) gives off some energy in the form of a positron (e⁺), i.e. a particle equivalent to an electron except that it has a positive charge instead of a negative charge. Since protons are positively charged, the emission of a positive charge makes the proton change into a neutron (so a neutron is a proton that has lost its charge and vice-versa). More collisions, at least the ones that stick, cause heavier elements such as D, ³H and He to form. Massive stars are capable of fusing even heavier nuclei such as C, O, Ne, etc.