What is the first step of thermonuclear fusion within the Sun to form helium-4? 1. The fission of hydrogen to form protons and neutrons.
2. Two lithium atoms, each with three protons and three neutrons, undergo fission to form hydrogen atoms with one proton and one neutron.
3. Two hydrogen nuclei, each with only one proton, fuse to form deuterium, a form of hydrogen with one proton and one neutron.
4. Electrons and protons fuse to form the nucleus of a helium atom.