Short answers, pulled from the story.
Ernest Rutherford discovered that the atom's mass and positive charge are concentrated in a tiny, dense nucleus. This finding shattered the plum pudding model and proved the atom is mostly empty space with a nucleus smaller than 3 times 10 to the minus 12 centimeters.
James Chadwick identified the neutron in 1932 as a neutral particle with a mass nearly identical to that of the proton. This discovery solved the puzzle of nuclear spin and allowed Dmitri Ivanenko to propose that the nucleus consists solely of protons and neutrons.
Arthur Eddington correctly speculated in 1920 that the source of stellar energy is the fusion of hydrogen into helium. This process releases vast amounts of energy according to Einstein's equation E equals mc squared and explains how stars shine for billions of years.
Hideki Yukawa proposed the existence of a virtual particle called a meson in 1935 to explain how the nucleus holds together. This theory introduced the strong nuclear force which binds nucleons together and prevents the nucleus from disintegrating under electromagnetic repulsion.
The valley of stability represents the balance between the number of protons and neutrons in a nucleus where too few or too many neutrons lead to radioactive decay. Radioactive decay processes such as alpha decay, beta decay, and gamma decay allow unstable nuclei to move toward stability.
Natural nuclear reactors operated in Oklo, Gabon, over 1.5 billion years ago. These reactors demonstrated that self-sustaining chain reactions can occur without human intervention under the right geological conditions.