Transcript: Physicists in the nineteenth century made various estimates of the age of the Sun, but they were fundamentally unaware of the most efficient energy source known. Early in the twentieth century physicists Rutherford and Becquerel began a systematic study of the phenomenon of radioactivity, a situation where atoms spontaneously emit both particles and radiation. Rutherford for example sealed a small amount of a radioactive substance in a tube that contained a pure vacuum. He returned months later to find that the tube contained helium gas and that the chemical properties of the radioactive substance had changed. Here was proof both that the atomic nucleus can emit energy and that chemicals can change fundamentally due to radioactive processes. The atomic nucleus could be transformed, and it could emit energy.