Although scientists identified and analyzed many of the elements in the Periodic Table in the nineteenth century, it was not until the mid-twentieth century that the origins of the heavier elements could be determined.
The lightest elements were produced by the Big Bang in the form of isotopes of hydrogen, helium and trace lithium nuclei. These nuclei were transformed into atoms some 400 million years later when the universe had cooled enough to allow atoms to form (that is, the bonding of electrons with atomic nuclei).
The earliest stars formed from the abundant hydrogen clouds. Stars are nuclear fusion furnaces, and the fusion converts hydrogen to helium, helium into the next heaviest element, and so on, in a process known as nucleosynthesis. At the end of a star's life when the star collapses, its gases comprised of these elements are strewn out into the void, forming stardust clouds. The next generation of stars incorporates those elements and the process continues.
This process was generally understood by the 1950s. But Fred Hoyle and other leading physicists were increasingly concerned to account for the heavy elements like uranium, plutonium and gold. Hoyle originally thought that ordinary stars like our Sun must produce such elements, but gradually realized this was not possible. By good fortune, Hoyle learned about supernovae, the explosion of super-massive stars. He was then able to hypothesize that supernovae were the source of the heavy elements. This is common knowledge today, but it took a long time for scientists to understand the origin of heavy elements.