IV.By 20 minutes after the Big Bang, essentially all of the matter in the universe was forged by nuclear fusion, into hydrogen and Helium (with small amounts of beryllium and boron). After 20 minutes the universe was too cool to support further fusion. See Steven Weinberg,First Three Minutes for details.
Formation of Stars and Forging of the Elements.11
When neutral atoms formed with the creation of darkness, the matter began to clump and heat up as gravity pulled it together. After about a billion years, the clumps heated up to millions of degrees and ignited into nuclear fusion. The hydrogen and helium fuzed to form heavier elements, beginning with the elements carbon, nitrogen and oxygen that are essential for life. The processes of element formation in the stars were first fully described in 1956 in a famous paper called B2FH14 after its authors' names. This paper described how each element is formed in star-burning (forming elements up to iron, nickel and copper) as well as in the cataclysmic supernova deaths of stars which caused a tremendous flux of radiation that forged all of the heavier elements through uranium, and filled the surrounding space with its debris.
Over time, this debris of supernovas again coalesced under the force of gravity and formed second and higher generation stars, including the sun and the solar system, which formed about 4.65 billion years ago.12
This description in B2FH of how the elements form in star interiors and in supernovas is a remarkable feat, because, of course, it is impossible to directly disect a star to see how it is constructed. But the conclusions of this paper have been confirmed repeatedly in high-energy experiments over the 60+ years since the first publicationIV.03, and there would be few genuine scientists that question its main conclusions (although there is a constant effort to test the validity of the claimed results).