Fred Hoyle noted that, even while the distribution of elements was fairly uniform, different stars had varying amounts of each element. To Hoyle, this indicated that they must have originated within the stars themselves. The abundance of elements peaked around the atomic number for iron, an element that could only have been formed under intense pressures and temperatures. Hoyle concluded that iron must have formed within giant stars. From this, in 1945 and 1946, Hoyle constructed the final stages of a star's life cycle. As the star dies, it collapses under its weight, leading to a stratified chain of fusion reactions: carbon-12 fuses with helium to form oxygen-16, oxygen-16 fuses with helium to produce neon-20, and so on up to iron. There was, however, no known method by which carbon-12 could be produced. Isotopes of beryllium produced via fusion were too unstable to form carbon, and for three helium atoms to form carbon-12 was so unlikely as to have been impossible over the age of the Universe. However, in 1952, physicist Ed Salpeter showed that a short enough time existed between the formation and the decay of the beryllium isotope that another helium had a small chance to form carbon, but only if their combined mass/energy amounts were equal to that of carbon-12. Hoyle, employing the anthropic principle, showed that it must be so, since he himself was made of carbon, and he existed. When the matter/energy level of carbon-12 was finally determined, it was found to be within a few percent of Hoyle's prediction.