From J. Silk, The Big Bang, 2nd Ed., © 1989. Reproduced by permission of W.H. Freeman Co., New York

As a massive hydrogen-rich star contracts and experiences greater pressures, helium is the first nuclear product within its core region. The energy released from fusion, along with continuing densification, yields higher temperatures (~ 10⁸ °K) that transmute this innermost helium into carbon while producing new helium at the next outer shell, but with hydrogen still dominant. Once carbon is formed in abundance, this helium is generated as an end product of the CNO cycle. In this, some C¹² reacts with protons to generate, in successive steps, nitrogen 13, 14, and 15 and then oxygen 15. After this last step, that unstable oxygen isotope can decay by fission, thereby releasing an alpha particle (He⁴, stripped of its electrons) causing the reversion to C¹².

Ever greater contraction, with concomitant temperature rises (> 10¹⁰ °K) , can progressively generate the elements listed in the figure up to iron (plus others of lesser atomic numbers) in amounts proportional to the comparative solar masses indicated. Thus, a star massive enough to ultimately achieve an iron core also contains elements of lower atomic numbers in its outer shells, broadly distributed in the relative positions shown in the figure, reflecting response to the fuel to outwardly decreasing densities and temperatures. Iron (atomic number Z [no. of protons]= 26; mass number A [number of protons + neutrons] = 56) is the heaviest element producible directly by stellar fusion. In fusion, nuclear binding energies for the new nuclides increase gradually up to iron but the mass of a fused nuclide is less than the sum of the fusing constituents; the missing mass is converted to energetic particles (E = mc²), released as gamma rays, neutrinos, positrons, and others.

Elements with A greater than Fe have decreasing binding energy and to form require energy input from non-fusion processes (principally neutron capture). Because those stars capable of synthesizing elements up to Fe have masses greater than 10 solar masses, these stars at their end stage of fusion will rapidly (over spans of hundreds of years) collapse and explode (fly apart) as novae or supernovae. This gives rise to intense neutron fluxes that manufacture various elements including those with A > 56 , most of which become rapidly dispersed into interstellar space. These heavier elements, along with H, He and the A < 56 elements (which include O, S, C, N, Fe, Mg, Ca, Al, Na, and K - the dominant constituents making up the planets), can thereafter collect into new nebulae (clouds) that may reorganize into additional stars, setting up further nucleosynthesis. The elements were mostly created in the first few billion years when rates of star formation, burning, and explosive destruction were higher than present, but the process of element production still goes on. Elemental materials not reincorporated in stars are available to organize into compounds that make up the dust, gases, and particles from which planetary bodies are assembled.