There are two main processes by which nuclei that fill out the range of β-stable nuclei are formed. The most important process (since it has to happen first) is the so-called r process, an explosion which subjects the nuclei found in old, large stars to a flood of neutrons. This spray of neutrons instantly populates the neutron-drip line and then almost immediately these highly unstable nuclei decay down into the valley of β stability. The other important process is the so-called s process, which occurs over long periods of time in intermediate-mass stars, mainly in the so-called Asymptotic giant branch (AGB). This slow neutron capture process runs right down the valley of β stability; the nuclei formed are close to the valley and decay into it before they have any chance of capturing another neutron.

There are other processes of far less importance, such as proton capture. Observation of stars that formed very early in the history of the universe show an abundance of elements characteristic of the r process alone. While the types of supernovae that explosively compress the core are the standard suspects for r-process events, it is increasingly likely that scenarios involving collisions of neutron stars with other dense or denser objects play a significant role.


Between about 10 to 350 seconds the entire universe had the right temperature and density, and comparable numbers of protons and neutrons, to do fusion in the ordinary nuclear way, which produced the primordial amounts of 4He, deuterium and 3He. Very, very tiny amounts of 7Be and 7Li were also generated. The incredibly rapid drop in density and temperature made this era of Big Bang nucleosynthesis very short and very limited. The astonishing fact that interstellar gas which has never been inside a star is already only 75% protons and the remaining 25% helium made it very clear to researchers in the late 1950s that the entire universe must at one time have been hot and dense enough to fuse protons and neutrons into helium, and that this would have had to occur over a time frame fairly short compared to the free neutron lifetime.

Pioneers of Cosmology