Following pioneering research into the
Big Bang and the formation of
helium in stars, an unknown process responsible for producing heavier elements found on Earth from
hydrogen and helium was suspected to exist. One early attempt at explanation came from
Subrahmanyan Chandrasekhar and Louis R. Henrich who postulated that elements were produced at temperatures between 6 billion and 8 billion
K. Their theory accounted for elements up to
chlorine, though there was no explanation for elements of
atomic weight heavier than 40
amu at non-negligible abundances. This became the foundation of a study by
Fred Hoyle, who hypothesized that conditions in the core of collapsing stars would enable nucleosynthesis of the remainder of the elements via rapid capture of densely packed free neutrons. However, there remained unanswered questions about equilibrium in stars that was required to balance beta-decays and precisely account for
abundances of elements that would be formed in such conditions. Their abundance table revealed larger than average abundances of natural isotopes containing
magic numbers of neutrons as well as abundance peaks about 10 amu lighter than
stable nuclei containing magic numbers of neutrons which were also in abundance, suggesting that radioactive neutron-rich nuclei having the magic neutron numbers but roughly ten fewer protons were formed. These observations also implied that rapid neutron capture occurred faster than
beta decay, and the resulting abundance peaks were caused by so-called
waiting points at magic numbers.
Alastair G. W. Cameron also published a smaller study about the
r-process in the same year. The stationary
r-process as described by the B2FH paper was first demonstrated in a time-dependent calculation at
Caltech by Phillip A. Seeger,
William A. Fowler and
Donald D. Clayton, who found that no single temporal snapshot matched the solar
r-process abundances, but, that when superposed, did achieve a successful characterization of the
r-process abundance distribution. Shorter-time distributions emphasize abundances at atomic weights less than , whereas longer-time distributions emphasized those at atomic weights greater than . Subsequent treatments of the
r-process reinforced those temporal features. Seeger et al. were also able to construct more quantitative apportionment between
s-process and
r-process of the abundance table of heavy isotopes, thereby establishing a more reliable abundance curve for the
r-process isotopes than B2FH had been able to define. Today, the
r-process abundances are determined using their technique of subtracting the more reliable
s-process isotopic abundances from the total isotopic abundances and attributing the remainder to
r-process nucleosynthesis. That
r-process abundance curve (vs. atomic weight) has provided for many decades the target for theoretical computations of abundances synthesized by the physical
r-process. The creation of free neutrons by electron capture during the rapid collapse to high density of a supernova core along with quick assembly of some neutron-rich seed nuclei makes the
r-process a
primary nucleosynthesis process, a process that can occur even in a star initially of pure H and He. This in contrast to the B2FH designation which is a
secondary process building on preexisting iron. Primary stellar nucleosynthesis begins earlier in the galaxy than does secondary nucleosynthesis. Alternatively the high density of neutrons within neutron stars would be available for rapid assembly into
r-process nuclei if a collision were to eject portions of a neutron star, which then rapidly expands freed from confinement. That sequence could also begin earlier in galactic time than would
s-process nucleosynthesis; so each scenario fits the earlier growth of
r-process abundances in the galaxy. Each of these scenarios is the subject of active theoretical research. Observational evidence of the early
r-process enrichment of interstellar gas and of subsequent newly formed stars, as applied to the abundance evolution of the galaxy of stars, was first laid out by
James W. Truran in 1981. He and subsequent astronomers showed that the pattern of heavy-element abundances in the earliest metal-poor stars matched that of the shape of the solar
r-process curve, as if the
s-process component were missing. This was consistent with the hypothesis that the
s-process had not yet begun to enrich interstellar gas when these young stars missing the
s-process abundances were born from that gas, for it requires about 100 million years of galactic history for the
s-process to get started whereas the
r-process can begin after two million years. These
s-process–poor,
r-process–rich stellar compositions must have been born earlier than any
s-process, showing that the
r-process emerges from quickly evolving massive stars that become supernovae and leave neutron-star remnants that can merge with another neutron star. The primary nature of the early
r-process thereby derives from observed abundance spectra in old stars See
Astrophysical sites below. ==Nuclear physics==