Where does gold come from?

Gold and the other heaviest stable elements are made by the rapid neutron-capture process, which requires enormous neutron densities; the leading site is the merger of two neutron stars, a connection supported by the 2017 multi-messenger observation of such an event.

What is the difference between the s-process and r-process?

Both build heavy elements by neutron capture, but the s-process captures neutrons slowly so unstable nuclei decay between captures, while the r-process captures them so rapidly that nuclei become very neutron-rich before decaying, reaching elements the s-process cannot make.

s-Process and r-Process Nucleosynthesis

Elements heavier than iron cannot be made by fusion; instead they are built up by nuclei capturing free neutrons, a process that runs slowly in evolved stars and rapidly in cataclysmic events such as neutron-star mergers.

Definition

The s-process and r-process are the slow and rapid neutron-capture nucleosynthesis pathways, respectively, that build elements heavier than iron through successive neutron captures and beta decays under low and high neutron densities.

Scope

The topic covers the two main neutron-capture processes: the slow s-process, in which neutron captures are slow compared with beta decay and which operates in asymptotic giant branch and massive stars, and the rapid r-process, in which captures vastly outpace beta decay in neutron-rich environments such as neutron-star mergers and certain supernovae, together with the abundance peaks each leaves behind.

Core questions

How are elements heavier than iron created?
What distinguishes the slow from the rapid neutron-capture process?
Where in the universe does each process occur?
Why do the element abundances show distinct s-process and r-process peaks?

Key concepts

neutron capture
beta decay
s-process
r-process
magic numbers
asymptotic giant branch
neutron-star merger

Key theories

The slow neutron-capture process: When neutron captures are slow compared with the beta decay of unstable nuclei, the buildup follows the valley of stability one step at a time; this s-process operates in helium-shell-burning asymptotic giant branch stars and produces roughly half the elements beyond iron.
The rapid neutron-capture process: Under very high neutron densities, nuclei capture many neutrons before they can decay, driving material far to the neutron-rich side of stability; this r-process, hosted in neutron-star mergers and rare supernovae, makes the heaviest elements including the actinides.

Mechanisms

A seed nucleus captures a free neutron to become a heavier isotope; if the new isotope is unstable it eventually beta-decays to the next element. When captures are slow the path hugs stability, but when neutrons are abundant the nucleus captures many before decaying, reaching very neutron-rich isotopes that decay back to stability after the neutron flux ends, leaving characteristic abundance peaks at the nuclear magic numbers.

Clinical relevance

Neutron-capture processes account for the origin of about half the elements heavier than iron, including gold, platinum, and uranium; identifying their sites, confirmed for the r-process by the 2017 neutron-star merger, is central to understanding galactic chemical evolution and the abundances measured in old stars.

History

The s-process and r-process were distinguished in the 1957 B2FH review and Cameron's independent work; the s-process site in asymptotic giant branch stars was established through later modeling, and the r-process site was strongly tied to neutron-star mergers after the 2017 gravitational-wave and electromagnetic detection of such a merger.

Debates

The dominant astrophysical site of the r-process: Whether neutron-star mergers alone produce the bulk of r-process elements, or whether rare supernovae such as magnetorotational explosions also contribute significantly, remains under study; abundance patterns in old stars and the timing of enrichment provide competing constraints.

Key figures

Margaret Burbidge
Alastair Cameron
Friedrich-Karl Thielemann
John Cowan

Seminal works

b2fh1957
cowan2021

Frequently asked questions

Where does gold come from?: Gold and the other heaviest stable elements are made by the rapid neutron-capture process, which requires enormous neutron densities; the leading site is the merger of two neutron stars, a connection supported by the 2017 multi-messenger observation of such an event.
What is the difference between the s-process and r-process?: Both build heavy elements by neutron capture, but the s-process captures neutrons slowly so unstable nuclei decay between captures, while the r-process captures them so rapidly that nuclei become very neutron-rich before decaying, reaching elements the s-process cannot make.