The problem
Some heavy elements, including gold, form when atomic nuclei capture neutrons rapidly. Which cosmic events supplied these elements throughout our Galaxy?[1]
Which astrophysical sites account for the Galaxy’s rapid-neutron-capture elements and their changing abundance patterns? The r-process requires neutron capture to outpace the relevant beta decays, driving nuclei away from stable compositions before they decay toward longer-lived products.
The merger-associated event GRB 230307A provides one observation. JWST spectroscopy 29 and 61 days after the burst includes a 2.15-micrometre emission line interpreted as tellurium, alongside a red kilonova consistent with lanthanide production. It supports heavy-element synthesis in a compact-object merger. It does not, by itself, measure every merger’s yield or establish the complete Galactic contribution.[1] [2]
Scope
Relative contributions, yields and time histories of r-process sites across Galactic chemical evolution.
What remains open
The 2026 review treats compact binary mergers as major contributors while retaining additional candidate sites. Old metal-poor stars and differing abundance patterns help test which events could have enriched material at different stages of Galactic evolution.
The accounting couples several unknowns: how frequently a site occurs, how much and which material it ejects, uncertain nuclear reactions, and how the ejecta enter later stars. Even the Solar System r-process pattern is inferred in part by subtracting a modeled slow-neutron-capture contribution. A match to one pattern does not uniquely select a site mixture.[1]
Why it matters
The answer would trace the origins of heavy elements to particular stellar deaths and mergers. It would explain how material made in rare events became part of later stars and, eventually, the Solar System.[1]
This is an origin story for physical ingredients such as gold and other heavy elements, connecting nuclei formed in brief events with material incorporated into later stellar systems. It also makes chemical abundance measurements a record of the Galaxy’s past.
An established site accounting would let researchers interpret different stellar abundance patterns in terms of event histories and nuclear conditions. The connection runs both ways: observations of old stars can test proposed explosive environments, while nuclear measurements constrain which elements those environments can produce.[1]
Definitions
3 terms
neutron capture
An atomic nucleus absorbs a neutron. In the r-process, repeated captures can occur faster than the relevant beta decays.[1]
beta decays
Radioactive changes that can turn a neutron in a nucleus into a proton. After rapid neutron capture, such decays move the products toward longer-lived nuclei.[1]
kilonova
Light produced by expanding merger ejecta as newly formed radioactive nuclei decay. Its spectrum can provide evidence about the elements produced.[2]
Sources
- The r-process: history, required conditions, astrophysical sites, and observations
Notes on The r-process: history, required conditions, astrophysical sites, and observations
Selected review introduction, timescale and conclusion passages discuss merger and additional-site contributions, uncertain yields, rates and nuclear inputs. Solar residuals partly depend on modeled slow-process subtraction; the complete reaction network was not audited.
- Heavy-element production in a compact object merger observed by JWST
Notes on Heavy-element production in a compact object merger observed by JWST
Selected abstract and main text report JWST spectra 29 and 61 days after GRB 230307A, including a 2.15 micrometre line interpreted as tellurium. This supports merger production, not the complete Galactic accounting; first online 2023 is distinct from the 2024 issue.