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Abstract

Half of all of the elements in the Universe that are heavier than iron were created by
rapid neutron capture. The theory underlying this astrophysical r-process was worked
out six decades ago, and requires an enormous neutron flux to make the bulk of the
elements¹. Where this happens is still debated². A key piece of evidence would be the
discovery of freshly synthesized r-process elements in an astrophysical site. Existing
models^3–5 and circumstantial evidence⁶ point to neutron-star mergers as a probable
r-process site; the optical/infrared transient known as a ‘kilonova’ that emerges in the
days after a merger is a likely place to detect the spectral signatures of newly created
neutron-capture elements^7–9. The kilonova AT2017gfo—which was found following the
discovery of the neutron-star merger GW170817 by gravitational-wave detectors¹⁰—was
the first kilonova for which detailed spectra were recorded. When these spectra were
first reported^11,12, it was argued that they were broadly consistent with an outflow of
radioactive heavy elements; however, there was no robust identification of any one
element. Here we report the identification of the neutron-capture element strontium in
a reanalysis of these spectra. The detection of a neutron-capture element associated
with the collision of two extreme-density stars establishes the origin of r-process
elements in neutron-star mergers, and shows that neutron stars are made of neutronrich
matter¹³.