  On 17 August 2017, less than two years after the direct detection of
gravitational radiation from the merger of two ~30 Msun black holes, a binary
neutron star merger was identified as the source of a gravitational wave signal
of ~100 s duration that occurred at less than 50 Mpc from Earth. A short GRB
was independently identified in the same sky area by the Fermi and INTEGRAL
satellites for high energy astrophysics, which turned out to be associated with
the gravitational event. Prompt follow-up observations at all wavelengths led
first to the detection of an optical and infrared source located in the
spheroidal galaxy NGC4993 and, with a delay of ~10 days, to the detection of
radio and X-ray signals. This paper revisits these observations and focusses on
the early optical/infrared source, which was thermal in nature and powered by
the radioactive decay of the unstable isotopes of elements synthesized via
rapid neutron capture during the merger and in the phases immediately following
it. The far-reaching consequences of this event for cosmic nucleosynthesis and
for the history of heavy elements formation in the Universe are also
illustrated.
