  The merger of two neutron stars is predicted to give rise to three major
detectable phenomena: a short burst of gamma-rays, a gravitational wave signal,
and a transient optical/near-infrared source powered by the synthesis of large
amounts of very heavy elements via rapid neutron capture (the r-process). Such
transients, named "macronovae" or "kilonovae", are believed to be centres of
production of rare elements such as gold and platinum. The most compelling
evidence so far for a kilonova was a very faint near-infrared rebrightening in
the afterglow of a short gamma-ray burst at z = 0.356, although findings
indicating bluer events have been reported. Here we report the spectral
identification and describe the physical properties of a bright kilonova
associated with the gravitational wave source GW 170817 and gamma-ray burst GRB
170817A associated with a galaxy at a distance of 40 Mpc from Earth. Using a
series of spectra from ground-based observatories covering the wavelength range
from the ultraviolet to the near-infrared, we find that the kilonova is
characterized by rapidly expanding ejecta with spectral features similar to
those predicted by current models. The ejecta is optically thick early on, with
a velocity of about 0.2 times light speed, and reaches a radius of about 50
astronomical units in only 1.5 days. As the ejecta expands, broad
absorption-like lines appear on the spectral continuum indicating atomic
species produced by nucleosynthesis that occurs in the post-merger fast-moving
dynamical ejecta and in two slower (0.05 times light speed) wind regions.
Comparison with spectral models suggests that the merger ejected 0.03-0.05
solar masses of material, including high-opacity lanthanides.
