  We present a simple analytic model, that captures the key features of the
emission of radiation from material ejected by the merger of neutron stars
(NS), and construct the multi-band and bolometric luminosity light curves of
the transient associated with GW170817, AT\,2017gfo, using all available data.
The UV to IR emission is shown to be consistent with a single
$\approx0.05$\,M$_\odot$ component ejecta, with a power-law velocity
distribution between $\approx 0.1\,c$ and $>0.3\,c$, a low opacity,
{$\kappa<1$\,cm$^2$\,g$^{-1}$}, and a radioactive energy release rate
consistent with an initial $Y_{\rm e}<0.4$. The late time spectra require an
opacity of $\kappa_\nu\approx0.1$\,cm$^2$\,g$^{-1}$ at 1 to $2\mu$m. If this
opacity is provided entirely by Lanthanides, their implied mass fraction is
$X_{\rm Ln}\approx10^{-3}$, approximately 30 times below the value required to
account for the solar abundance. The inferred value of $X_{\rm Ln}$ is
uncertain due to uncertainties in the estimates of IR opacities of heavy
elements, which also do not allow the exclusion of a significant contribution
to the opacity by other elements (the existence of a slower ejecta rich in
Lanthanides, that does not contribute significantly to the luminosity, can also
not be ruled out). The existence of a relatively massive, $\approx
0.05$\,M$_\odot$, ejecta with high velocity and low opacity is in tension with
the results of numerical simulations of NS mergers.
