  In the first billion years after the Big Bang, sources of ultraviolet (UV)
photons are believed to have ionized intergalactic hydrogen, rendering the
Universe transparent to UV radiation. Galaxies brighter than the characteristic
luminosity $L^{*}$ do not provide enough ionizing photons to drive this cosmic
reionization. Fainter galaxies are thought to dominate the photon budget;
however they are surrounded by neutral gas that prevents the escape of the
Lyman-$\alpha$ photons, which has been the dominant way to identify them so
far. JD1 was previously identified as a triply-imaged galaxy with a
magnification factor of 13 provided by the foreground cluster Abell 2744, and a
photometric redshift of $z\sim10$. Here we report the spectroscopic
confirmation of this very low luminosity ($\sim0.05 L^{*}$) galaxy at $z=9.79$,
observed 480 Myr after the Big Bang, by means of the identification of the
Lyman break and redward continuum, as well as multiple $\gtrsim4\sigma$
emission lines, with the Near-InfraRed Spectrograph (NIRSpec) and Near-InfraRed
Camera (NIRCam) instruments. The combination of the James Webb Space Telescope
(JWST) and gravitational lensing shows that this ultra-faint galaxy ($M_{\rm
UV}=-17.35$) -- with a luminosity typical of the sources responsible for cosmic
reionization -- has a compact ($\sim$150 pc) and complex morphology, low
stellar mass (10$^{7.19}$ M$_\odot$), and subsolar ($\sim$0.6 $Z_{\odot}$)
gas-phase metallicity.
