  JWST observations of galaxies at $z\gtrsim 8$ suggest that they are more
luminous and clumpier than predicted by most models, prompting several
proposals on the physics of star formation and feedback in the first galaxies.
In this paper, we focus on the role of ultraviolet (UV) radiation in regulating
star formation by performing a set of cosmological radiation hydrodynamics
simulations of one galaxy at sub-pc resolution with different radiative
feedback models. We find that the suppression of cooling by far UV (FUV)
radiation (i.e., $\mathrm{H_2}$ dissociating radiation) from Pop II stars is
the main physical process triggering the formation of compact and massive star
clusters and is responsible for the bursty star formation observed in
metal-poor galaxies at $z\gtrsim 10$. Indeed, artificially suppressing FUV
radiation leads to a less intense continuous mode of star formation distributed
into numerous, but low-mass open star clusters. Due to the intense FUV field,
low-metallicity clouds remain warm ($\sim 10^4\,\mathrm{K}$) until they reach a
relatively high density ($\gtrsim 10^3\,\mathrm{cm^{-3}}$), before becoming
self-shielded and transitioning to a colder ($\sim 100\,\mathrm{K}$), partially
molecular phase. As a result, star formation is delayed until the clouds
accumulate enough mass to become gravitationally unstable. At this point, the
clouds undergo rapid star formation converting gas into stars with high
efficiency. We, therefore, observe exceptionally bright galaxies (ten times
brighter than for continuous star formation) and subsequent quenched "dead"
galaxies that did not form stars for tens of Myrs.
