  The most distant galaxies detected by JWST are assembling in a Universe that
is less than 5\% of its present age. At these times, the progenitors of
galaxies like the Milky Way are expected to be about 10,000 times less massive
than they are now, with masses quite comparable to that of massive globular
clusters seen in the local Universe. Composed today primarily of old stars and
correlating with the properties of their parent dark matter halos, the first
globular clusters are thought to have formed during the earliest stages of
galaxy assembly. In this article we explore the connection between star
clusters and galaxy assembly by showing JWST observations of a strongly lensed
galaxy at zspec = 8.304, exhibiting a network of massive star clusters (the
'Firefly Sparkle') cocooned in a diffuse arc. The Firefly Sparkle exhibits the
hallmarks expected of a future Milky Way-type galaxy captured during its
earliest and most gas-rich stage of formation. The mass distribution of the
galaxy seems to be concentrated in ten distinct clusters, with individual
cluster masses that straddle the boundary between low-mass galaxies and
high-mass globular clusters. The cluster ages suggest that they are
gravitationally bound with star formation histories showing a recent starburst
possibly triggered by the interaction with a companion galaxy at the same
redshift at a projected distance of $\sim$2 kpc away from the Firefly Sparkle.
The central star cluster shows nebular-dominated spectra consistent with high
temperatures and a top-heavy initial mass function, the product of formation in
a very metal poor environment. Combined with abundance matching that suggests
that this is likely to be a progenitor of galaxies like our own, the Firefly
Sparkle provides an unprecedented case study of a Milky Way-like galaxy in the
earliest stages of its assembly in only a 600 million year old Universe.
