  Shell galaxies make a class of tidally distorted galaxies, characterised by
wide concentric arc(s), extending out to large galactocentric distances with
sharp outer edges. Recent observations of young massive star clusters in the
prominent outer shell of NGC 474 suggest that such systems host extreme
conditions of star formation. In this paper, we present a hydrodynamic
simulation of a galaxy merger and its transformation into a shell galaxy. We
analyse how the star formation activity evolves with time, location-wise within
the system, and what are the physical conditions for star formation. During the
interaction, an excess of dense gas appears, triggering a starburst, i.e. an
enhanced star formation rate and a reduced depletion time. Star formation
coincides with regions of high molecular gas fraction, such as the galactic
nucleus, spiral arms, and occasionally the tidal debris during the early stages
of the merger. Tidal interactions scatter stars into a stellar spheroid, while
the gas cools down and reforms a disc. The morphological transformation after
coalescence stabilises the gas and thus quenches star formation, without the
need for feedback from an active galactic nucleus. This evolution shows
similarities with a compaction scenario for compact quenched spheroids at
high-redshift, yet without a long red nugget phase. Shells appear after
coalescence, during the quenched phase, implying that they do not host the
conditions necessary for in situ star formation. The results suggest that
shell-forming mergers might be part of the process of turning blue late-type
galaxies into red and dead early-types.
