  Despite the sphericity of the collapsing stellar core, the birth conditions
of neutron stars can be highly non spherical due to a hydrodynamical
instability of the shocked accretion flow. Here we report the first laboratory
experiment of a shallow water analogue, based on the physics of hydraulic
jumps. Both the experiment and its shallow water modeling demonstrate a robust
linear instability and nonlinear properties of symmetry breaking, in a system
which is one million times smaller and about hundred times slower than its
astrophysical analogue.
