  We introduce a simple variational principle for coherent material vortices in
two-dimensional turbulence. Vortex boundaries are sought as closed stationary
curves of the averaged Lagrangian strain. Solutions to this problem turn out to
be mathematically equivalent to photon spheres around black holes in cosmology.
The fluidic photon spheres satisfy explicit differential equations whose
outermost limit cycles are optimal Lagrangian vortex boundaries. As an
application, we uncover super-coherent material eddies in the South Atlantic,
which yield specific Lagrangian transport estimates for Agulhas rings.
