The addition of soap to vortexing water produces bubble structures around the vortices’ ‘throats’
that resemble galaxies of stars. Such soap-bubble models can be used to suggest the prediction that,
at the heart of spiral galaxies may lie supermassive black holes. Combined with earlier work on
illuminating black holes, these simple models could be useful classroom demonstrations of the
extreme consequences of space-time distortion predicted by general relativity.