Collapse of a uniform density sphere with an isothermal equation of state and dimensionless isothermal sound speed c_s = 1. The figure shows the analytic solution for pressure-less freefall collapse (solid red lines) and the radial position of the 90%, 50% and 10% mass radii (blue dashed lines). The black dot-dashed line shows the analytic solution for the progression of the rarefaction wavefront.