The red line shows the radius of a SN shock R_SN(t) as a function of time. Up to roughly 100 yr the SN shock expands freely, then the Sedov-Taylor expansion sets in. The available maximum radius for SNe R_SN,max(t) as a function of time is shown for the case of a spherical halo by the blue line. This radius decreases in time, as the number of SN in the protogalactic core increases. The first SN shocks collide after a time of approximately 0.36 Myr.