  Simple geometrical considerations show that the collision zone in high energy
nuclear collisions may be divided into a central part (``core''), with high
energy densities, and a peripheral part (``corona''), with smaller energy
densities, more like in pp or pA collisions. We present calculations which
allow to separate these two contributions, and which show that the corona
contribution is quite small (but not negligible) for central collisions, but
gets increasingly important with decreasing centrality. We will discuss
consequences concerning results obtained in heavy ion collisions at the
Relativistic Heavy Ion Collider (RHIC) and the Super Proton Synchrotron (SPS).
