Most luminous supernovae produced by shocks

The extremely luminous supernova SN2006gy is explained as some other peculiar supernovae: light is produced by a radiative shock propagating in a dense circumstellar envelope. This envelope is formed by a previous weak explosion at a stage of hydrodynamic instability due to creation of electron-positron pairs in stellar interiors. The problems in the theory and observations of supernovae created by multiple explosions are briefly reviewed.