Resumen : The Nobel Prize in Physics 2011 was recently awarded to astronomers Saul Perlmutter, Brian Schmidt and Adam Riess, for the discovery of the accelerating expansion of the Universe. The observation of supernovae, which are exploding stars, is the base of this discovery. In this colloquium, Prof. Mario Hamuy will summarize the key role of the Calán / Tololo project, held in Chile, prior to this discovery.