These largely historical remarks were meant for those in the audience that had not followed the developments in recent years.

One of the great achievements of astrophysics in this century was the discovery of the source of the solar (and stellar) energy: the fusion of hydrogen into helium. It became also clear that once the conditions for hydrogen burning become unfavourable the star switches to the burning of helium: a contraction of the central region increases the temperature and pressure until the helium starts to burn into carbon and oxygen. In this way one imagines that stars during their lifetime go through successive phases of atomic fusion, with the endproduct of the previous phase as the fusion element for the one following. This succession can continue until the iron core is reached; thereafter the fusion becomes exothermic, and is no longer a viable source of energy; perhaps a supernova should follow here. As we know now (and will be discussed here), this succession is interrupted by mass loss; most stars do not develop beyond the second core, one of carbon and oxygen.