Episodes in the discovery of variations in the chemical composition of stars and galaxies

Five episodes from the 1950s are reviewed that led to a fundamental change in prevalent hypotheses of how, when, and where the chemical elements were formed. The episodes are (1) the discovery that all stars do not have the s ame abundance ratios of heavy elements to hydrogen, and the inventions of m ethods to detect and measure the variations, culminating in the fundamental paper by Chamberlain and Aller; (2) Roman's discovery of the ultraviolet e xcess in 17 high-velocity F subdwarfs, and the subsequent discovery of the excess in globular cluster stars; (3) the development of the understanding of the physics of the H-R diagram and the evolution of stars, explaining th e various sequences in that diagram; (4) Hoyle's idea of the recyclying of the products of nucleosynthesis from stellar interiors of evolving stars th rough the interstellar medium by stellar mass loss and supernovae explosion s; and (5) the development of the theory of the various nuclear processes i n stellar interiors by a small number of different buildup mechanisms (equi librium burning, slow neutron capture using normally produced neutrons, pro ton capture, and fast neutron capture in supernova explosions) that can pro duce all the chemical elements.