Introduction

By 1945, many of the physical processes involved in the evolution of stars on the main sequence were beginning to be understood, but there remained an enormous amount of detailed work to be undertaken before a precise comparison between theory and observation could be made. To build detailed models of the stars, three types of data are required. The first is the equation of state of the material of the star; the second are accurate nuclear reaction rates; and the third is the opacity of stellar material for the transfer of radiation. These quantities need to be known for the wide ranges of temperature and density encountered inside the stars. Then, the problems of radiation transfer through the body of the star and its surface layers have to be solved so that meaningful comparisons can be made between the theory and observations. As a result, the astrophysicists had to have access to a very wide range of data from nuclear, atomic and molecular physics, which began to become available with the great expansion in the funding for the physical sciences after the Second World War.

Then, there was the need to develop models for the evolution of stars from one region of the Hertzsprung–Russell diagram to another. It was a daunting task, but there was light at the end of the tunnel with the development of high-speed digital computers in the 1950s and 1960s, which was to convert the study of the structure and evolution of the stars into a precise astrophysical science. The new wavebands brought important new insights into many of the key phases of stellar evolution using techniques which could not have been imagined by the pioneers of the first half of the twentieth century.