1 - Rate of formation of stars suitable for the development of intelligent life, R*, pre-1961

Summary

Abstract

The rate of star formation, past and present, in the galaxy is an important astronomical question that lies at the heart of our modern understanding of the nature of stars, their distribution in space and time, and the history of the galaxy itself. Here we look at history, to 1961, concentrating on the question of how stars form and what is the rate of formation of stars in the galaxy. It is related to what astronomers call the luminosity function for stars, or the frequency distribution of luminosities for all stars in the galaxy. Our recounting will encompass the correlation era of stellar statistics, when astronomers began to conceptualize evolutionary models for the galaxy: that the luminosity function changes with time, and that the rate of change is determined not only by the rate of star formation but also by the nature and rate of the evolution of stars once they have been formed. We will show that, throughout much of the first half of the twentieth century, these rates were debated and revised many times, first by the recognition that there exists a main sequence of stars, which was interpreted as the distribution of stars existing in some form of quasi-stability. Later came the recognition of giant stars and instability, then the mass–luminosity law for stars, then an awareness of the energy sources of the stars and the subsequent revision of the evolutionary place and nature of giant stars, and finally, the fact that populations of stars existed within the galaxy that were generationally distinct. Based on this review, we will then argue that Drake estimated R* (regarded by most as the number of stars forming per year in the galaxy) in a manner consistent with the state of knowledge at the time.