Chapter 3

In this chapter of A Brief History of Time, Hawking discusses the discovery and understanding of the Milky Way and other celestial bodies. He explains how astronomers have been able to determine the types and compositions of stars and galaxies through the study of their light spectra and thermal spectra. He also discusses the concept of the expanding universe, which was supported by the observations of the red shift in the spectra of distant objects and the microwave radiation measurements in the 1960s. This expanding universe suggests that all objects in the universe must have once been much closer together, implying that the universe had a beginning and may have an end at some point in the future. Hawking also discusses his own contributions to the study of the big bang theory, which he developed with Roger Penrose. He explains how the big bang theory has been supported by various observations and experiments, and how it has helped scientists to better understand the origin and evolution of the universe.