This book presents a detailed pedagogical account of the equation of state and its applications in several important and fast growing topics in theoretical physics, chemistry and engineering. This book is the storv of the analysis of starlight by astronomical spectroscopy. It describes the development of the subject from the time of Joseph Fraunhofer, who, in 1814, used a telescope-mounted prism to observe the spectral light emitted from several bright stars. He discovered that light was missing at certain colours (wavelengths) in the starlight, and these so-called spectral lines were subsequently shown to hold clues to the nature of the stars themselves. The book explains how the classification of stars using their line spectra developed into a major branch of astronomy whilst new methods in astrophysics made possible the approximate quantitative analysis of spectral lines in the 1920s and 1930s. After the Second World War these techniques were considerably improved when computers were programmed to model the structure of the outer layers of stars. Basic concepts in spectroscopy and spectral analysis are also covered and. finally. Dr Hearnshaw comments on the stellar spectroscopy of some individual star.