Until a few decades ago, the interior of a star was something of a “black box” for the observing astronomer, shielded from the prying eyes of astronomers by a photosphere that was only wafer-thin compared to its diameter. What was hidden underneath could only be deduced by means of theoretical considerations—and with considerable success. The path to a logically consistent physical theory of the inner structure of stars and stellar evolution, which was paved with trials and tribulations, has already been described in detail in the first chapter of this book.