The Great Correlation Era in Astronomy

Before astrophysics became truly physical in the 1920s, it was typically described as either "photographic" or "spectroscopic," where empirical mapping campaigns made it possible to intercompare the observed properties of the stars. From this effort, many correlations were established including the HR diagram, spectroscopic parallaxes, a mass-luminosity relation and a period-luminosity relation. But what did they mean?, some astronomers asked. Some speculated about what they implied about stellar development, or about the history of the structure and nature of the universe. But what did these correlations really mean, physically? And were they even real? We explore this question and explore how it resulted in the creation of modern astrophysical practice.