As historian David DeVorkin has written, by the early twentieth century, American astrophysicists were becoming world leaders in astronomical natural history thanks to the unparalleled power of astrophysics to reveal the nature of celestial bodies. “Akin to the naturalist, the typical American professional astronomer was collector and classifier. Instead of museum shelves and cases, astronomers stored their systematic observations in plate vaults and letterpress log books, and displayed them in catalogues sponsored by universities and observatories.” In particular, as more and more stellar spectra were gathered, they hinted at numerous variations in the nature of the stars. Once stellar physics was understood later in the twentieth century the reasons became clear: stellar structure depended on mass, temperature, and luminosity, and the range of all of these physical variables was enormous. Moreover, stars existed in a variety of different ages, and (it turned out) in a variety of stages. Stars were born, lived, and died; once the concept of stellar evolution was accepted, the problem was determining which stars were in which stages and how the physics worked under varying conditions, among the greatest puzzles in the history of science. In short, William Herschel’s gardens, first explored with his work on the nebulae, were luxuriant beyond his wildest dreams when it came to the stars themselves.