Beyond the Galaxy: The Development of Extragalactic Astronomy 1885–1965, Part 2 (original) (raw)

2009, Journal for the History of Astronomy

Astronomy underwent a series of transformations between the late nineteenth century and the 1960s that remade the discipline in scientific, technological and institutional terms. 1 In this paper the focus will be on the evolution of observational extragalactic astronomy in these years. It is important to emphasize, however, that at the end of the nineteenth and start of the twentieth century nearly all astronomers would have regarded the pursuit of extragalactic astronomy as pointless. In 1890, Agnes Clerke (1842-1907), in what must be now one of the most famous quotations in the history of modern astronomy, contended in her The system of the stars that "No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of coordinate rank with the Milky Way". 2 She repeated this confident declaration in the second edition of the book published in 1905. 3 But within a few years of this new edition several astronomers were challenging this view. By the late 1920s it had been completely overturned. In 1931 the prominent Dutch astronomer Willem de Sitter (1872-1934) declared that never "in all the history of science has there been a period when new theories and hypotheses arose, flourished, and were abandoned in so quick succession as in the last fifteen or twenty years". 4 De Sitter was reflecting on a period of turmoil in physics and astronomy in which alongside the upheavals wrought by quantum theory and general relativity, the centuries-old debate on the nature of nebulae had finally been settled. The resolution of the debate was due mainly to the efforts of a small group of astronomers who in the first three decades of the twentieth century exploited large optical telescopes and state-of-the-art ancillary instruments at good observing sites in the west of the United States to observe a class of nebulae, the spiral nebulae, with unprecedented clarity. The spirals, they proved, were external galaxies. This finding was then linked to general relativity by a handful of theorists who thereby forged the entirely unexpected discovery of the expansion of the universe. Following the discussion of these developments, I will track the main changes in extragalactic astronomy to the mid-1960s. By this time extragalactic astronomy was very widely pursued. Cosmology had become a much more respectable enterprise than it had been in the 1920s, 1930s, 1940s, and even 1950s, decades in which the properties of stars and their life histories had dominated the interests of astrophysicists in the United States and elsewhere. While the paper will concentrate on observational researches, at some points theoretical issues will also be discussed in order the better to explain the shifting directions and goals of observational investigations.