More Details on Hubble and Shapley

I noticed Sidney van den Bergh’s letter on Edwin Hubble and Harlow Shapley in the September 2004 issue of Physics Today (page 15), and I believe a clarification about the “Different Views” of these two observational astronomers is in order. Any reader not familiar with the history of galactic astronomy would be deprived of a most important footnote, namely the distinguishing definitions of the universe in the early 20th century.

Van den Bergh is essentially correct when he states that the discovery of “the existence of galaxies beyond the outer limits of our Milky Way system” is much to the credit of these men. However, he does not qualify that into the 1920s there were two diverging theories about the extent of the universe: the old theory that defined the Milky Way galaxy as the universe, including all the “spiral nebulae” observed for over a century and denoted as gas clouds, within the Milky Way boundary; and the new one, that these other spirals were, on the contrary, external individual galaxies, coined as “island universes.”

Shapley was the central Figure of the former opinion based on his conclusions while observing globular clusters at Mount Wilson during the years 1914 to 1917. He declared that these clusters—and essentially all others—were confined within the Milky Way boundary. To the north, at Lick Observatory of the University of California, Berkeley, Heber D. Curtis had been studying for some 10 years the spiral nebulae that Shapley assumed without investigation to be gas nebulae, at distances similar to those of the clusters Shapley had studied. Curtis concluded they were spiral galaxies, and he became the chief proponent of the external universe view that the Milky Way was only one of many galaxies in a much larger universe. The scale of the universe became the central theme of the 1920 meeting of the National Academy of Sciences in Washington, DC. What has been called the “Great Debate” during that meeting was presentations by Shapley for the old theory and Curtis for the new. But it was never a debate; Shapley did not want to get into what Curtis projected enthusiastically to be a “scrap” over the two views. Shapley presented a safe astronomy lecture, whereas Curtis successfully made his point with a powerful, comprehensive research presentation.

The real debate was in the seesaw of papers that characterized the two views thereafter into the 1920s. Hubble’s work at Mount Wilson began in 1919, where he used the recently completed and largest reflector in the world, the Hooker 100-inch, to also concentrate on the spiral nebulae. He and Shapley had very personal and professional differences that developed in the short time they were both at Mount Wilson. Hubble became convinced that the nebulae of the controversy were galaxies, and his proof came in early 1924 when he determined that the so-called “Andromeda Nebula” was a separate galaxy far beyond the Milky Way.

So Shapley’s universe was profoundly smaller than the one Hubble revealed as an emerging cosmos—though the general applications of Shapley’s work deserve all the credit of astronomical history.