By the early 20th century, it was possible to see and measure a very different Universe than was accessible to the Ancients, or even to Galileo and Newton. It had many more stars, and it had beautiful collections of stars (then called “nebulae”) with crazy patterns—like spirals. (See the modern image of M31 shown here in color, for a good example of a “Spiral” galaxy.)
Spectroscopy hinted that faraway objects were composed of elements not unlike those in the Sun, and that they were moving with large speeds. Thanks to the explosion of new photographs and spectra, debate and uncertainty about the true size and extent of the Universe began to spread. Famously, in 1920, Harlow Shapley and Heber Curtis debated whether observed “spiral nebulae” were just at the edge of our Milky Way, which essentially was the whole Universe (Shapley) or were in fact their own “island universes” in sea of galaxies, of which the Milky Way was just one (Curtis).
Harlow Shapley took the incorrect Milky-Way-is-everything position in 1920. This position seems odd in hindsight, given that only a short while earlier (in 1912) Henrietta Leavitt, a colleague of Shapley’s at the Harvard College Observatory, published what is now known as “Leavitt’s Law,” that allows measurement of distances using a relation between periodic changes in the brightness of Cepheid variable stars and their intrinsic brightness. Shapley in fact over-estimated distances in his work, but still clung to a gigantic Milky Way being the full Universe. Somewhat ironically, it was also Leavitt’s Law that let Edwin Hubble, who shared Heber Curtis’ view that spiral nebulae were galaxies beyond the Milky Way, measure the distances that later unlocked the secrets revealed by the Hubble Law.