Science wars: The island universe

April 26, 1920, two astronomers publicly debated the structure of the cosmos. Is the Milky Way everything there is, or is it just one of many “island” universes?

I sometimes feel guilty about looking back at great debates in science. We all know who won, and the result seems so obvious in hindsight. But at the time the participants were grappling with the complex boundaries of established knowledge, so we should cut them some slack and admire their exploration of the possible.

A hundred years ago, the boundaries of the possible were at the border of the Milky Way. A scientist named Harlow Shapley had recently demonstrated that the Milky Way galaxy was ten times larger than had previously been estimated – and that our solar system was not at its centre. Both were remarkable findings that gave us a much better idea of the scale of the universe. To put it bluntly, it’s much larger than anyone thought before.

Because the Milky Way was so ridiculously large, Shapley seems to have been of the opinion that the Milky Way was the whole universe. There was just one problem with that idea: spiral nebulae. These funny smudges had been observed for many hundreds of years, and a few historical figures (for example, the 18th century philosophers Emanuel Swedenborg and Immanuel Kant) had suggested that the Milky Way may be just one of several independent collections of stars. Those spiral nebulae were not part of the Milky Way. Instead, we live in a cosmos of island universes.

Heber Curtis believed that those weird smudges were far beyond the Milky Way. If Shapley’s recalculation of the size of the Milky Way was intimidatingly large, Curtis’ calculations about the distance to these island universes was mind-boggling. Nevertheless, Curtis had facts on his side. Star explosions (novae) in the spiral nebulae were much fainter than in the rest of the Milky Way – which would make sense if the nebulae contained thousands of their own stars and were much further away. The spiral nebulae had dust clouds near their centre, much like the Milky Way.

The two scientists faced off in a public debate hosted by the Smithsonian in Washington, D. C. There wasn’t actually a lot of debate (Shapley was angling for a position at the Harvard College Observatory and so soft-pedalled his arguments) but the event was later seen as so significant to the field of astronomy that today it is known simply as the Great Debate.

Curtis was correct, of course: the so-called spiral nebulae are actually island universes. Today we call them galaxies, and there are billions of them.

• Great Debate
• The Great Debate of Shapley and Curtis — 100 years later
• Galaxy