Shapley-Curtis debate

The Great Debate, also called the Shapley–Curtis Debate, was held on 26 April 1920 at the Smithsonian Museum of Natural History, between the astronomers Harlow Shapley and Heber Curtis. It concerned the nature of so-called spiral nebulae and the size of the universe; Shapley believed that distant nebulae were relatively small and lay within the outskirts of the Earth's home galaxy, while Curtis held that they were in fact independent galaxies, implying that they were exceedingly large and distant.

The two scientists first presented independent technical papers about "The Scale of the Universe" during the day and then took part in a joint discussion that evening. Much of the lore of the Great Debate grew out of two papers published by Shapley and by Curtis in the May 1921 issue of the Bulletin of the National Research Council. The published papers each included counter arguments to the position advocated by the other scientist at the 1920 meeting.

In the aftermath of the public debate, scientists have been able to verify individual pieces of evidence from both astronomers but on the main point of the existence of other galaxies, Curtis has been proven correct.

Shapley was arguing in favor of the Milky Way as the entirety of the then known universe. He believed that "spiral nebulae" such as Andromeda were simply part of the Milky Way. He could back up this claim by citing relative sizes—if Andromeda were not part of the Milky Way, then its distance must have been on the order of 10⁸light years—a span most astronomers would not accept. Adriaan van Maanen was also providing evidence to Shapley's argument. Van Maanen was a well-respected astronomer of the time who claimed he had observed the Pinwheel Galaxy rotating. If the Pinwheel Galaxy were in fact a distinct galaxy and could be observed to be rotating on a timescale of years, its orbital velocity would be enormous and there would clearly be a violation of the universal speed limit, the speed of light. Also used to back up his claims was the observation of a nova in the Andromeda "nebula" that had briefly outshone the entire nebula, constituting a seemingly impossible output of energy were Andromeda in fact a separate galaxy.

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