Великая дискуссия Шейпли и Кертиса о галактиках.

1920 debate between Harlow Shapley and Heber Curtis on whether there were other galaxies
other topics known as "great debates"
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 these nebulae were relatively small and lay within the outskirts of the Milky Way galaxy (then thought to be the entire universe), 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 counterarguments 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 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 108 light years—a span most contemporary astronomers would not accept. Adriaan van Maanen, a well respected astronomer of the time, also provided evidence supporting Shapley's argument. Van Maanen claimed he had observed the Pinwheel Galaxy rotating, He showed that there were more novae in Andromeda than in the Milky Way. From this, he could ask why there were more novae in one small section of the galaxy than the other sections of the galaxy, if Andromeda were not a separate galaxy but simply a nebula within Earth's galaxy. This led to supporting Andromeda as a separate galaxy with its own signature age and rate of nova occurrences. Curtis also noted the large radial velocities of spiral nebulae that suggested they could not be gravitationally bound to the Milky Way in a Kapteyn model Universe. In addition, he cited dark lanes present in other galaxies similar to the dust clouds found in Earth's own galaxy, explaining the zone of avoidance. Curtis stated that if van Maanen's observation of the Pinwheel Galaxy rotating were correct, he himself would have been wrong about the scale of the universe and that the Milky Way would fully encompass it.

Later in the 1920s, Edwin Hubble showed that Andromeda was far outside the Milky Way by measuring Cepheid variable stars, proving that Curtis was correct. It is now known that the Milky Way is only one of as many as an estimated 200 billion (2e11) to 2 trillion (2e12) or more galaxies in the observable universe. Also, astronomers generally accept that the nova Shapley referred to in his arguments was in fact a supernova, which does indeed temporarily outshine the combined output of an entire galaxy. On other points, the results were mixed (the actual size of the Milky Way is in between the sizes proposed by Shapley and Curtis), or in favor of Shapley (the Sun was near the center of the galaxy in Curtis's model, while Shapley correctly placed the Sun in the outer regions of the galaxy). It later became apparent that van Maanen's observations were incorrect—one cannot actually see the Pinwheel Galaxy rotate during a human lifespan.

The format of the great debate has been used subsequently to argue the nature of fundamental questions in astronomy. In honor of the first "Great Debate", the Smithsonian has hosted four more events.