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Lord Rosse discovers that many of the nebulae have a spiral structure. He and John Herschel were at odds over the nature of nebulae, with Rosse believing they were unresolved clusters of stars. (The contention was related to the theory of evolution, with Rosse not believing that new stars were still forming.)
In 1900, Cornelius Easton argued that the Milky Way had a spiral structure and that many of the nebulae formed part of this structure. (This had first been suggested in the middle of the 19th century.)
The observations of E. E. Barnard lent weight to the idea that the dark regions of the sky were actually obscuring matter (rather than tunnel-like holes suggested by John Herschel).
The 1920 Shapley–Curtis Debate that would lead to the categorization of many nebulae as Island Universes (or galaxies) per Edwin Hubble.
Max Wolf was the first person to estimate the distance to interstellar clouds. His approximation compared the number of stars per square degree (and their magnitudes) in a cloud region to elsewhere.
In 1927, Herman Zanstra determined how the neutral hydrogen in H II regions became excited.
The 1927 explanation by Ira Sprague Bowen of the forbidden lines in the O III spectrum within nebula. (Previously attributed to "nebulium".)