Harlow Shapley
In 1920 Heber Doust Curtis and Harlow Shapley participated in the Great Debate on the nature of nebulae and galaxies, and the size of the universe. Shapley believed that distant nebulae were relatively small and lay within the Milky Way galaxy. Curtis advocated the now-accepted view that nebulae were farther away, and that other galaxies apart from the Milky Way therefore existed.