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Astronomer Harlow Shapley thought the Milky Way constituted the entire universe while his rival, Herbert Curtis, had performed rough measurements of the distance to neighboring Andromeda, also known as the Messier 31, that suggested we live in an island universe and a teaming with large distance galaxies. And on a dark night, Andromeda had always been visible to the naked eye, but over the years, sky watchers have debated whether it was a constellation, to the nebula or another galaxy.