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Loading contentIn 1920 Harlow Shapley and Heber Curtis debated the scale of the universe: whether the spiral nebulae lie within the Milky Way or are separate 'island universes'. Edwin Hubble settled the question a few years later by measuring the distance to Andromeda.
Facts on this topic will be cited from these primary and reference sources.
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General reference for the history of astronomy, biographies, and cultural context.