What conceptual problem did the grouping of objects like M31 (Andromeda) and M42 create for astronomy?
Answer
It fueled the Great Debate over whether they were local gas clouds or distant 'island universes'.
Because observers lacked the tools to distinguish between nearby nebulae and distant galaxies, Messier's mixed catalog entry for objects like M31 instigated the Great Debate of the 1920s regarding the scale of the universe.
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