What was the central scientific disagreement during the Great Debate of 1920?

Answer

Whether spiral nebulae were within the Milky Way or were independent galaxies

The Great Debate pitted Heber Curtis, who argued spiral nebulae were independent galaxies far outside the Milky Way, against Harlow Shapley, who maintained they were smaller objects contained within our galaxy's vast boundaries.

What was the central scientific disagreement during the Great Debate of 1920?
astronomytheorystarMilky Way