What astronomical debate regarding spiral nebulae did Hubble's spectroscopic measurements help resolve?

Answer

Whether they were star clusters in the Milky Way or distant galaxies.

Before Edwin Hubble applied precise spectroscopic techniques, there was significant debate among astronomers regarding the true nature of the faint, fuzzy patches identified as spiral nebulae. The prevailing uncertainty was whether these objects were relatively small star clusters located nearby, contained within the boundaries of our own Milky Way galaxy, or if they were actually enormous, entirely separate island universes situated millions of light-years away. By measuring the spectral lines—specifically finding pronounced redshifts—Hubble could determine their recession velocities and distances, providing the necessary evidence to confirm they were external galaxies far outside the Milky Way, fundamentally altering the perceived scale of the cosmos.

What astronomical debate regarding spiral nebulae did Hubble's spectroscopic measurements help resolve?

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