Which astronomer argued that spiral nebulae were entire 'island universes' separate from the Milky Way?
Answer
Heber Curtis
Heber Curtis posited the view that the fuzzy patches of light, like Andromeda, were independent galaxies existing far beyond the confines of our own.

Related Questions
What was the central scientific conflict known as the Great Debate centered around in the early 1900s?Which astronomer argued that spiral nebulae were entire 'island universes' separate from the Milky Way?What critical piece of technology was essential for Edwin Hubble to successfully measure the distance to Andromeda?The breakthrough in calculating the distance to Andromeda relied upon identifying what specific type of star?What specific physical relationship made the distance calculation using Cepheid variables possible?What was Hubble's initial calculated distance estimate for the Andromeda Nebula (M31)?What was the immediate implication of Hubble proving Andromeda was a separate galaxy?Hubble's discovery that the universe contained many galaxies set the stage for which subsequent major revelation?What relationship is demonstrated by Hubble's Law?What field of astronomy was provided foundational expertise by Hubble's proof that other galaxies existed?