The astronomer who argued in the Great Debate that spiral nebulae were separate galaxies was Heber Curtis.
At the 26 April 1920 debate, Curtis opposed Harlow Shapley’s view that the Milky Way encompassed the known universe. Curtis maintained that objects such as the Andromeda Nebula were distant “island universes”—galaxies comparable to, but separate from, the Milky Way. He cited the apparent structure of spiral nebulae, their high radial velocities, and the frequency of novae observed in Andromeda.
The debate did not settle every question. Shapley was closer to the truth about the Sun’s position in the Milky Way, while Curtis underestimated the Galaxy’s size. Curtis was nevertheless right on the central issue: spiral nebulae were external galaxies. Edwin Hubble’s observations of Cepheid variables in Andromeda later supplied decisive distance evidence in the 1920s.
A common mix-up is to credit Hubble with originating the separate-galaxies idea. Hubble provided crucial confirmation; Curtis had defended it decades earlier, during a debate that helped launch modern extragalactic astronomy.