The Shapley-Curtis Debate:  1920
Are spiral nebulae really other
galaxies, or just swirling clouds of
gas and dust within our own
galaxy?
Many spiral galaxies had much
larger radial velocities than other
objects within our own galaxy
Answered by new observations:
In 1923/4 Edwin Hubble photographed and identified Cepheids in the
Andromeda Galaxy:
Distance to Cepheids cleared showed Andromeda was outside our own
galaxy