“The evidence at present available points strongly to the conclusion that the spirals are individual galaxies, or island universes, comparable with our own galaxy in dimension and in number of component units.” -Heber Curtis, 1920
In many different areas of life, we settle disagreements by popular vote. But in science, we know better; regardless of whether people accept the conclusions or not, the Universe simply is the way it is, and it’s up to us to listen to the evidence to uncover scientific truths. It often happens, however, that everyone can agree on the pieces of evidence, yet disagree as far as what those pieces of evidence mean.
In 1920, this was exactly the case when it came to the nature of spiral nebulae: were they protostars, or were they galaxies all unto themselves? A great debate took place, where two extremely well-respected astronomers argued some very different interpretations of six pieces of data. While the arguments were interesting and a vote was held, the outcome (the wrong side won) didn’t matter. Three years later, the deciding evidence did.