“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.
One theory was that these spiral nebulae were molecular clouds that collapsed into a disk, began rotating and funneling mass into the center, where they would eventually form stars. Images credit (from L-to-R): NASA and The Hubble Heritage Team (STScI/AURA). Acknowledgment: C. R. O’Dell (Vanderbilt University); ESA: C. Carreau; Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF.
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.
The star in the great Andromeda Nebula that changed our view of the Universe forever, as imaged first by Edwin Hubble in 1923 and then by the Hubble Space Telescope nearly 90 years later. Image credit: NASA, ESA and Z. Levay (STScI) (for the illustration); NASA, ESA and the Hubble Heritage Team (STScI/AURA) (for the image).