Edwin Hubble
On December 1st, 1925, a scientific paper called "Cepheids in Spiral Nebulae" was published. In it, Edwin Hubble concluded that the stars he was looking at were too far away to belong to our galaxy. For the first time in history we humans realized that stars group themselves in galaxies—farther away than anything we ever imagined
Source: Sagan, Carl. Cosmos. New York: Random House, 1980
Public Domain
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!