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![Andromeda nebula, photographed at the Yerkes Observatory circa 1900. To modern eyes, this is clearly a galaxy. At the time, though, it was described as "a mass of glowing gas." (From the book Astronomy of To-Day)](https://cdn.discovermagazine.com/assets/image/4940/Screen-Shot-2017-01-02-at-11.48.14-AM-1024x797-xs.webp "Andromeda nebula, photographed at the Yerkes Observatory circa 1900. To modern eyes, this is clearly a galaxy. At the time, though, it was described as "a mass of glowing gas." (From the book Astronomy of To-Day)")
