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Published online by Cambridge University Press: 14 August 2015
In 1924 Hubble presented the first generally convincing evidence, from the identification of variable stars, that some of the brightest “spiral nebulae” are galaxies of stars well outside the bounds of our own Galaxy. This led him to reconsider the idea that the faint “spiral nebulae,” which were known to be much more abundant than bright ones, might be similar objects at greater distances.