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• Andromeda Nebula Lies Outside Milky Way Galaxy
  Hubble's observations of Cepheid variable stars in Andromeda show that it is actually separate from our Milky Way galaxy.
• "Great Debate" Resolved
  Hubble's distance to the Andromeda Nebula resolves the debate held in 1920 between Harlow Shapley and Heber Curtis over whether the spiral nebulae were part of our Galaxy or outside it.
• Universe is Expanding
  Hubble's careful distance calculations coupled with redshift measurements show that the Universe is expanding.
• Sidebar: The Minds atop Mount Wilson
  Biographical information about Hubble and his assistant, Humason.
• Sidebar: Classifying Nebulae
  Brief overview of Hubble's system of classifying galaxies.
• In Their Own Words
  Snippets of papers published by other astronomers during this time. The snippet here is from Hubble about observing distant objects in the Universe.

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