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We know our immediate neighborhood rather intimately. With increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial.

Edwin Hubble, 1936

Questions to Explore:
  • What are the different kinds of galaxies?
  • How have astronomers determined the distances to galaxies?
  • What makes astronomers think that galaxies contain vast amounts of dark matter?
  • How do we think galaxies formed?
  • Why do nearly all galaxies have redshifted spectra?
  • Why do the redshifts of galaxies increase with their distances?

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