In the 1900s, better telescopes showed astronomers more about the universe. The Solar System is in a galaxy made of billions of stars, which we call the Milky Way. Other galaxies exist outside it, as far as we can see. This started a new kind of astronomy called cosmology, in which astronomers study what these galaxies are made of and how they are spread out. By measuring the redshift of galaxies, cosmologists soon discovered that the Universe is expanding (see: Hubble).
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