                        Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our
      fascinating universe is featured, along with a brief explanation
                    written by a professional astronomer.

                                2020 April 26

                     Edwin Hubble Discovers the Universe
     Image Credit & Copyright: Courtesy Carnegie Institution for Science

   Explanation: How big is our universe? This very question, among others,
   was debated by two leading astronomers 100 years ago today in what has
   become known as astronomy's Great Debate. Many astronomers then
   believed that our Milky Way Galaxy was the entire universe. Many
   others, though, believed that our galaxy was just one of many. In the
   Great Debate, each argument was detailed, but no consensus was reached.
   The answer came over three years later with the detected variation of
   single spot in the Andromeda Nebula, as shown on the original glass
   discovery plate digitally reproduced here. When Edwin Hubble compared
   images, he noticed that this spot varied, and so wrote "VAR!" on the
   plate. The best explanation, Hubble knew, was that this spot was the
   image of a variable star that was very far away. So M31 was really the
   Andromeda Galaxy -- a galaxy possibly similar to our own. The featured
   image may not be pretty, but the variable spot on it opened a door
   through which humanity gazed knowingly, for the first time, into a
   surprisingly vast cosmos.

     Centennial Celebration: Astronomy's Great Debate was 100 Years Ago
                                    Today
                  Tomorrow's picture: Another Great Debate?
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                             & Michigan Tech. U.

