                        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.

                               2023 October 6

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

   Explanation: How big is our universe? This question, among others, was
   debated by two leading astronomers in 1920 in what has since 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 on October 6, 1923 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. Annotated
   100 years ago, the featured image may not be pretty, but the variable
   spot on it opened a window through which humanity gazed knowingly, for
   the first time, into a surprisingly vast cosmos.

                  Tomorrow's picture: once and future stars
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                           NASA Science Activation
                             & Michigan Tech. U.

