                        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.

                              2019 September 9

                          M31: The Andromeda Galaxy
              Image Credit & Copyright: Amir H. Abolfath (TWAN)

   Explanation: How far can you see? The most distant object easily
   visible to the unaided eye is M31, the great Andromeda Galaxy, over two
   million light-years away. Without a telescope, even this immense spiral
   galaxy appears as an unremarkable, faint, nebulous cloud in the
   constellation Andromeda. But a bright yellow nucleus, dark winding dust
   lanes, luminous blue spiral arms, and bright red emission nebulas are
   recorded in this stunning six-hour telescopic digital mosaic of our
   closest major galactic neighbor. While even casual skygazers are now
   inspired by the knowledge that there are many distant galaxies like
   M31, astronomers seriously debated this fundamental concept only 100
   years ago. Were these "spiral nebulae" simply outlying gas clouds in
   our own Milky Way Galaxy or were they "island universes" -- distant
   galaxies of stars comparable to the Milky Way itself? This question was
   central to the famous Shapley-Curtis debate of 1920, which was later
   resolved by observations favoring Andromeda being just like our Milky
   Way Galaxy -- a conclusion making the rest of the universe much more
   vast than many had ever imagined.

                   Tomorrow's picture: pluto in true color
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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.

