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 30

                          Andromeda Island Universe
                    Image Credit & Copyright: Yuzhe Xiao

   Explanation: The most distant object easily visible to the unaided eye
   is M31, the great Andromeda Galaxy some two and a half million
   light-years away. But without a telescope, even this immense spiral
   galaxy - spanning over 200,000 light years - appears as a faint,
   nebulous cloud in the constellation Andromeda. In contrast, a bright
   yellow nucleus, dark winding dust lanes, expansive blue spiral arms and
   star clusters are recorded in this stunning telescopic image. While
   even casual skygazers are now inspired by the knowledge that there are
   many distant galaxies like M31, astronomers debated this fundamental
   concept 100 years ago. Were these "spiral nebulae" simply outlying
   components of our own Milky Way Galaxy or were they instead "island
   universes", distant systems 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 of M31 in favor of
   Andromeda, island universe.

   Experts Debate: How will humanity first discover extraterrestrial life?
                      Tomorrow's picture: galaxies away
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                      A service of: ASD at NASA / GSFC
                             & Michigan Tech. U.

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