Astronomy Picture of the Day

   [1]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
    [2]See Explanation. Clicking on the picture will download the highest
                        resolution version available.

                     Edwin Hubble Discovers the Universe
      Image Credit & [3]Copyright: Courtesy [4]Carnegie Institution for
                                   Science

   Explanation: How big is our universe? This question, [5]among others,
   was debated by two leading astronomers in 1920 in what has since become
   known as [6]astronomy's Great Debate. Many astronomers then believed
   that our [7]Milky Way Galaxy was the entire universe. Many others,
   though, believed that our galaxy was just [8]one of many. In the
   [9]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 [10]Andromeda Nebula, as shown on the
   [11]original glass discovery plate digitally reproduced here. When
   [12]Edwin Hubble compared images, he noticed that this [13]spot varied,
   and on October 6, 1923 wrote "VAR!" on the plate. The best explanation,
   Hubble knew, was that this spot was the [14]image of a variable star
   that was very far away. So M31 was really the [15]Andromeda Galaxy -- a
   galaxy possibly similar to our own. Annotated 100 years ago, the
   [16]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 [17]surprisingly vast cosmos.

                  Tomorrow's picture: once and future stars
     __________________________________________________________________

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     __________________________________________________________________

    Authors & editors: [29]Robert Nemiroff ([30]MTU) & [31]Jerry Bonnell
                                 ([32]UMCP)
          NASA Official: Phillip Newman [33]Specific rights apply.
              [34]NASA Web Privacy Policy and Important Notices
                A service of: [35]ASD at [36]NASA / [37]GSFC,
                         [38]NASA Science Activation
                           & [39]Michigan Tech. U.

References

   1. https://apod.nasa.gov/apod/archivepix.html
   2. https://apod.nasa.gov/apod/image/2004/HubbleVarOrig_Carnegie_2880.jpg
   3. https://obs.carnegiescience.edu/PAST/m31var
   4. https://obs.carnegiescience.edu/
   5. http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1995PASP..107.1133T
   6. https://apod.nasa.gov/debate/debate100th.html
   7. https://solarsystem.nasa.gov/resources/285/the-milky-way-galaxy/
   8. https://apod.nasa.gov/apod/ap020901.html
   9. https://en.wikipedia.org/wiki/Great_Debate_(astronomy)
  10. https://apod.nasa.gov/apod/ap170303.html
  11. https://obs.carnegiescience.edu/PAST/m31var
  12. https://en.wikipedia.org/wiki/Edwin_Hubble
  13. https://apod.nasa.gov/apod/ap960406.html
  14. https://hubblesite.org/contents/media/images/2011/15/2851-Image.html
  15. https://apod.nasa.gov/apod/ap190909.html
  16. https://obs.carnegiescience.edu/PAST/m31var
  17. https://apod.nasa.gov/apod/ap200405.html
  18. https://apod.nasa.gov/apod/ap231005.html
  19. https://apod.nasa.gov/apod/archivepix.html
  20. https://apod.nasa.gov/apod/lib/apsubmit2015.html
  21. https://apod.nasa.gov/apod/lib/aptree.html
  22. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  23. https://apod.nasa.gov/apod/calendar/allyears.html
  24. https://apod.nasa.gov/apod.rss
  25. https://apod.nasa.gov/apod/lib/edlinks.html
  26. https://apod.nasa.gov/apod/lib/about_apod.html
  27. http://asterisk.apod.com/discuss_apod.php?date=231006
  28. https://apod.nasa.gov/apod/ap231007.html
  29. http://www.phy.mtu.edu/faculty/Nemiroff.html
  30. http://www.phy.mtu.edu/
  31. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  32. http://www.astro.umd.edu/
  33. https://apod.nasa.gov/apod/lib/about_apod.html#srapply
  34. https://www.nasa.gov/about/highlights/HP_Privacy.html
  35. https://astrophysics.gsfc.nasa.gov/
  36. https://www.nasa.gov/
  37. https://www.nasa.gov/centers/goddard/
  38. https://science.nasa.gov/learners
  39. http://www.mtu.edu/