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.

                                2025 August 4
      [2]The Andromeda Galaxy is shown just right of center, while some
    unusual blue arcs appear to its left. Please see the explanation for
                         more detailed information.

                         Blue Arcs Toward Andromeda
                 Image Credit & Copyright: [3]Ogle et [4]al.

   Explanation: What are these gigantic blue arcs near the Andromeda
   Galaxy (M31)? [5]Discovered in 2022 by amateur astronomers, the faint
   arcs -- dubbed [6]SDSO 1 -- span nearly the same angular size as M31
   itself. At first, their origin was a mystery: are they actually [7]near
   the Andromeda Galaxy, or alternatively [8]near to our Sun? Now, over
   550 hours of combined exposure and a collaboration between amateur and
   professional astronomers has revealed strong evidence for their true
   nature: SDSO 1 is not [9]intergalactic, but a new class of
   [10]planetary nebula within our galaxy. Dubbed a Ghost Planetary Nebula
   (GPN), SDSO 1 is the first recognized member of a new subclass of faded
   [11]planetary nebulas, along with seven others also [12]recently
   identified. Shown in blue are extremely faint [13]oxygen emission from
   the shock waves, while the [14]surrounding red is a hydrogen-emitting
   trail that indicates the GPN's age.

                 Tomorrow's picture: complex stellar jumble
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    Authors & editors: [26]Robert Nemiroff ([27]MTU) & [28]Jerry Bonnell
                                 ([29]UMCP)
          NASA Official: Amber Straughn [30]Specific rights apply.
            [31]NASA Web Privacy, [32]Accessibility, [33]Notices;
                A service of: [34]ASD at [35]NASA / [36]GSFC,
                         [37]NASA Science Activation
                           & [38]Michigan Tech. U.

References

   1. https://apod.nasa.gov/apod/archivepix.html
   2. https://apod.nasa.gov/apod/image/2508/M31Oxy_Collab_5415.jpg
   3. https://arxiv.org/abs/2507.15834
   4. https://www.astrobin.com/ns2x09/
   5. https://iopscience.iop.org/article/10.3847/2515-5172/acaf7e
   6. https://www.astrobin.com/1d8ivk/
   7. https://apod.nasa.gov/apod/ap230117.html
   8. https://www.reddit.com/r/CatsBeingCats/comments/17psiw8/cats_basking_in_a_sun_bath/#lightbox
   9. https://science.nasa.gov/asset/hubble/intergalactic-absorbtion-clouds/
  10. https://apod.nasa.gov/apod/ap250713.html
  11. https://science.nasa.gov/category/universe/nebulae/planetary-nebulae/
  12. https://arxiv.org/abs/2507.15834
  13. https://www.astronomy.com/science/oxygen-the-color-of-life/
  14. https://apod.nasa.gov/apod/ap221024.html
  15. https://apod.nasa.gov/apod/ap250803.html
  16. https://apod.nasa.gov/apod/archivepix.html
  17. https://apod.nasa.gov/apod/lib/apsubmit2015.html
  18. https://apod.nasa.gov/apod/lib/aptree.html
  19. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  20. https://apod.nasa.gov/apod/calendar/allyears.html
  21. https://apod.com/feed.rss
  22. https://apod.nasa.gov/apod/lib/edlinks.html
  23. https://apod.nasa.gov/apod/lib/about_apod.html
  24. https://asterisk.apod.com/discuss_apod.php?date=250804
  25. https://apod.nasa.gov/apod/ap250805.html
  26. http://www.phy.mtu.edu/faculty/Nemiroff.html
  27. http://www.phy.mtu.edu/
  28. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  29. http://www.astro.umd.edu/
  30. https://apod.nasa.gov/apod/lib/about_apod.html#srapply
  31. https://www.nasa.gov/about/highlights/HP_Privacy.html
  32. https://www.nasa.gov/general/accessibility/
  33. https://www.nasa.gov/privacy/
  34. https://astrophysics.gsfc.nasa.gov/
  35. https://www.nasa.gov/
  36. https://www.nasa.gov/centers/goddard/
  37. https://science.nasa.gov/learners
  38. http://www.mtu.edu/