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. 2019 September 9 [2]See Explanation. Clicking on the picture will download the highest resolution version available. M31: The Andromeda Galaxy Image Credit & Copyright: [3]Amir H. Abolfath ([4]TWAN) Explanation: How far can you see? The most distant object easily visible to the unaided eye is M31, the great [5]Andromeda Galaxy, over two million [6]light-years away. Without a telescope, even this immense [7]spiral galaxy appears as an unremarkable, faint, [8]nebulous cloud in the [9]constellation Andromeda. But a bright yellow nucleus, dark winding dust lanes, luminous blue spiral arms, and bright red emission nebulas are recorded in [10]this stunning six-hour telescopic digital mosaic of our closest major galactic neighbor. While even [11]casual skygazers are now inspired by the knowledge that there are many distant galaxies like M31, astronomers seriously debated this fundamental concept [12]only 100 years ago. Were these "spiral nebulae" simply outlying gas clouds in our own [13]Milky Way Galaxy or were they "island universes" -- distant galaxies of stars comparable to the [14]Milky Way itself? This question was central to the famous [15]Shapley-Curtis debate of 1920, which was later resolved by observations favoring Andromeda being just like our [16]Milky Way Galaxy -- a conclusion making [17]the rest of the universe much more vast than many had ever imagined. Tomorrow's picture: pluto in true color __________________________________________________________________ [18]< | [19]Archive | [20]Submissions | [21]Index | [22]Search | [23]Calendar | [24]RSS | [25]Education | [26]About APOD | [27]Discuss | [28]> __________________________________________________________________ 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]Michigan Tech. U. References 1. https://apod.nasa.gov/apod/archivepix.html 2. https://apod.nasa.gov/apod/image/1909/M31_Abolfath_3000.jpg 3. http://amir.torgheh.ir/about.php 4. http://www.twanight.org/newTWAN/index.asp 5. https://en.wikipedia.org/wiki/Andromeda_Galaxy 6. https://youtu.be/MX3PIkbTQwQ 7. https://en.wikipedia.org/wiki/Spiral_galaxy 8. https://apod.nasa.gov/apod/ap160322.html 9. https://en.wikipedia.org/wiki/Andromeda_(constellation) 10. http://amir.torgheh.ir/picture.php?/1237/category/14 11. https://i.pinimg.com/736x/17/e0/6c/17e06cb8ddaa2fc90b7a416c0d0349ef--pugs-on-the-beach.jpg 12. http://www.nasonline.org/about-nas/history/archives/milestones-in-NAS-history/the-great-debate-of-1920.html 13. https://solarsystem.nasa.gov/resources/285/the-milky-way-galaxy/ 14. https://apod.nasa.gov/apod/ap190617.html 15. https://apod.nasa.gov/diamond_jubilee/debate_1920.html 16. https://apod.nasa.gov/apod/ap180729.html 17. https://apod.nasa.gov/apod/ap180508.html 18. https://apod.nasa.gov/apod/ap190908.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=190909 28. https://apod.nasa.gov/apod/ap190910.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. http://www.mtu.edu/