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. 2020 April 30 [2]See Explanation. Clicking on the picture will download the highest resolution version available. Andromeda Island Universe Image Credit & [3]Copyright: [4]Yuzhe Xiao Explanation: The most distant object easily visible to the unaided eye is [5]M31, the great [6]Andromeda Galaxy some two and a half million light-years away. But without a telescope, even this immense spiral galaxy - spanning [7]over 200,000 light years - appears as a faint, nebulous cloud in the [8]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 [9]many distant galaxies like M31, astronomers [10]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 [11]Shapley-Curtis debate of 1920, which was later [12]resolved by observations of M31 in favor of Andromeda, [13]island universe. Experts Debate: [14]How will humanity first discover extraterrestrial life? Tomorrow's picture: galaxies away __________________________________________________________________ [15]< | [16]Archive | [17]Submissions | [18]Index | [19]Search | [20]Calendar | [21]RSS | [22]Education | [23]About APOD | [24]Discuss | [25]> __________________________________________________________________ Authors & editors: [26]Robert Nemiroff ([27]MTU) & [28]Jerry Bonnell ([29]UMCP) NASA Official: Phillip Newman [30]Specific rights apply. [31]NASA Web Privacy Policy and Important Notices A service of: [32]ASD at [33]NASA / [34]GSFC & [35]Michigan Tech. U. References 1. https://apod.nasa.gov/apod/archivepix.html 2. https://apod.nasa.gov/apod/image/2004/M31Dec2019final1YuzheB2.jpg 3. http://antwrp.gsfc.nasa.gov/apod/lib/about_apod.html#sraplyy 4. mailto:%20xiaoyuzhe19944%20[at]%20gmail%20[dot]%20com 5. http://www.messier.seds.org/xtra/history/m-m31_42.html 6. http://www.messier.seds.org/m/m031.html 7. https://apod.nasa.gov/apod/ap061228.html 8. http://www.hawastsoc.org/deepsky/and/index.html 9. https://apod.nasa.gov/apod/ap070208.html 10. http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?1995PASP%2E%2E107%2E1133T 11. http://antwrp.gsfc.nasa.gov/diamond_jubilee/debate20.html 12. https://apod.nasa.gov/apod/ap200426.html 13. http://pages.astronomy.ua.edu/keel/goodies/data_resources/galaxies.text 14. https://apod.nasa.gov/debate/debate100th.html 15. https://apod.nasa.gov/apod/ap200429.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.nasa.gov/apod.rss 22. https://apod.nasa.gov/apod/lib/edlinks.html 23. https://apod.nasa.gov/apod/lib/about_apod.html 24. http://asterisk.apod.com/discuss_apod.php?date=200430 25. https://apod.nasa.gov/apod/ap200501.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://astrophysics.gsfc.nasa.gov/ 33. https://www.nasa.gov/ 34. https://www.nasa.gov/centers/goddard/ 35. http://www.mtu.edu/