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. 2021 October 17 [2]The picture shows single distant quasar that appears four times due to the gravitational lens effect of an intervening galaxy. Please see the explanation for more detailed information. The Einstein Cross Gravitational Lens Image Credit & [3]License: [4]J. Rhoads ([5]Arizona State U.) et al., [6]WIYN, [7]AURA, [8]NOIRLab, [9]NSF Explanation: Most galaxies have a single nucleus -- does this galaxy have four? The strange answer leads [10]astronomers to conclude that the nucleus of the surrounding galaxy is not even visible in [11]this image. The central [12]cloverleaf is rather light emitted from a background [13]quasar. The [14]gravitational field of the visible foreground galaxy [15]breaks light from this distant [16]quasar into four distinct images. The [17]quasar must be [18]properly aligned behind the center of a massive galaxy for a [19]mirage like this to be evident. The general effect is known as [20]gravitational lensing, and this specific case is known as the [21]Einstein Cross. Stranger still, the images of the [22]Einstein Cross vary in relative brightness, enhanced occasionally by the additional [23]gravitational microlensing effect of specific stars in the foreground galaxy. Tomorrow's picture: earthshine fireworks __________________________________________________________________ [24]< | [25]Archive | [26]Submissions | [27]Index | [28]Search | [29]Calendar | [30]RSS | [31]Education | [32]About APOD | [33]Discuss | [34]> __________________________________________________________________ Authors & editors: [35]Robert Nemiroff ([36]MTU) & [37]Jerry Bonnell ([38]UMCP) NASA Official: Phillip Newman [39]Specific rights apply. [40]NASA Web Privacy Policy and Important Notices A service of: [41]ASD at [42]NASA / [43]GSFC & [44]Michigan Tech. U. References 1. https://apod.nasa.gov/apod/archivepix.html 2. https://apod.nasa.gov/apod/image/2110/qso2237_wiyn_1024.jpg 3. https://noirlab.edu/public/copyright/ 4. https://isearch.asu.edu/profile/858089 5. https://sese.asu.edu/ 6. https://www.noao.edu/wiyn/wiyn.html 7. https://www.aura-astronomy.org/ 8. https://noirlab.edu/ 9. https://www.nsf.gov/ 10. https://aas.org/careers/career-in-astronomy 11. https://noirlab.edu/public/images/noao-q2237/ 12. https://en.wikipedia.org/wiki/Four-leaf_clover 13. https://en.wikipedia.org/wiki/Quasar 14. https://spaceplace.nasa.gov/what-is-gravity/en/ 15. http://www.youtube.com/watch?v=BkBNf_nFuhM 16. https://apod.nasa.gov/apod/ap000419.html 17. https://imagine.gsfc.nasa.gov/ask_astro/quasar.html 18. https://lh3.ggpht.com/_LlfXWxcpJyU/SkXvJ2cC99I/AAAAAAAAMxE/hC0Z002bgv4/7%20Maine%20Coon%20Kittens%5B9%5D.jpg 19. https://apod.nasa.gov/apod/ap990331.html 20. https://en.wikipedia.org/wiki/Gravitational_lens 21. http://www.astr.ua.edu/keel/agn/qso2237.html 22. https://apod.nasa.gov/apod/ap961215.html 23. https://en.wikipedia.org/wiki/Gravitational_microlensing 24. https://apod.nasa.gov/apod/ap211016.html 25. https://apod.nasa.gov/apod/archivepix.html 26. https://apod.nasa.gov/apod/lib/apsubmit2015.html 27. https://apod.nasa.gov/apod/lib/aptree.html 28. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search 29. https://apod.nasa.gov/apod/calendar/allyears.html 30. https://apod.nasa.gov/apod.rss 31. https://apod.nasa.gov/apod/lib/edlinks.html 32. https://apod.nasa.gov/apod/lib/about_apod.html 33. http://asterisk.apod.com/discuss_apod.php?date=211017 34. https://apod.nasa.gov/apod/ap211018.html 35. http://www.phy.mtu.edu/faculty/Nemiroff.html 36. http://www.phy.mtu.edu/ 37. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html 38. http://www.astro.umd.edu/ 39. https://apod.nasa.gov/apod/lib/about_apod.html#srapply 40. https://www.nasa.gov/about/highlights/HP_Privacy.html 41. https://astrophysics.gsfc.nasa.gov/ 42. https://www.nasa.gov/ 43. https://www.nasa.gov/centers/goddard/ 44. http://www.mtu.edu/