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. 2026 May 21 [2]On a dark background, galaxies are shown as fuzzy white dots. A bright blue spiral expands from the center. Clicking on the picture will download the highest resolution version available. A Collision of Galaxy Clusters Image Credit: X-ray: [3]NASA/[4]CXC/[5]CfA/[6]C. Watson et al.; Optical: [7]PanSTARRS; Image Processing: [8]NASA/[9]CXC/[10]SAO/[11]N. Wolk and [12]P. Edmonds Text: [13]Cecilia Chirenti ([14]NASA [15]GSFC, [16]UMCP, [17]CRESST II) Explanation: This big beautiful [18]spiral shines in [19]X-ray light. It is about 20 times larger than our [20]Galaxy. It belongs to [21]Abell 2029, a [22]galaxy cluster one billion [23]light-years away. (To see only the galaxies, hover your cursor over the image, or follow this [24]link.) Galaxy clusters are the [25]largest structures in the universe that are [26]supported by gravity. Abell 2029 is formed by [27]thousands of galaxies, surrounded by a huge cloud of hot gas and the equivalent of hundreds of trillions times the mass of the Sun in [28]dark matter. The spiral is made of gas, mostly hydrogen and helium, heated to tens of millions of degrees. It was found in a [29]recent study that used data from NASA's [30]Chandra X-ray Observatory to show that Abell 2029 had a collision with a smaller cluster four billion years ago. The [31]collision affected the gravitational field and caused the intracluster gas to slosh, like wine moving in a [32]wine glass, shaping the spiral. Tomorrow's picture: stellar winds __________________________________________________________________ [33]< | [34]Archive | [35]Submissions | [36]Index | [37]Search | [38]Calendar | [39]RSS | [40]Education | [41]About APOD | [42]Discuss | [43]> __________________________________________________________________ Authors & editors: [44]Robert Nemiroff ([45]MTU) & [46]Jerry Bonnell ([47]UMCP) NASA Official: Amber Straughn [48]Specific rights apply. [49]NASA Web Privacy, [50]Accessibility, [51]Notices; A service of: [52]ASD at [53]NASA / [54]GSFC, [55]NASA Science Activation & [56]Michigan Tech. U. References 1. https://apod.nasa.gov/apod/archivepix.html 2. https://apod.nasa.gov/apod/image/2605/a2029_lg.jpg 3. https://www.nasa.gov/ 4. https://science.nasa.gov/mission/chandra/ 5. https://www.cfa.harvard.edu/ 6. https://iopscience.iop.org/article/10.3847/1538-4357/ae2026 7. https://panstarrs.ifa.hawaii.edu/ 8. https://www.nasa.gov/ 9. https://science.nasa.gov/mission/chandra/ 10. https://www.cfa.harvard.edu/about/about-smithsonian-astrophysical-observatory 11. https://www.cfa.harvard.edu/people/nancy-ra-wolk 12. https://www.cfa.harvard.edu/people/peter-edmonds 13. https://science.gsfc.nasa.gov/sci/bio/cecilia.chirenti 14. https://www.nasa.gov/ 15. https://www.nasa.gov/goddard/ 16. https://www.astro.umd.edu/people/cecilia-chirenti 17. https://cresst2.umd.edu/ 18. https://chandra.si.edu/photo/2026/a2029/ 19. https://science.nasa.gov/ems/11_xrays/ 20. https://www.youtube.com/watch?v=MX3PIkbTQwQ&t=44s 21. https://en.wikipedia.org/wiki/Abell_2029 22. https://chandra.harvard.edu/xray_sources/galaxy_clusters.html 23. https://en.wikipedia.org/wiki/Light-year 24. https://apod.nasa.gov/apod/image/2605/a2029_optical.jpg 25. https://science.nasa.gov/universe/galaxies/large-scale-structures/ 26. https://apod.nasa.gov/apod/ap190226.html 27. https://apod.nasa.gov/apod/ap240327.html 28. https://science.nasa.gov/dark-matter/ 29. https://iopscience.iop.org/article/10.3847/1538-4357/ae2026 30. https://science.nasa.gov/mission/chandra/ 31. https://apod.nasa.gov/apod/ap000306.html 32. https://images.unsplash.com/photo-1700455350393-34e80e3e0997 33. https://apod.nasa.gov/apod/ap260520.html 34. https://apod.nasa.gov/apod/archivepix.html 35. https://apod.nasa.gov/apod/lib/apsubmit2015.html 36. https://apod.nasa.gov/apod/lib/aptree.html 37. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search 38. https://apod.nasa.gov/apod/calendar/allyears.html 39. https://apod.com/feed.rss 40. https://apod.nasa.gov/apod/lib/edlinks.html 41. https://apod.nasa.gov/apod/lib/about_apod.html 42. https://asterisk.apod.com/discuss_apod.php?date=260521 43. https://apod.nasa.gov/apod/ap260522.html 44. http://www.phy.mtu.edu/faculty/Nemiroff.html 45. http://www.phy.mtu.edu/ 46. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html 47. http://www.astro.umd.edu/ 48. https://apod.nasa.gov/apod/lib/about_apod.html#srapply 49. https://www.nasa.gov/about/highlights/HP_Privacy.html 50. https://www.nasa.gov/general/accessibility/ 51. https://www.nasa.gov/privacy/ 52. https://astrophysics.gsfc.nasa.gov/ 53. https://www.nasa.gov/ 54. https://www.nasa.gov/centers/goddard/ 55. https://science.nasa.gov/learners 56. http://www.mtu.edu/