The galaxy was once a normal spiral galaxy before it had a head-on collision with a smaller companion about 200 million years ago. When the nearby galaxy passed through the Cartwheel Galaxy, the force of the collision caused a powerful shock wave through the galaxy. Moving at high speed, the shock wave swept up gas and dust, creating a starburst around the galaxy's centre (which was undisturbed). This explains the bluish ring around the center, brighter portion. It can be seen that the galaxy is beginning to retake the form of a normal spiral galaxy, with arms spreading out from a central core.
There are other possible explanations of the ring-like appearance.
- Moore, Patrick 2000. The data book of Astronomy. CRC Press, 318. ISBN 0-7503-0620-3
- "Amazing Space - fast facts: Cartwheel Galaxy". Amazing Space. 2008. http://amazing-space.stsci.edu/resources/fastfacts/cartwheel_galaxy.php.p=Astronomy+basics@,eds,astronomy-basics.php&a=,eds. Retrieved 2009-07-03.
- Amram P. et al (1998). "The Hα kinematic of the Cartwheel galaxy". Astron Astrophys. 330: 881–93.
- Zwicky F (1941). Theodore van Karman Anniversary volume: contribution to applied mechanics and related subjects. Pasadena, California: California Institute of Technology. p. 137.
- Griv E (2005). "Origin of the Cartwheel Galaxy: disk instability?". Astrophys Space Sci. 299 (4): 371–85. doi:10.1007/s10509-005-3423-5.
- Mayya YD et al. (2005). "The discovery of spiral arms in the starburst galaxy M82". Ap J. 628 (1): L33. doi:10.1086/432644.
- That is, 200 million years before the image. We see the galaxy as it was 500 million years ago, and the collision took place 700 million years ago.
- "Cartwheel Galaxy". College of Southern Nevada. http://sites.csn.edu/planetarium/galaxy.html. Retrieved 2009-07-03.
- Platt, Jane (2006). "Cartwheel galaxy makes waves in new NASA image". NASA. http://www.nasa.gov/mission_pages/galex/galex-20060111.html. Retrieved 2009-05-15.