The Cartwheel Galaxy, seen by the NASA/ESA Hubble Space Telescope, its shape the result of a violent galactic collision. A smaller galaxy passed right through the large disc galaxy and produced shock waves that swept up gas and dust — much like the ripples produced when a stone is dropped into a lake — sparking regions of intense star formation (appearing blue). The outermost ring of the galaxy, 1.5 times the size of our Milky Way, marks the shock wave’s leading edge. See https://www.spacetelescope.org/images/potw1036a/
