The Cartwheel Galaxy (also known as ESO 350-40 or PGC 2248) is a lenticular ring galaxy about 500 million light-years away in the constellation Sculptor. It has a D25 isophotal diameter of , and a mass of about solar masses; its outer ring has a circular velocity of . It was discovered by Fritz Zwicky in 1941. Zwicky considered his discovery "one of the most complicated structures awaiting its explanation on the basis of stellar dynamics." An estimation of the galaxy's span resulted in a conclusion of 150,000 light years, slightly smaller than the Andromeda Galaxy. The Third Reference Catalogue of Bright Galaxies (RC3) measured a D25 isophotal diameter for the Cartwheel Galaxy at about 60.9 arcseconds, giving it a diameter of based on a redshift-derived distance of . The large Cartwheel Galaxy is the dominant member of the Cartwheel Galaxy group, consisting of four physically associated spiral galaxies. The three companions are referred to in several studies as G1, the smaller irregular blue Magellanic spiral; G2, the yellow compact spiral with a tidal tail; and G3, a more distant spiral often seen in wide field images. SN 2021afdx, a type II supernova in the Cartwheel Galaxy, was discovered in November 2021. Structures The structure of the Cartwheel Galaxy is noted to be extremely complicated and heavily disturbed. The Cartwheel consists of two rings: the outer ring, the site of massive ongoing star formation due to gas and dust compression; and the inner nucleic ring that surrounds the galactic center. A ring of dark absorbing dust is also present in the nucleic ring. Several optical arms or “spokes” are seen connecting the outer ring to the inner, and are theorized to be reforming spiral arms following the collision. Observations show the presence of both non-thermal radio and optical spokes, but the two do not overlap and have been shown to be unassociated with each other, and thus are different structures. Evolution The galaxy was once a normal spiral galaxy before it apparently underwent a head-on “bullseye” style collision with a smaller companion approximately 200-300 million years prior to how we see the system today. When the nearby galaxy passed through the Cartwheel Galaxy, the force of the collision caused a powerful gravitational shock wave to expand through the galaxy, like a rock being tossed into a sandbed. Moving at high speed, the shock wave swept up and compressed gas and