The Cartwheel Galaxy is the result of a collision between a small and a big galaxy. After the small galaxy had passed through the big one, waves of star formation started spreading through the big galaxy from the collision point. The main acting force, gravitation, makes the waves of density move towards the periphery causing an alternate formation of hot bright young stars that form the ring that can be seen in the picture. Our answer - In reality it is a process of energy formation in a stellar cluster.