The galaxy was a normal spiral galaxy before it had a collision with a smaller galaxy about 200 million years ago. When the nearby galaxy passed through the Cartwheel Galaxy, the power of the collision sent a strong shock wave through the galaxy. Moving at high speed, the shock wave moved gas and dust. This made a starburst around the galaxy's center. This explains the blue colored ring around the center, bright section. It can be seen that the galaxy is beginning to take the shape of a normal spiral galaxy again, as spiral arms (The spokes) are spreading out from a central core.