The unique shape of the cartwheel galaxy with its huge rim - larger than the diameter of the milky way galaxy - is likely due to a collision with one of the smaller galaxies on the lower left several hundred million years ago 車輪星系獨特的外觀及龐大的外緣(比銀河系的直徑還要大)可能起源于幾億年前與左下某個小星系的碰撞。