Cartwheel galaxy. Optical image of the Cartwheel galaxy (centre) and its companions (lower right). The ring shape of the Cartwheel is due to a collision with one of the smaller galaxies. As the smaller galaxy passed through the Cartwheel, it caused a shock wave to spread outwards in a similar fashion to ripples on a pool of water. The shock wave led to a burst of star formation in the galaxy as it spread outward, leading to the bright ring seen here. Before the collision, the Cartwheel was probably a normal spiral galaxy. The ring is some 150, 000 light years in diameter. This galaxy lies around 500 million light years from Earth in the constellation Sculptor.
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