Cartwheel Galaxy (ESO 350-40). Around 100 million years ago, a smaller galaxy passed directly through the centre of the Cartwheel Galaxy. The resulting shock waves propagated through the galaxy, causing bursts of star formation. The bright blue ring marks the region where huge, powerful ultraviolet-emitting stars have formed. The inner orange ring is the second wave, but this has far less star formation than the first wave. The Cartwheel Galaxy is around 500 million light years away, in the constellation of Sculptor. Image obtained on 11 September 2015.