Cartwheel galaxy, multi-wavelength image. Around 100 million years ago, a smaller galaxy passed directly through the centre of the Cartwheel galaxy. This caused shock waves to propagate out through the galaxy, which caused 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. It is not known if the colliding galaxy was one of the two at lower left. The images were taken in X-rays by the Chandra Space Telescope (purple), in UV by GALEX (blue), in green light by the Hubble Space Telescope, and in infrared by the Spitzer Space Telescope (red).