Colliding galaxies. Supercomputer simulation showing the gas content of two colliding spiral galaxies. Temperature is represented by colour, blue (coolest) to red (hottest). Brightness represents density. Two galaxies approach and merge (top pair), then separate (second pair), before merging into a large elliptical galaxy (bottom). During the merger, gravity drives gas into the centre, causing a burst of new star formation. The incoming gas also feeds a supermassive black hole which is thought to power a quasar. The black hole increases in mass until it releases enough energy to heat the gas and expel it into space. As the new galaxy is gas- poor, little new star formation occurs. Simulation created in 2005 at the Max Planck Institute for Astrophysics, Germany.
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