The systematic rotation of stars and gas about the centre of a galaxy. Rotation is most obvious in spiral and lenticular galaxies, and is also seen in some (but not all) ellipticals and irregulars. The rotation maintains the structure of the galaxy against further gravitational collapse. Galaxies exhibit differential rotation, the time taken for one revolution increasing with distance from the centre. The Sun orbits the centre of our Galaxy about once every 230 million years, at a speed of 220 km/s. See also rotation curve.