Galaxies come in a range of colours and sizes. In spiral galaxies such as the Milky Way, most stars are in a thin disc. The disc of the Milky Way has a radius of about 15 kpc, and contains ? 6 × 1010M of stars. The disc stars rotate around the centre of the galaxy on nearly circular orbits, with rotation speed vc ? 220 km s?1 almost independent of radius r (unlike themotion of planets in the solar system where vc ? 1/r1/2). Such flat rotation curves require a very extended mass distribution, much more extended than the observed light distribution, indicating the presence of large amounts of invisible dark matter. Curiously, the spiral arms do not rotate with the same speed as the stars, hence stars may move in and out of spiral arms. Stars are born in the large molecular gas clouds that are concentrated in spiral arms. Massive short-lived stars light-up their natal gas with their ionising radiation, and such ‘HII’ regions of ionised hydrogen follow the arms as beads on a string. The blue light from these massive and hot stars affects the colour of the whole galaxy, one reason why spirals are blue. Remember that the luminosity L of a star is a strong function of its mass M, with approximately L ? M3 on the main sequence, hence a single 100M star outshines nearly 106 solar-mass stars: the colour of a galaxy may be affected by how many massive (hence young) stars it harbours.