A cometary globule is a dense cloud of gas and dust. This image shows CG 4, which is 1,300 light years away. Cometary globules are so called because of their superficial resemblance to comets, with a bright-rimmed head and long diffuse tail, which can be up to 10 light years in length. Nearby young stars ionize hydrogen in the head of the dust cloud, making it glow red, and their radiation sweeps the dust into a long, tenuous tail. The blue colour is the result of light-scattering by tiny dust particles. This photograph was taken with the Anglo-Australian Telescope.