North America nebula (top to bottom left) and Pelican nebula (bottom right). These are emission nebulae, huge clouds of gas and dust that glow as the hydrogen they contain is ionised by radiation from the hot young stars that have formed within them. The two nebular are separated by a molecular cloud of dark dust. The nebulae lie 1800 light years from Earth in the constellation Cygnus. Different colours represent different ionised gases; sulphur is red, hydrogen is green and oxygen is blue.