Pacman nebula (NGC 281). This is an emission nebula, a huge cloud of gas and dust that glows as the hydrogen it contains is ionised by radiation from the hot young stars that have formed within it. Silhouetted against the bright nebula are several small, dense blobs known as Bok globules. These are compact regions of gas and dust in the latter stages of the process of star formation. The nebula lies some 10,000 light years from Earth in the constellation Cassiopeia. Different colours represent different ionised gases; sulphur is red, hydrogen is green and oxygen is blue.