Emission nebula Sharpless 157 (centre). The bubble nebula (light blue surrounded by yellow) is also seen at upper left. 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. This nebula lies in the constellation Perseus. Different colours represent different ionised gases; sulphur is red, hydrogen is green and oxygen is blue.