Bubble nebula (NGC 7638). This nebula is named for the circular shell of gas (blue) at centre. This hollow shell has been excavated out of the surrounding nebula by the powerful stellar wind of a massive star within it. The entire nebula glows as the gases it contains are ionised by radiation from hot young stars that have formed within it. The nebula lies 11,000 light years from Earth in the constellation Cassiopeia.