Bubble Nebula. Optical image of the Bubble Nebula (NGC 7635), an emission nebula. North is towards lower right. An emission nebula is a cloud of gas that glows due to the ionisation of the hydrogen it contains by the radiation from hot stars within it. This nebula takes its name from the spherical ring of gas at upper centre. This ring has been formed by the powerful stellar wind of a massive star at its centre. This wind has excavated a hollow bubble from the nebula surrounding the star. The bubble is thought to be around 10 light years in diameter. It lies around 11,000 light years from Earth in the constellation Cassiopeia.