Omega nebula (M17), optical image. This nebula, also called the Swan nebula, is a huge cloud of gas and dust in which stars formed, and are still forming. Ultraviolet radiation from the hot young stars that formed within it ionises the remaining gases in the nebula, causing them to glow. The colours here relate to the ions responsible for the light emission: red is from ionised sulphur, green from hydrogen and blue from oxygen. The Omega nebula lies around 6000 light years from Earth in the constellation Sagittarius.