Emission nebula NGC 6559, optical image. Emission nebulae are clouds of gas and dust that are ionised by radiation from hot stars within them, causing them to glow red. Around the bright stars are regions of reflection nebulosity, where fine dust particles reflect the light of the stars. These regions appear blue as short_wavelength blue light scatters more than red. There are also dark dust lanes silhouetted against the bright nebula. This nebula lies 5000 light years from Earth in the constellation Sagittarius.