The space between the stars in the galaxies is not completely empty. There are scant traces of dust and gas, particularly hydrogen. In places the gas an dust accumulates to create relatively dense clouds, or nebulae. When a cloud is lit up by stars embedded within it, we call it an emission nebula. When we see one by light reflected from nearby stars, we call it a reflection nebula. Sometimes the cloud is not lit up at all. We only see it, as a dark nebula, when it blots out the light from background stars. The most prominent bright nebula, which we can see with the naked eye, is the Orion nebula. Other interesting ones include the North American nebula in Cygnus, the Trifid and Omega nebula in Sagittarius, and the Ring Nebula in Lyra. The last one looks like a smoke ring. It is a mass of gas puffed off when a central star exploded many years ago. The best-known dark nebulae are the Horsehead nebula in Orion and the Coal Sack in Crux, the Southern Cross.
