Nebulae are interstellar clouds of molecular material, most commonly hydrogen, helium, other ionized gases and dust. There are the emission nebulae that are “excited” by a nearby star or stars and there are dark nebulae which are made up of the same material but not energized by a nearby star and thus remain dark. To see a classic example of this interaction just look at the Horsehead nebula that is shown in the title area of this website. The red glowing part of the scene is the emission nebula IC 434. In front of the IC 434 is the dark nebula Barnard 33 (the horsehead itself). Other types of nebula come into being when stars destriy themselves in a supernova (and example is the Crab Nebula (M1) or cast off an outer shell of gas (thus creating what is called “planetary nebulae”).