A nebula is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula referred to any diffuse astronomical object, including galaxies beyond the Milky Way. Most nebulae are of vast size; some are hundreds of light-years in diameter. A nebula’s appearance depends on how it is lit up. Emission nebulae are illuminated by nearby stars; reflection nebulae have no nearby star to illuminate them and appear blue because they reflect the blue light from a nearby star; and absorption nebulae block out the light from behind them, making them appear dark.
Nebulae are often categorized according to their shape: planetary nebulae have round or elliptical shapes, emission nebulae often have irregular shapes, and supernova remnants tend to be filamentary. The Eagle Nebula (M16) is an example of an emission nebula; the Dumbbell Nebula (M27) is an example of a planetary nebula; and the Crab Nebula (M1) is an example of a supernova remnant.
Nebulae form when stars eject material during their death throes, or when two galaxies collide and trigger star formation. In either case, the resulting shock waves compress clouds of interstellar gas and dust until they collapse under their own gravity and form new stars. As these young stars heat up and begin to shine, they ionize the surrounding gases—stripping electrons off atoms—and make the Nebulae visible at optical wavelengths.
Some Nebulae are also sites of ongoing star formation. These “star factories” contain dense clumps of gas that collapse to form new stars. The Orion Nebula (M42), for example, is one such region where several hundred low-mass protostars are currently forming within a larger cloud of gas and dust