Nebulae are vast clouds of dust and/or gas that either reflect the light of nearby stars or emit light due to the gas molecules being excited by the radiation of nearby stars. Some are a combination of both reflection and emission nebulae. Some nebulae are also "dark" nebulae which block the light of background stars. There are also "planetary" nebulae which are emission nebulae created when a giant star near the end of its life cycle throws off its outer layers which are then excited by the white dwarf star that then remains. Nebulae are generally the most colorful objects in space.