Diffuse nebulae are clouds of interstellar matter , i.e. thin and widely dispersed collections of gas and dust . In contrast to the planetary nebulae , diffuse nebulae are irregularly shaped and are not necessarily stimulated by one, but mostly by several stars to shine or reflect . Diffuse nebulae themselves (without their stars) can contain up to 100,000 solar masses of matter .
- Some of these young stars are often very massive and so hot that their high radiation energy can stimulate the surrounding gas in the nebula (mostly hydrogen ) to glow. Such nebulae are called emission nebulae .
- If the stars are not hot enough, their light is reflected by the surrounding dust and can be observed as a white or blue glowing reflection nebula .