Their appearance differs widely, depending not only on the temperature and density of the material observed but also on how the material is spatially situated with respect to the observer. Dark nebulae appear as irregularly shaped black patches in the sky and blot out the light of the stars lying beyond them. These clouds are the sites where new stars are formed through the gravitational collapse of some of their parts. Bright nebulae are comparatively dense clouds of gas within the diffuse interstellar medium.
Reflection nebulae reflect the light of a nearby star from their constituent dust grains.
The gas of reflection nebulae is cold, and such objects would be seen as dark nebulae if it were not for the nearby light source. The star must be of stellar type O or B, the most massive and hottest of normal stars in the Galaxy, in order to produce enough of the radiation required to ionize the hydrogen. That is to say, a red giant has shed its outer envelope in a less violent event than a supernova explosion and has become an intensely hot star surrounded by a shell of material that is expanding at a speed of tens of kilometres per second. Planetary nebulae typically appear as rather round objects of relatively high surface brightness. Supernova remnants are the clouds of gas expanding at speeds of hundreds or even thousands of kilometres per second from comparatively recent explosions of massive stars.