THE STUFF OF STARS
Nebulas can be placed into two broad categories: bright and dark. Bright nebulas are glowing clouds of dust and gas that would be invisible if they were not associated with stars.
A bright nebula can be simply a reflection nebula, where starlight reflects off the nebula's dust, like a lighthouse beam illuminating a fog bank. When a nebula is near a very hot star, however, the intense ultraviolet radiation from that star excites, or ionizes, atoms within the gas, causing the cloud itself to emit light and become visible as an emission nebula. Emission nebulas are apparent in star-forming regions—good examples are the Great Nebula in Orion (M42) and the Lagoon Nebula (M8). Emission/reflection nebulas are a mixture of the two nebula types.
In photographs, reflection nebulas are a cool blue color and emission nebulas glow red, but our eyes are not sensitive enough to detect these colors through a telescope.
Planetary nebulas, such as the Ring Nebula (M57) and the Dumbbell Nebula (M27), are the remains of stars that puffed away their outer envelopes in the final phases of their lives. The Veil Nebula
(NGC 6960) and the Crab Nebula (Ml) are both supernova remnants— the gaseous shards of stars that ended more dramatically by exploding.
Dark nebulas are clouds of dust and gas dense enough to obscure the light from background stars. They are most visible when silhouetted against bright nebulas—two examples are the famous Horsehead
Nebula (1C 434) and the dark lanes in the Lagoon Nebula
(M8). Infrared telescopes often reveal the presence of new stars within these dark clouds.