nebula

The term nebula is currently used for a cloud of gas and dust. A general classification according to their visible appearance divides them into bright nebulae versus dark nebulae. A dark nebula includes dust blocking any light from behind, often a star-forming region. A bright nebula may be a reflection nebula, essentially what would be a dark nebula, but is reflecting light from a nearby source (e.g., star) or a bright nebula may be an emission nebula, emitting light itself, such as an HII region. Planetary nebulae and supernova remnants are also nebulae that emit light. The term diffuse emission nebula is used for those without distinct boundaries.

The term nebula was coined for (stationary) fuzzy objects, i.e., things that appear quite different from stars, planets, the Moon, comets (which move across the sky like planets), or the Sun. This included distant groups of stars, e.g., stellar clusters and galaxies. The name may still be used occasionally for stellar clusters, but use of the term for galaxies is now virtually gone, even though it was formerly the ordinary term for them (before they were considered to be galaxies), a usage that is still seen in historical writing, such as references to Andromeda as the Great Andromeda Nebula, and the Messier Catalog's original name, Catalogue of Nebulae and Star Clusters.

The word nebula is also used within some phrases that indicate particular kinds of "fuzzy" objects, such as planetary nebulae and protoplanetary nebulae.