cloud

The term cloud (or interstellar cloud) in astronomy is used to refer to regions with a higher density of gas and/or dust. Some have been visible and observed over the centuries, originally termed nebulae, though the term nebulae was also used for visible galaxies and stellar clusters. Among the types of clouds:

• molecular cloud (and giant molecular cloud).
• HI region.
• HII region.

Star formation takes place within clouds (star-forming regions) that have grown cool and dense, some terms being:

• Bok globule
• dense core
• filament

Other terms for various kinds clouds (in some cases, classified by appearance from Earth):

• dark nebula
• bright nebula
• reflection nebula, light nebula, cometary nebula.
• emission nebula
• evaporating gas globule
• high-velocity cloud
• infrared dark cloud
• photodissociation region
• planetary nebula
• Herbig-Haro object
• pulsar wind nebula
• supernova remnant
• infrared cirrus
• intergalactic HI cloud
• Lyman-alpha blob
• protogalaxy
• radio relic
• Strömgren sphere
• radio halo
• radio phoenix

Note that the Magellanic Clouds (the Small Magellanic Cloud and Large Magellanic Cloud), despite their names, are actually galaxies rather than clouds in the above sense. Also, the Oort Cloud is not the above kind of cloud: it is the name of a region with many comets.