The Triangulum Emission Nebula NGC 604 lies in a spiral arm of Galaxy M33, 2.7 million light-years from Earth. This nebula is a region in which stars are forming.

A nebula (Latin for "mist"; plur. nebulae) is an interstellar cloud of dust and gas. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy is sometimes referred to as the Andromeda Nebula).

Nebulae can be classified by how they are illuminated:

• Diffuse nebulae are illuminated nebulae
  • Emission nebulae are internally illuminated clouds of ionized gas. Two of the most common types of emission nebula are H II regions and Planetary nebulae
  • Reflection nebulae are illuminated by reflections from nearby stars. An example is the nebulosity inside the Pleiades star cluster.
• Dark nebulae are unilluminated. They can be detected when they obscure stars or other nebulae. Famous examples include the Horsehead nebula in Orion, and the Coalsack Nebula in the Southern Cross.

HII regions are the birthplace of stars. They are formed when very diffuse molecular clouds begin to collapse under their own gravity, often due to the influence of a nearby supernova explosion. The cloud collapses and fragments, forming sometimes hundreds of new stars. The newly-formed stars ionize the surrounding gas to produce an emission nebula.

Other nebulae are formed by the death of stars. A star that undergoes the transition to a white dwarf blows off its outer layer to form a planetary nebula. Novae and supernovae can also create nebulae known as nova remnants and supernova remnants respectively.

See also:

• Solar nebula
• Timeline of the interstellar medium and intergalactic medium
• Crab nebula
• Messier object
• Images of nebulae

The Nebula Awards, annually given out by the Science Fiction and Fantasy Writers of America, are named after nebulae.