Question: What is diffuse nebulae?
About the article "Nebula", section "Diffuse nebulae".

Candidate passages:
  Lists of nebulae * List of dark nebulae * List of diffuse nebulae * List of planetary nebulae * List of protoplanetary nebulae  
Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.
In visible light these nebulae may be divided into emission and reflection nebulae.
Emission nebulae emit spectral line radiation from ionized gas (mostly ionized hydrogen); they are often called HII regions (the term "HII" is used in professional astronomy to refer to ionized hydrogen).
Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them.
Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called "dark nebulae".
Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae.
Edwin Hubble distinguished between the emission and reflection nebulae in 1922.
Reflection nebulae are usually blue because the scattering is more efficient for blue light than red (this is the same scattering process that gives us blue skies and red sunsets).
Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.
Some 500 reflection nebulae are known.
Among the nicest of the reflection nebulae are those surrounding the stars of the Pleiades.
A blue reflection nebula can also be seen in the same area of the sky as the Trifid Nebula.
The giant star Antares, which is very red (spectral class M1), is surrounded by a large, red reflection nebula.
Reflection nebulae may also be the site of star formation. 
Astronomers divide nebulae into four major groups, all of which are represented in the sections that follow:  Diffuse nebulae.
Most nebulae can be described as diffuse nebulae, which means that they span extended areas and are not circumscribed by well-defined boundaries.
In visible light these nebulae may be further divided into emission and reflection nebulae.
Emission nebulae emit spectral line radiation from ionized gas, principally hydrogen (H II), with a predominant red emission line at 656 nm.
Reflection nebulae do not themselves emit significant amounts of visible light, but are near stars and reflect light from them, with blue light being the most efficiently scattered.
Planetary nebulae.
Planetary nebulae form from the gaseous shells that are ejected from low-mass red giant stars as they transform into white dwarfs.
The Sun is expected to spawn a planetary nebula about 12 billion years after its formation.
Supernova remnants.
A supernova occurs when a high-mass star reaches the end of its life: When nuclear fusion in the core of the star stops, the star collapses.
The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.
The expanding shell of gas forms a supernova remnant, a special type of diffuse nebula.
Dark nebulae.
A dark nebula is a type of diffuse nebula that is so dense it obscures the light from a background emission or reflection nebula (for example, the Horsehead Nebula) or it blocks out background stars (for example, the Snake Nebula).  
The Sadr region, or IC 1318, is the diffuse emission nebula surrounding Sadr or Gamma Cygni.
Sadr lies in the center of Cygnus's cross.
The Sadr region is one of the surrounding nebulous regions; others include the Butterfly Nebula and the Crescent Nebula.
It contains many dark nebulae in addition to the emission diffuse nebulae.
Sadr itself has approximately a magnitude of 2.2.
The nebulous regions around the region are also fairly bright.