Emission Nebula

An emission nebula is a cloud of ionized gas (i.e. a plasma) emitting light of various colors. The most common source for ionization are high-energy photons emitted from a nearby hot star. Among the several different types of emission nebula are H II regions, in which star formation is taking place and young, massive stars are the source of the ionising photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.
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A cloud of interstellar gas which is glowing because it absorbs, and then re-emits, electromagnetic radiation from hot stars embedded within it. The ultraviolet radiation from these embedded stars (usually of spectral classification O and B) ionises the hydrogen in the cloud, hence these clouds are sometimes called HII regions. The predominant wavelength of radiation that is released by the hydrogen gas is 656 nm which is in the red end of the electromagnetic spectrum. Thus the emission nebulae are a characteristic pinkish-red in colour. As the solar winds from the ionising star blow away the central regions, the nebula dissipates. Emission nebulae are usually isolated regions of much larger interstellar clouds.