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 of nebulae. Emission nebulae emit spectral line radiation from iozonid gas (mostly ionized hydrogen they are often called HII region (the term "HII" is used in professional astronomy to refer to ionized hydrogen).
Reflection nebulae do not themselves 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 infered emission, chiefly from dust within the nebulae
.#credit: http://en.wikipedia.org/wiki/Nebula
Reflection nebulae do not themselves 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 infered emission, chiefly from dust within the nebulae
.#credit: http://en.wikipedia.org/wiki/Nebula
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