Emission and reflection nebulae are two distinct types of nebulae that differ primarily in their composition and the mechanisms by which they are illuminated. Here are the key differences between them:
Emission Nebulae:
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Composition: Emission nebulae are primarily composed of ionized gas, mostly hydrogen, but may also contain other elements in smaller amounts.
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Illumination Mechanism: They emit their own light. The light is produced when ultraviolet radiation from nearby hot stars ionizes the gas, causing it to glow as electrons recombine with protons.
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Color: Typically, emission nebulae appear red due to the strong H-alpha emission line of hydrogen (at around 656 nm), along with other emission lines from different elements.
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Examples: Famous examples include the Orion Nebula (M42) and the Lagoon Nebula (M8).
Reflection Nebulae:
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Composition: Reflection nebulae are made up of dust and gas but do not contain ionized gas that emits light on its own.
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Illumination Mechanism: They do not emit light; instead, they reflect light from nearby stars. The light that is scattered is predominantly from stars that are not hot enough to ionize the gas.
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Color: Reflecting nebulae often appear blue, as blue light is scattered more efficiently than other colors by the dust particles.
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Examples: An example of a reflection nebula is the nebula surrounding the Pleiades star cluster (M45).
Summary
In summary, emission nebulae shine due to ionized gas emitting light from nearby hot stars, resulting in a red coloration, while reflection nebulae appear blue as they reflect light from starlight without emitting their own light.
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