What process causes an Emission Nebula to produce its own visible, often reddish, glow?
Answer
Ionization of hydrogen by powerful ultraviolet light, followed by electron recombination.
Emission nebulae are characterized by their ability to generate their own light, which typically appears reddish due to the abundance of hydrogen. This process is initiated by proximity to extremely hot, young, massive stars. These stars emit powerful beams of ultraviolet radiation that possess enough energy to strip electrons away from the surrounding hydrogen atoms, resulting in a state known as ionization. When these free electrons subsequently manage to recombine with the positively charged hydrogen nuclei, energy is released in the form of photons, causing the entire cloud structure to visibly glow intensely.
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