What physical state of gas produces the bright glow observed in an emission nebula?
Answer
Plasma resulting from electron recombination after excitation.
An emission nebula is characterized by its bright, intrinsic glow, which results from intense local energy input, typically ultraviolet radiation from nearby hot, young stars. This radiation excites the gas, stripping electrons from the atoms, which forces the gas into a plasma state—a superheated, ionized gas. As the excited electrons recombine with the atoms, they release energy in the form of photons across the electromagnetic spectrum. For instance, the recombination of ionized hydrogen often results in the characteristic reddish glow seen in these types of nebulae. This process is distinct from scattering light or having gas simply exist in a cold, molecular state.
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