What excites the hydrogen gas in emission nebulae to cause glowing?
Answer
Intense ultraviolet radiation from very hot, young stars within or near the cloud.
Emission nebulae shine because they are actively interacting with high-energy photons originating from nearby stellar objects, specifically very hot, young stars. These stars emit copious amounts of intense ultraviolet radiation. When this high-energy radiation strikes the surrounding, relatively cooler hydrogen gas, it strips electrons from the protons, ionizing the gas. As these free electrons subsequently recombine with the protons, they release energy in the form of visible light, often resulting in the characteristic reddish hue associated with hydrogen emission spectral lines.
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