Why do H II regions glow at characteristic wavelengths?

H II regions glow at characteristic wavelengths because they consist of ionized hydrogen gas, which is heated and excited by the intense ultraviolet radiation emitted by nearby hot, massive stars.12

These stars, which have surface temperatures exceeding 20,000 K, provide the energy necessary to strip electrons from hydrogen atoms. Within these regions, the ionized gas—composed of protons and free electrons—emits radiation as the electrons recombine with ions or interact with the surrounding plasma. Because the hydrogen is ionized, it becomes transparent to a wide range of photons, allowing for the presence of various ionic species that contribute to the nebula's specific spectral signatures. While these regions are often observed at optical wavelengths, they also emit radio waves, which are particularly useful for studying the gas distribution when interstellar dust obscures visible light.123