What physical process allows the gases in a planetary nebula to glow with visible light?
Answer
Ionization of gas atoms by ultraviolet radiation from the white dwarf
The vibrant colors observed in planetary nebulae are the result of ionizing radiation. The white dwarf at the center emits intense ultraviolet radiation as it cools. This radiation travels outward and collides with the previously ejected gaseous cloud. These collisions strip electrons from the atoms in the gas, a process called ionization. When these ionized atoms capture electrons back, they release energy in the form of visible light, with different elements producing different colors.
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