What physical process leads to the characteristic red color of a red giant star?
Answer
The surface temperature drops as the outer layers expand
As the outer envelope of a star expands, the gas within those layers spreads out over a much larger volume, leading to a decrease in density. Thermodynamics dictates that as gas expands, it loses thermal energy and cools down. This drop in surface temperature shifts the peak wavelength of light emitted by the star toward the longer, red end of the visible spectrum. Even though the star is generating substantial energy from shell burning, the lower surface temperature is what gives it its distinct reddish appearance.
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