What causes the surface cooling responsible for imparting the 'red' color to a red supergiant?
Answer
The outward expansion forces the outer layers far from the central core, causing them to cool down.
The characteristic reddish hue is a direct consequence of the star's physical restructuring during expansion. When the outer gaseous envelope is forced outward due to extreme pressure from shell fusion, these layers move a much greater distance from the hot, central furnace. This increased separation leads to significant cooling of the star's surface temperature. This relatively low surface temperature is what causes the star to glow with the distinctive, deep reddish or orange color associated with red supergiants.
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