In a red giant, how does the vastly increased luminosity relate to the surface temperature?

Answer

Surface temperature decreases because energy spreads over a larger volume

The total energy output (luminosity) increases due to the enormous surface area, but the surface temperature drops because that energy is spread out over this much larger volume.

In a red giant, how does the vastly increased luminosity relate to the surface temperature?

#Videos

Why Do Red Giant Stars Get Bigger? - YouTube

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