Question

What causes a red giant star's luminosity?

Answer 1

The red giants are very luminous because they are so large, although their surface temperature is lower than that of the Sun.

Explanation:

In the red giant phase, the star's core gets hotter and its luminosity increases greatly. As the star expands, the photosphere's surface area increases dramatically, With the star's energy being emitted by a much larger radiating surface, the energy output per unit area decreases, thereby lowering the surface temperature.