Well, i've seen in almost all sites and books that Red Giants are more luminous than the star itself... Particularly, let us take our sun as an example... First of all, we know that due to the He- Fusion and the enormous pressure acting on the H- on the surface, it IGNITES or fuses faster... Thus, the energy is vast causing it's Surface area to increase... This is the main cause of y red giants appear more luminous.. However, to my point of view .. L= σ A T^4 ... The temperature will decrease by this increase in volume and the area is increasing.. If we take each by scale. Temperature should rule the way the red giant luminates.. If there is an increase in area, temperature will decrease with the scale of FOUR... Shouldn't it be less luminous ... I need clarification in this part... Thanks in advance to whoever helps!