Why is a 1 solar-mass red giant more luminous than a 1 solar-mass main sequence star?

A 1 solar-mass red giant is more luminous than a 1 solar-mass main sequence star primarily due to its larger radius and the characteristics of its energy production system.

As stars evolve from the main sequence to the red giant stage, they experience significant changes in their internal structure. In the red giant phase, despite having the same mass, the star has expanded tremendously. The outer layers of the red giant have expanded significantly, which increases its surface area dramatically. Even though the temperature of the surface may be lower compared to main sequence stars, the sheer increase in size leads to a much greater luminosity overall. The luminosity of a star is proportional to the square of its radius multiplied by the fourth power of its surface temperature. Thus, the vast increase in radius of a red giant compensates for any lower surface temperature.

While fusion processes do continue in both states, the stars in the red giant phase can ignite helium fusion and other processes that might not be occurring in main sequence stars. However, it is the size and its impact on light emission that plays the crucial role in the greater luminosity.

In summary, a red giant’s larger radius allows it to emit more light compared to a main sequence star of the same mass