\[L=4 \pi \sigma R^2T^4\]
where
\[R=Radius \: of \: Star\]
\[T=Surface \: Temperature\]
\[\sigma = Stefan's \: Constant =5.67 \times 10^{-8} W/K^4 /m^2\]
Increasing Luminosity of the Sun
The Sun is getting bigger and brighter, very slowly. It's temperature is stable, has been so for billions of years and will be so for billions more, but because of the luminosity, temperature radius relationship