<P> By treating the star as an idealized energy radiator known as a black body, the luminosity L and radius R can be related to the effective temperature T by the Stefan--Boltzmann law: </P> <Dl> <Dd> L = 4 π σ R 2 T e f f 4 (\ displaystyle L = 4 \ pi \ sigma R ^ (2) T_ (eff) ^ (4)) </Dd> </Dl> <Dd> L = 4 π σ R 2 T e f f 4 (\ displaystyle L = 4 \ pi \ sigma R ^ (2) T_ (eff) ^ (4)) </Dd> <P> where σ is the Stefan--Boltzmann constant . As the position of a star on the HR diagram shows its approximate luminosity, this relation can be used to estimate its radius . </P>

How do you know if a star is main sequence