Equation 8: Radius of a Star

rRadius of the Star (m)
LLuminosity of the star (W)
TSurface Temperature of the Star (K)

The surface temperature of a star is determined by it's spectrum.

As such, the radius of a distant star can be determined by its spectrum and luminosity.
In conjunction with Equation 7, the radius of a star can also be determined by its spectrum and absolute magnitude.

Our star, sol, is a G2 main sequence (V) star, with a surface temperature of 5782.75 K, luminosity of 3.86 x1026 W, and a radius of 6.956 x 108 m.

The visible-light spectrum of our sun can be seen here.