The Stefan-Boltzmann law states that the power radiated by a black body is directly proportional to the fourth power of its absolute temperature. This means that if we know the power radiated by a black body, we can calculate its temperature by solving the equation P = σ * T^4, where P is the power radiated, σ is the Stefan-Boltzmann constant (5.67 * 10^-8 W/m^2 K^4), and T is the temperature in kelvins.
In this case, the power radiated by the sun is 6.5 * 10^26 W, so we can calculate its temperature by solving the equation 6.5 * 10^26 = 5.67 * 10^-8 * T^4. This gives us T = 5750 K, which is the surface temperature of the sun.