The amount of radiant power produced by the sun is approximately 3.9 x 1026 W. Assuming the sun to be a perfect blackbody (e = 1) sphere with a radius of 6.96 x 108 m, find its surface temperature (in kelvins).

Answer 1

here:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html

NOW, that is the power PER UNIT AREA
so

3.9 *10^26 / (4 pi r^2) = power per unit area
= constant*T^4
as shown in Stefan/ B link