[UPSC CSE 1990] Black Body Radiation (Q492, 20M)

Define solar constant and say which of the values $1.34\text{ W/m}^2$ , $1.34 \times 10^3\text{ W/m}^2$ , $1.34 \times 10^5\text{ W/m}^2$ is valid for it. Calculate the total energy radiated by the sun in one second and hence the decrease in its mass per second.