Stars emit massive amounts of energy per second and
so the power of a star is enormous. We assume that a star behaves as a perfectly 'black body' in
other words it is a perfect radiator of radiation at its surface temperature.

The Stefan-
Boltzmann law states that the power emitted by a black body of surface area A and with a surface
temperature T (K) is given by the equation:

Power = σAT^{4} where σ is a constant
(5.7x10^{-8} Wm^{-2}K^{-4}).

(Note: we are assuming here that the
temperature of the surroundings (deep space) has a temperature of 0 K)

If we assume
that a star is roughly spherical then A = 4πr^{2} for a
star of radius r.

The power of a star is therefore:

Consider our Sun. It is a star of surface temperature 6000 K, and a radius 6.96x10

Power output of the Sun = 7.16x10

An alternative way of finding out the power output of the Sun is to use the solar constant.

See: Solar constant

It is interesting to compare this power output with that of Canopus (a Carinae). Canopus has a surface temperature of 7500 K and a radius of 2x10