Luminosity and Brightness

The total energy output of the Sun or any other star is related to its brightness, but the brightness depends also on the distance because of the inverse square law. The luminosity is the true energy output; the apparent brightness is a function of both luminosity and distance.

In addition, the brightness depends on the part of the spectrum to which our eyes or a detector are sensitive. A star that is dim at visible wavelengths might be very bright at UV wavelengths. The Sun appears bright to us because it is luminous, it is close to us relative to other stars, and it emits most of its light at visible wavelengths.