Luminosity is a direct measure of the total energy radiated away by a star, expressed in joules/second or watts. If one has the flux and the distance, then one can calculate the luminosity of a star. That is:
which is usually expressed in units of the solar luminosity (4 × 10 26 watts = 1 L ⊙). Luminosity (sometimes called absolute brightness) and absolute magnitude are two different ways of expressing a measurement of the same aspect of a star. From the definition of magnitude, therefore, one has
The span of absolute magnitude –10 < M < +20 covers the full range of absolute stellar brightnesses for normal stars.