Luminosity is the total amount of energy that an object, such as a star, radiates into space every second. It depends on both the surface area and the surface temperature of an object, so that, for example, two stars with the same surface temperature but different luminosity must differ in size.

 

Luminosity, L, is measured in watts (W) or in terms of solar luminosity L_sun (3.9 × 10²⁶ watts) and is related to bolometric magnitude (M _bol) by the formula:

 

    M _bol – 4.72 = –2.5 log (L/L _sun)

 

For example, in the case of Sirius L/L _sun = 23, so that M _bol (Sirius) = 4.72 – 2.5 log 23 = 1.32.

 

The luminosity of a blackbody (which most stars closely approximate) of temperature T and radius R is given by the Stefan-Boltzmann equation:

 

    L = 4πR ²σT ⁴

 

where σ is the Stefan-Boltzmann constant (5.67 × 10^–8 W/m²/K⁴).