A measurement of the total amount of energy given off from a star: Luminosity is the total amount of energy radiated by a luminous object per unit time, for example one second. Common units are watts.

Luminosity can also be described as the speed at which a star's energy is emitted in all directions.

An empirical relation between the mass and luminosity, both usually expressed in solar units, for main sequence stars: The mass-luminosity relation was predicted by Eddington in 1924.

For sunlike stars, the luminosity varies as the 3.5 power of the mass.

The power is smaller for lower mass stars.

A relation obeyed by cepheid variable stars (highly luminous yellow or orange super giant stars that varies regularly in brightness), and which states that the period of the changes in luminosity varies directly with the luminosity of the star.