Luminosity is an absolute measure of radiated electromagnetic energy (light) per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical objects.
The Sun has an intrinsic luminosity of 3.83×1026 watts. In astronomy, this amount is equal to one solar luminosity, represented by the symbol L⊙. A star with four times the radiative power of the Sun has a luminosity of 4 L⊙.
In astronomy, magnitude is measure of the brightness of an object, usually in a defined passband. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.
Light sources of different magnitudes. A very bright satellite flare can be seen in the night sky.