Luminosity

Luminosity is an absolute measure of radiated electromagnetic energy 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 luminous astronomical object.

In SI units, luminosity is measured in joules per second, or watts. In astronomy, the luminosity of the Sun, L_☉ is often used as a unit of luminosity. Luminosity can also be stated in the astronomical magnitude system: the absolute bolometric magnitude (M_bol) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band.

In contrast, the term brightness in astronomy is generally used to refer to an object's apparent brightness to an observer. Apparent brightness depends on the luminosity of the object, the distance between the object and observer, and on any absorption of light along the path from object to observer. Apparent magnitude is a logarithmic measure of apparent brightness. Distance can often be estimated as the luminosity distance if the luminosity is thought to be known, but this can be inaccurate.