Luminosity

Image of galaxy NGC 4945. It shows the huge luminosity of the central star clusters, suggesting there are 10 to 100 supergiant stars in each of these, packed into regions just a few parsecs across.

Luminosity is the amount of power given off by an astronomical object.

Stars and other objects emit energy in the form of radiation. It is measured in joules per second, which are equal to watts. A watt is one unit of power. Just as a light bulb is measured in watts, the Sun can also be measured in watts. The sun gives off 3.846×10²⁶ W. This amount of power is known as 1 sol, the symbol for which is $L_{\odot }$.

There are other ways to describe luminosity. The most common is apparent magnitude, which is how bright an object looks to an observer on Earth. It only applies to light, that is, visible wavelengths. Apparent magnitude is contrasted with absolute magnitude, which is an object's intrinsic brightness at visible wavelengths, irrespective of distance. The apparent magnitude is the less, for objects more than 32.6 light years away.

When talking about the total power output across all wavelengths, that is called bolometric magnitude.