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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 a measure of brightness. Brightness is how much light is given off.

In astronomy, the total amount of energy emitted by a star or other astronomical object is its luminosity. 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 number is known as 1 sol, the symbol for which is .

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.

If it is necessary to talk about the total power output across all wavelengths, that is called bolometric magnitude.