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 \displaystyle{ L_\odot }.