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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 watts. A watt is one unit of power. Just as a light bulb is measured in watts, so too is the Sun. The sun gives off 3.846×1026 W. This number is known as 1 sol, the symbol for which is [math]L_\odot[/math].
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. Naturally this 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.