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.
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