In astronomy, luminosity is not quite the same thing as brightness. Luminosity measures the total amount of energy emitted</u/l> by a star or other astronomical object in SI units of joules per second, which are watts. A watt is one unit of power, and just as a light bulb is measured in watts, so is the Sun, which has a total power output of 3.846×10 W. This number is the basic metric used in astronomy: it is known as 1 solar luminosity, the symbol for which is formula_1. 