Luminosity is the total energy an object radiates each second, an intrinsic property that does not depend on how far away the observer stands. It differs from apparent brightness, which falls off with the square of distance. Comparing the two is how astronomers measure distance: if the true luminosity of a source is known, its observed faintness reveals how far away it lies. Objects with reliably known luminosity, such as Type Ia supernovae and Cepheid variables, are called standard candles and hold up the cosmic distance ladder.