Foe is a unit of energy equal to 10⁴⁴ joules.

To measure the immense amount of energy produced by a supernova, specialists occasionally use a unit of energy known as a foe which is short hand for (Fifty One Ergs) or 10⁵¹ ergs. This unit of measure is convenient because a supernova typically releases about one foe of observable energy. By comparision the total output of the sun over its entire lifespan is about 0.1 foe.