A foe (or bethe) is a unit of energy on the order of the energy release of a supernova. It is defined as (i.e., a shorthand for) 1051 ergs or 1044 joules. The name is an acronym for "fifty one ergs", i.e., the last three words of the phrase "ten to the power of fifty one ergs". It can be used for supernova studies, or as a shorthand for other processes dealing with similar amounts of energy. For example, a specific supernova might be quoted as releasing 1.3 foe of energy, or a star might be quoted as releasing 2.3 foe over the course of its lifetime. The Sun is expected to release on the order of a foe.
Models of supernovae attempt to simulate the reactions from different circumstances to the release of on the order of 1051 ergs, which serves as one test of the model's plausibility.