Add the unit 'Foe'

Author: sharkdp

book/src/prelude/list-units.md

@@ -48,6 +48,7 @@ and — where sensible — units allow for [binary prefixes](https://en.wikipedi
 | `Energy` | [Calorie](https://en.wikipedia.org/wiki/Calorie) | `cal`, `calorie`, `calories` |
 | `Energy` | [Electron volt](https://en.wikipedia.org/wiki/Electronvolt) | `electronvolt`, `electronvolts`, `eV` |
 | `Energy` | [Erg](https://en.wikipedia.org/wiki/Erg) | `erg`, `ergs` |
+| `Energy` | [Foe](https://en.wikipedia.org/wiki/Foe_(unit)) | `foe` <br/> (`use extra::astronomy`) |
 | `Energy` | [Hartree](https://en.wikipedia.org/wiki/Hartree) | `hartree`, `hartrees` <br/> (`use units::hartree`) |
 | `Energy` | [Joule](https://en.wikipedia.org/wiki/Joule) | `J`, `joule`, `joules` |
 | `Energy` | [Planck energy](https://en.wikipedia.org/wiki/Planck_energy) | `planck_energy` |

numbat/modules/extra/astronomy.nbt

@@ -2,6 +2,7 @@ use physics::constants
 use units::si
 use units::time
 use units::astronomical
+use units::cgs
 
 @name("Light-second")
 @description("The distance that light travels in one second.")
@@ -114,3 +115,8 @@ unit jansky: SpectralFluxDensity = 1e-26 W / m^2 / Hz
 @url("https://en.wikipedia.org/wiki/Solar_flux_unit")
 @aliases(solarfluxunits, sfu: short)
 unit solarfluxunit: SpectralFluxDensity = 1e4 Jy
+
+@name("Foe")
+@description("A unit of energy equal to 10⁵¹ ergs. Used to express the energy released by supernovae. The word is an acronym derived from '(ten to the power of) fifty-one ergs'.")
+@url("https://en.wikipedia.org/wiki/Foe_(unit)")
+unit foe: Energy = 1e51 erg