A unit of energy = 10⁵¹ ergs or 10⁴⁴ joules, in concept equal to the energy released in all forms by a garden-variety supernova. Symbol, B. It is named for the Nobel-prize-winning physicist Hans Bethe.

a smiling Hans Bethe

The bethe is a renaming by Steven Weinberg of an earlier unit called the foe, the name being an acronym for Fifty-One Ergs, which was coined by Bethe.

Notice that “foe” suggests that these extremely distinguished physicists were thinking in terms of ergs, not joules. The bethe also illustrates a shortcoming of the original concept of SI, in that nature exceeds the range of magnitudes of its prefixes, both large and small.


Following the death of Hans Bethe last year, I have proposed a new unit called the bethe, where 1 B is 10⁵¹ ergs or 10⁴⁴ J.

Stephen Weinberg.
A Bethe unit.
Physics World, vol. 19, no. 2, page 17 (2006).



For the positrons to leak out of the expanding nebula at the required rate necessary to produce the modified 56d exponential, the mass of the ejecta from a one foe (10⁵¹ erg in kinetic energy) explosion must be small, Mejec = 0.4M with Mejec proportional to KE0.5.

S. A. Colgate, C. L. Fryer and K. P. Hand.
Low mass SN Ia and the late light curve.
Conference paper: NATO advanced study institute conference on thermonuclear supernovae, Barcelona (Spain), 19-30 Jun 1995. Report LA-UR-95-4080.


Around the same time that prompt models were being developed, scientists realized that there was far more energy available to power supernova explosions than what was typically measured as the kinetic energy of the debris. Based on observations, the explosive energies of supernovae typically tally to about 10⁵¹ ergs, or 1 “foe.” Hans Bethe coined the acronym for (ten to the) fifty-one ergs.

Marc Herant, Stirling A. Colgate, Willy Benz and Chris Fryer.
Neutrinos and Supernovae.
Los Alamos Science, no. 25, pages 64-79, (1997).
Page 71.


The formation of a new black hole or compact neutron star is heralded by an energy release of ≈10⁵³ erg. More than 99% of this energy is carried away by neutrinos within a few seconds. Roughly 1% (10⁵¹ erg = 1 foe) of the energy is converted into the kinetic energy of the ejected stellar envelope, and even less (0.01 foe) is emitted as optical light on a time scale of months.

Dieter H. Hartmann.
Proceedings of the National Academy of Sciences of the USA, vol. 96, pages 4752–4755 (April 1999).
Page 4752.
doi: 10.1073/pnas.96.9.4752.


To get a sense of the energy involved in a typical supernova explosion, imagine that our sun burned its entire supply of hydrogen—enough fuel to sustain it for more than 10 billion years—within a few seconds. That enormous amount of energy is quantified by its own physical unit: one bethe (named after Nobel Prize winner Hans Bethe).

Daniel Kasen.
Stellar fireworks.
Scientific American, vol. 314, no. 6 (June 2016).
Page 39.

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