# N-body units

Quantity | Expression |
---|---|

Length (R) | |

Mass (M) |

**N-body units** are a completely self-contained system of units used for N-body simulations of self-gravitating systems in astrophysics. In this system, the base physical units are chosen so that the total mass, *M*, the gravitational constant, *G*, and the virial radius, *R*, are normalized. The underlying assumption is that the system of *N* objects (stars) satisfies the virial theorem. The consequence of standard N-body units is that the velocity dispersion of the system, *v*, is and that the dynamical or crossing time, *t*, is .
The use of standard N-body units was advocated by Michel Hénon in 1971.^{[1]} Early adopters of this system of units included H. Cohn in 1979^{[2]} and D. Heggie and R. Mathieu in 1986.^{[3]} At the conference MODEST14 in 2014, D. Heggie proposed that the community abandon the name "N-body units" and replace it with the name "Hénon units" to commemorate the originator.^{[4]}

## References

- ↑ Hénon, Michel (1971). "The Monte Carlo Method".
*Astrophysics and Space Science*.**14**(1): 151–167. Bibcode:1971Ap&SS..14..151H. doi:10.1007/BF00649201. - ↑ Cohn, Haldan (1979). "Numerical integration of the Fokker-Planck equation and the evolution of star clusters".
*The Astrophysical Journal*.**234**: 1036–1053. Bibcode:1979ApJ...234.1036C. doi:10.1086/157587. - ↑ Heggie, D. C.; Mathieu, R. D. (1986). "Standardised units and time scales". In Hut, P.; McMillan, S.
*The Use of Supercomputers in Stellar Dynamics*. Lecture Notes in Physics.**267**. pp. 233–235. Bibcode:1986LNP...267..233H. doi:10.1007/BFb0116419. - ↑ http://www.astro.uni-bonn.de/~sambaran/DS2014/Modest14_Talks/Heggie.pdf