''N''-body units are a completely self-contained

system of units A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement i ...

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 The gravitational constant (also known as the universal gravitational constant, the Newtonian constant of gravitation, or the Cavendish gravitational constant), denoted by the capital letter , is an empirical physical constant involved in ...

, ''G'', and the

virial radius In astrophysics, the virial mass is the mass of a gravitationally bound astrophysical system, assuming the virial theorem applies. In the context of galaxy formation and dark matter halos, the virial mass is defined as the mass enclosed within ...

, ''R'', are normalized. The underlying assumption is that the system of ''N'' objects (stars) satisfies the

virial theorem In mechanics, the virial theorem provides a general equation that relates the average over time of the total kinetic energy of a stable system of discrete particles, bound by potential forces, with that of the total potential energy of the system. ...

. The consequence of standard ''N''-body units is that the velocity dispersion of the system, ''v'', is

\scriptstyle \frac\sqrt

and that the dynamical or crossing time, ''t'', is

\scriptstyle 2\sqrt

. The use of standard ''N''-body units was advocated by Michel Hénon in 1971. Early adopters of this system of units included H. Cohn in 1979 and D. Heggie and R. Mathieu in 1986. At the conferenc
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.

References

External links

N-Body Units

STELLAR DYNAMICS by D.C. Heggie, p.6–7
{{systems of measurement Physical cosmology Systems of units 1971 introductions