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

system of units A system of units of measurement, also known as a system of units or 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 defi ...

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 is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's general relativity, theory of general relativity. It ...

, ''G'', and the virial radius, ''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 a conservative force (where the work done is independent of path), with ...

. 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