Globular star clusters of the Milky Way and satellites from McLaughlin & van der Marel (2005). Log half-mass relaxation time (in years) is plotted against log total mass (in Solar mass). Globular clusters Ω Centauri and M 54 are shown as a magenta square and cyan triangle, respectively. Assuming a mean stellar mass of 0.5 M_⊙, star clusters with a mass above 10⁵ M_⊙ (shaded in light blue) contain 2 × 10⁵ stars and can be simulated only with great computational effort, and they are limited to a handful of realisations. This is especially true if a realistic binary fraction is included, at variance with smaller systems (grey shaded area) that are well within the capabilities of current direct N-body codes. Star clusters with a relaxation time over the typical globular cluster age (≈10 Gyr) can be regarded as collisionless and are shaded in peach and dark green. According to this definition, Ω Centauri and M 54 are only slightly collisional, but clearly not accessible to modelling through direct N-body simulations.