Zwicky 1933 estimated that there are N = 800 galaxies in the cluster, each having observed stellar mass m = 10 9 M ⊙ ⊙ (suggested by Hubble), and the cluster has radius R = 10 6 ly . He also measured the radial velocities of the galaxies by doppler shifts in galactic spectra to be ⟨ ⟨ v r 2 ⟩ ⟩ = ( 1000 km/s ) 2 . Assuming equipartition of kinetic energy, ⟨ ⟨ v 2 ⟩ ⟩ = 3 ⟨ ⟨ v r 2 ⟩ ⟩ .