He approximated the Coma cluster as a spherical "gas" of N stars of roughly equal mass m , which gives ⟨ ⟨ T ⟩ ⟩ = 1 2 N m ⟨ ⟨ v 2 ⟩ ⟩ . The total gravitational potential energy of the cluster is U = − − ∑ ∑ i < j G m 2 r i , j , giving ⟨ ⟨ U ⟩ ⟩ = − − G m 2 ∑ ∑ i < j ⟨ ⟨ 1 / r i , j ⟩ ⟩ . Assuming the motion of the stars are all the same over a long enough time (ergodicity), ⟨ ⟨ U ⟩ ⟩ = − − 1 2 N 2 G m 2 ⟨ ⟨ 1 / r ⟩ ⟩ .