The total mass is divergent, but it is often useful to take the edge of the halo to be the virial radius, R vir, which is related to the "concentration parameter", c, and scale radius via R v i r = c R s (Alternatively, one can define a radius at which the average density within this radius is Δ Δ times the critical or mean density of the universe, resulting in a similar relation: R Δ Δ = c Δ Δ R s . The virial radius will lie around R 200 to R 500 , though values of Δ Δ = 1000 are used in X-ray astronomy, for example, due to higher concentrations.)