Here, φ φ ( ξ ξ ) is interpreted as the dimensionless mass, defined by m ( r ) = 4 π π α α 3 ρ ρ c φ φ ( ξ ξ ) . The relevant initial conditions are φ φ ( 0 ) = 0 and θ θ ( 0 ) = 1 . The first equation represents hydrostatic equilibrium and the second represents mass conservation.