Suppose that you gathered a ball of interstellar gas that was equal to the size of earth (a radius of about 6000 km). if this gas has a density of 1 hydrogen atom per cm3, typical of the interstellar medium, how would its mass compare to the mass of a bowling ball (5 or 6 kg)? how about if it had the typical density of the local bubble, about 0.01 atoms per cm3? the volume of a sphere is v = (4/3)πr3.