The density of air in a child's balloon $20 \mathrm{cm}$ in diameter is roughly the same as the density of air at sea level, $10^{19}$ particles/cm $^{3}$. To how large a diameter would you have to expand the balloon to make the gas inside the same density as the interstellar medium, about 1 particle/$\mathrm{cm}^{3} ?($ Hint: The volume of a sphere is $\left.\frac{4}{3} \pi R^{3} \cdot\right)$