Suppose the Milky Way contains 10 billion $M_{\odot}$ of dark matter at radii between 8 and 9 kpc from its center.
a. What is the density of matter in the "shell" between the radii of 8 and $9 \mathrm{kpc} ?$ (Hint: Subtract the volume of a sphere of radius $8 \mathrm{kpc}$ from the volume of a sphere of radius $9 \mathrm{kpc} .$
b. How much mass of dark matter would there be in a spherical volume with a radius equal to the Earth's radius? Compare your result to the mass of the Sun.