Calculate average density of the earth in (a) kg/cm3 and (b) slugs/ft3, assuming our planet to be perfect sphere. Five billion years from now our sun will reduce in size and become a neutron star. At this time the sun's diameter will be about 2.16% what it is today. (c) What will be the average density of the sun in kg/cm3 in 5 billion years? neutron stars from as remnants left after of certain supernovae-explosions of huge stars. Typically, neutron stars are about 20 km in diameter and have around the same mass as our sun. What is typical neutron star density in kg/cm3?