Question: how dense are neutron stars?
Answer: A neutron star is so dense that one teaspoon (5 milliliters) of its material would have a mass over 5.5×1012 kg, about 900 times the mass of the Great Pyramid of Giza. ... The entire mass of the Earth at neutron star density would fit into a sphere of 305 m in diameter (the size of the Arecibo Observatory).