Neutron stars are so dense that it has more mass concentrated in about small radius of about 20-30kms.
What is Neutron star?
Neutron stars are the celestial objects which are formed after the collapse of a giant star after a supernova explosion. The giant star before collapse would weigh about 20 to 29 solar masses.
How dense can these be?
Imagine something of the mass of the Sun so compact and dense that it is of the size of a moon or even dense as that of size of a city.
The gravitational pull on such Neutron stars is so strong that it could even bend radiations of star while in the process of Gravitational lensing.
In comparison to earth the gravitational force of Neutron stars is about 2 x 1011 times more.
These Neutron stars over a period of time with increase of solar mass have to capacity to transform to quark star and eventually Black holes.