Neutron stars are remarkable, and also terribly bizarre. Imagine our Sun being compressed to the size of Chicago, and you begin to understand just how remarkably bizarre neutron stars are. Even though they are tiny, typically only about 25 km (15 mi) in diameter, neutron stars are more massive than our sun. In fact, aside from black holes, neutron stars are the densest objects in the universe. An average neutron star will have a density around 5 x 1017 kg/m3, which is roughly the equivalent of all of humanity being squashed into a single sugar cube (that's pretty dense).
Learn about these amazing structures here: