Besides black holes, these are the densest things in the universe. They can have diameters anywhere from 6-20 miles and have masses comparable to the size of the sun and often many times more.
The density of a neutron star is 1 quadrillion the times that of water.
A neutron star is so dense that one teaspoon of its material would have a mass over about 900 times the mass of the Great Pyramid of Giza.
Since gravity is related to mass, this means the gravity is extremely powerful. If an object were to fall from a height of one meter, it would only take one microsecond to hit the surface of the neutron star, and would do so at around 2000 kilometers per second, or 7.2 million kilometers per hour.
The temperature inside a newly formed neutron star is from around 1011 to 1012 kelvin, but that falls fast as the massive amounts of neutrinos emitted bring a lot of that energy with them. Within a few years it’s a mere 10^6 kelvin. For reference, that’s around the temperature of the sun.
