Neutron stars are the densest things in the universe. They are the collapsed cores of massive stars that have exploded as supernovae. Neutron stars are extremely dense, with a mass of about 1.4 solar masses (the mass of the Sun) compressed to a diameter of about 20 kilometers (12 miles). This means that neutron stars have a density of about 10^14 grams per cubic centimeter, which is about 100 trillion times denser than water.

In comparison, the density of the Earth is about 5.5 grams per cubic centimeter, and the density of the Sun is about 1.4 grams per cubic centimeter. Even the densest known white dwarfs, which are the remnants of smaller stars that have collapsed, have a density of only about 10^6 grams per cubic centimeter.

The density of neutron stars is so high that they are held together by the force of gravity alone. The gravitational force between two protons in a neutron star is about 10^36 times stronger than the electrostatic force that repels them. This means that neutron stars are extremely stable and can withstand enormous pressures.

Neutron stars are also very hot, with temperatures of about 10^6 Kelvin (1 million degrees Celsius). This heat is generated by the gravitational collapse of the star's core. Neutron stars also emit powerful radio waves and X-rays.

Neutron stars are fascinating objects that are still not fully understood. They are a testament to the extreme forces that can exist in the universe.