Neutron stars are the collapsed cores of massive stars that have undergone a supernova explosion. They are extremely dense, with a mass of about 1.4 solar masses (the mass of the Sun) compressed into a volume of about 10 kilometers (6 miles) in diameter. This gives them a density of about 10^14 grams per cubic centimeter, or about 100 trillion times the density of water.

The gravity on the surface of a neutron star is so strong that it would crush a human being into a thin layer of atoms. The escape velocity from a neutron star is also very high, about 100,000 kilometers per second (62,000 miles per second). This means that nothing, not even light, can escape from the surface of a neutron star.

Neutron stars are powered by their rotation. As they spin, they generate a magnetic field that can produce radio waves and X-rays. Some neutron stars also emit bursts of gamma rays.

Neutron stars are often found in binary star systems, in which they orbit another star. In some cases, the neutron star can draw matter from the companion star, which can lead to the formation of a black hole.