At the end of a star life, it would undergo a supernova explosion. The outer layer of the star will be blown of after the explosion and form a supernova remnant. In the center area, the star collapses under intense gravity so much so that the protons and electrons fuses to form neutrons. A neutron star is formed. The neutron star is about 1.4 times the mass of a sun. A black hole is more massive than a neutron star.

A Pulsar is a rotating neutron star. It is around the size of a large city, very dense and spherical. It is very magnetized emitting electromagnetic radiation. It also shoots out gamma ray, X-ray and visible light.(Credit:blogs.scientificamerican.com)