A supernova leaves behind a tiny object termed a neutron star, far smaller than the Earth. It is the core of the star that exploded. The protons and electrons of its atoms have been squashed together by the force of the supernova, to from the atomic particles known as neutrons.
A neutron star is unimaginably dense. It contains the mass of the Sun, squeezed into a ball 20km (12 1/2 miles) across. Neutron stars are so small that they can spin very rapidly, once a second or faster. Some of them flash each time they spin, like a lighthouse. Such flashing neutron stars are termed pulsars.