Strange compact objects Figure 3. In this Hubble image, the white arrow points to a neutron star. It was detected by X-rays. When a massive star explodes not all the material is ejected into space. Matter in a neutron star is so compressed that one drop of it would weigh millions of tons on Earth.
Gravitation crushes protons and electrons together despite their electrostatic repulsion.
Thus a neutron star is actually a huge atomic nucleus essentially constituted of neutrons. Stellar black holes. Beyond this critical mass, the internal pressure against gravitational collapse is no longer sufficient and the star becomes a black hole. Nothing, not even light, can escape from a black hole. Virtually all types of compact objects are significant sources of high-energy emission because the enormous gravitational field accelerates matter in the vicinity to extreme velocities.