The core of a massive star that is 1.5 to 4 times as massive as our Sun ends up as a neutron star after the supernova. Neutron stars spin rapidly giving off radio waves . The core of a massive star that has 10 or more times the mass of our Sun remains massive after the supernova. No nuclear fusion is taking place to support the core, so it is swallowed by its own gravity. It has now become a black hole which readily swallows any matter and energy that comes too near it.
Some black holes have companion stars whose gases they pull off.
As the gases are pulled down into the black hole, they heat up and give off energy in the form of X-rays . Black holes are detected by the X-rays which are given off as matter falls down into the hole. The Answer Show me the Level 1 version of this page.