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. The fast particles emit gamma-radiation.
Many riddles remain to be solved.
For example, on 27 August 1998 a powerful flash of gamma-rays turned night into day in the Earth's outer atmosphere. NGC 4261, with its black-hole heart. The innermost regions of the galaxy NGC 4261. A massive black hole is located at the heart of this galaxy. From observations of radio and infrared radiation we know that the heart of our Galaxy is a site of violent activity.