How can a black hole be detected even though its gravitational field is too strong for light to escape?

We can infer the presence of black holes and study them by detecting their effect on matter nearby.

Scientists detect their effect on the matter nearby, say if a normal star passes near a black hole, the black hole will pull it towards itself tearing it apart, as the attracted matter accelerates it emits x-rays that radiate in space till it reaches the Earth.