In the vast universe， where to find a black hole？

A large mass of stars will form a black hole after the explosion. When a black hole constantly engulfed the surrounding material， these substances are generally issuedaelectromagnetic signals of"help". The scientists are using these signals to determine the location of the black hole. There are a lot of black holes in the universe， according to the size of the mass of the black hole， scientists divide it into a small black hole， medium black hole and large black hole. In fact， there is a large mass of black hole in the center of our Milky Way galaxy， and its mass is almost 300 million times of the mass of the sun. While the core of active galaxies and quasars is a supermassive black hole， and its quality can often reach trillions of times， billions of times or even tens of billions of times more than the mass of the sun. We can find the black hole in the core of the X - ray binary system， the center of the cluster or the core of the galaxy.

（Answered by Li Qingkang）