A giant star explodes and the core collapses into a small black spot. Black holes are the remains of giant dead stars, but they are also spectacular. 1. There can be millions around usA team of astronomers from the University of California made a complex calculation to do a cosmic census. They found that our galaxy is probably overflowing with black holes, contrary to what astronomers thought. That means that the Milky Way can contain up to 100 million black holes. 2. They are incredibly heavyBlack holes arise as so-called red super giants, which are hundreds of times heavier than the sun, collapse and explode as supernova. A black hole with a diameter of one centimeter weighs 3,370,000,000,000,000,000,000,000 kilograms - half the earth's mass. Because of that mass, gravity is so great that even light cannot escape. 3. They are invisibleBecause no light escapes and they are very small compared to their enormous mass, black holes are very difficult to observe. Only special telescopes can detect them based on the behavior of stars and gases around them. When a black hole and a star rotate close to each other, light is produced with a very high energy, and that can be absorbed by special equipment.