When the biggest stars explode as supernovas, their cores collapse in on themselves under gravity to become black holes. These regions have so much gravity that not even light can escape their incredibly powerful pull. Astronomers can’t see black holes, but they can detect them. This is because matter spiralling into a black hole emits X-rays, which can be seen by X-ray telescopes.
Supermassive black holes seem to be responsible for the exceptional energy output of active galaxies such as quasars. They have a mass millions of times greater than the Sun’s. Matter drawn in from surrounding gas clouds or stars forms an accretion disc, which emits light and other radiation. A central jet emits energy too.
If you were unfortunate enough to wander near a steep black hole, its enormous gravity would pull you in, in the same way that it sucks in light and matter. As you got nearer, the strength of gravity would increase so rapidly that it would tug at your feet more than your head, and you would stretch out long and thin, like a piece of spaghetti.