A black hole is formed when a sun becomes weak and collapses into it self meaning anything within its gravitational pull is sucked in like a vacuum and is compressed making all that matter and energy occupy a small place. from what iv been told in school that more mass means more gravity so no matter how big an object maybe that does not mean its gravitational pull is high. so with all that mass in a Small area its gravity is very strong making it impossible for even light to escape from it. so a black hole may not be big at all it could be the size of our moon and still have enough gravitational force to pull something in as far away as our closest star.