Black Holes
A black hole is nothing but empty space. It is also a lot of matter packed into a small amount of room. If you made the sun 10 times bigger and then squeezed it into the size of New York City thats how much matter are in them. This causes a huge gravitational field. This field is so big not even light can escape. Black holes are created when a star dies and it leaves its core that could be 10 times bigger than the sun. Lets say a star moves toward the black hole it tears its atoms apart and sucks it in. Most black holes are made when a Super Nova explosions kills the star this causes the star to collapse because the explosion is so big. Stellar Black holes are almost impossible to detect. These can get up to 24 times the size of the sun. There are also "Super Massive" Black holes that could be billions the size of the sun.
http://science.nasa.gov/astrophysics/focus-areas/black-holes/
