So, today I learned about neutron stars at http://www.space.com/22180-neutron-stars.html
Anyway, they're created when giant stars die and their protons and electrons fuse together to form neutrons. They're the size of a city, but they have a mass that's about 40% larger than our own sun. Think of them as the fat midgets of celestial objects. They happen when a star goes supernova and only its core is left, which gravity puts extreme pressure on, forcing their protons and electrons together. Neutron stars are so dense that a single teaspoon of one would weigh about a billion tons. Again, fat celestial midget. Gravity on these bad boys is two billion times greater than on Earth. It's so strong that it bends its own radiation. The supernova that created them makes them spin really fast, with a top speed of 43,000 times a minute, although they slow over time.