As heavy as neutron stars are, they too can only contract so far. This is because, as crushed as they are, the neutrons also resist the inward pull of gravity, just as a white dwarf's electrons do. A black hole is one of the last options that a star may take. After a black hole is created, the gravitational force continues to pull in space debris and other types of matters to help add to the mass of the core, making the hole stronger and more powerful. The most defining quality of a black hole is its emission of gravitational waves so strong they can cause light to bend toward it.
Gravitational waves are disturbances in the curvature of space-time caused by the motions of matter.
Though gravitational waves pass straight through matter, their strength weakens as the distance from the original source increases. Although many physicists doubted the existence of gravitational waves, physical evidence was presented when American researchers observed a binary pulsar system that was thought to consist of two neutron stars orbiting each other closely and rapidly. Radio pulses from one of the stars showed that its orbital period was decreasing. In other words, the stars were spiraling toward each other, and by the exact amount predicted if the system were losing energy by radiating gravity waves. Most black holes tend to be in a consistent spinning motion as a result of the gravitational waves.