What object has gravity so strong that not even light can escape it?

A black hole is an object in space where gravity is so intense that nothing, not even light, can escape its gravitational pull. This phenomenon occurs because a black hole's mass is concentrated in an extremely small area, resulting in an escape velocity that exceeds the speed of light. The boundary surrounding a black hole is known as the event horizon; once an object crosses this boundary, it cannot return, making the interior of a black hole invisible to the outside universe.

The other celestial objects listed have significant gravitational pull but not to the extent that it prevents light from escaping. Neutron stars, while incredibly dense and having a strong gravitational field, do allow light to escape, as their gravity does not reach the extreme levels found in black holes. White dwarfs have similar properties; they are remnants of stars that have exhausted their nuclear fuel, and they too do not possess gravity strong enough to trap light. Pulsars, which are rotating neutron stars emitting beams of radiation, also allow light to escape.

In summary, the defining characteristic of a black hole is its event horizon, which leads to the unique property of not allowing any form of matter or radiation, including light, to escape once it gets too close.