The equations of gravity tell us that the escape velocity for a spherical object (such as a star or planet) is given by
vesc = ¥GM/R. Thus, if we make an object more massive (larger M) or compress it (smaller R), then we increase the
escape velocity. Taking this to its extreme, we can imagine taking an object as massive as the Sun and compressing it
down to a radius of only 3 km, which implies an enormous density (a teaspoonful of this material on Earth would weigh
many billions of tonnes!). This object would then be so massive and yet so small that the escape velocity at its surface
would be equal to the speed of light. Since nothing in the Universe can travel faster than light, we infer that nothing can
escape this object’s strong gravitational field. Such a remarkable entity is what we call a black hole.