051.1 The Event Horizon

Light falls in a gravitational field, just like everything else does.

As the mass of a neutron star increases, the strength of gravity near its surface becomes enough to prevent even an outward light ray from escaping.

Because nothing can travel faster than light, nothing else can escape either and a 'point of no return' or event horizon develops around the star.

Because the event horizon absorbs all light (and anything else) that comes near it, it is called a black hole.