Consider what happens when you observe a particular object, such as a page of this book. Light, either from the sun or a lamp, is continually being reflected off the page, and some of this light will enter your eyes. This will stimulate the retina, thereby causing a signal to be sent via the optic nerve to your brain. The brain then deciphers this signal and ‘reads’ the words that are printed. The key point is that light has to be reflected if you are to see the words. In effect, it must escape from the surface of the page.

Suppose that we are located on the surface of a star and, using a cannon, fire a tennis ball upwards. The distance travelled by the ball is determined by the speed it has when it is released. If the ball is moving sufficiently fast, it can escape completely from the star's influence. In that case, it need never fall back to the surface.

What would happen if the star began to collapse? Its density would gradually increase, and the force of gravity near its surface would become stronger. This means that the ball would have to be released with a greater speed than before in order to escape. If the collapse were to proceed unhindered, the density and gravitational pull of the star would soon become extremely high.