In the end, all sources of energy will be completely consumed, and the star will cease to shine. Its ultimate fate will depend on its mass. Stars that start their lives with masses similar to that of the Sun will end their lives as white dwarfs. Much more massive stars will become neutron stars or black holes.

Brown Dwarf

Objects with masses less than 1/12 the mass of the Sun never become hot enough to ignite and maintain nuclear reactions and therefore cannot be considered true stars. A brown dwarf has a mass between 1/100 and 1/12 times the mass of the Sun and may produce energy for a brief time by means of nuclear reactions involving deuterium, but it will not become hot enough to fuse protons to form helium. An object with a mass less than that of a brown dwarf is a planet.

Black Hole

The velocity required to escape the gravitational pull of a body is called its escape velocity. If the escape velocity exceeds the speed of light, light will be unable to escape from such a "star" and the object is invisible. Such an object is called a black hole.

To calculate what happens when the gravitational force becomes so large, the theory of general relativity is required - sadly this is beyond this beginners guide to stars! However, we can use our knowledge of the Sun to do some thought experiments. If we try to compress the Sun to make it shrink in diameter until light is unable to escape to the surface we find we need a radius of about 3 km.

This radius is called the Schwarzschild radius and the surface with the highest escape velocity is the event horizon. The event horizon is the boundary of the black hole and all that is inside is hidden from us. We have no images of black holes but observations of binary stars suggest that black holes do exist.

Neutron Star

When the first pulsar was discovered in 1967, there was speculation that the pulsed signal originated from an intelligent civilization since the pulses peaked at very regular intervals. By now, more than 400 pulsars have been discovered and their periods are approximately in the range of 0.001 to 10 sec. Their periods are related to the rotation period of the neutron star: as they age, they lose energy, their rotation slows and their spin period increases.