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 tith masses similar to that of the Sun will end their lives as white dwarfs. Massive stars will become neutron stars or possibly black holes or may even be completely disrupted.

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 they cannot be considered true stars. A brown dwarf (or infrared dwarf) has a mass between 1/100 and 1/12 times the mass of the Sun and it 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 at star is called the escape velocity. If the escape velocity exceeds the speed of light and if light is composed of particles, one might expect it to feel the influence of gravity. If so, 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, and we don't have enough room to dive into that now. However, we can use our daily knowledge of the Sun to do some thought experiments. If we try to compress the Sun to make it shrink in diameter until light aswell as everything else is trapped inside, unable to escape through that surface where the escape velocity equals the speed of light, it will have a radius of about 3 km.

This radius is called the Schwarzschild radius and the surface with the high 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, forever. We have no images of black holes but observations of binary stars suggest that a few black holes do exist.

Neutron Star

When the first pulsar was discovered, in 1967, there was speculation that it might be a signal from an intelligent civilization since pulsars emit pulses of radiation at regular intervals. By now more than 400 pulsars have been discovered and their periods are approximately in the range of 0.001 to 10 seconds. The periods are related to the rotation period of the neutron star. As they age, they lose energy, the rotation slows and their periods increase.