When a star with a mass similar to that of the Sun exhausts its store of nuclear energy, it can only contract and release more of its potential energy and eventually it will have an enormous density. These are the extremely compact white dwarfs stars. Gradually a white dwarf cools off and after a few hundered million years in existence it has a luminosity about 1 percent of the Sun's.

Eventually a white dwarf will cease to shine at all and it will become a black dwarf instead - a cold mass of degenerate gas floating through space. We have not yet observed a black dwarf and it may be that the Galaxy is not old enough for any star to have yet had time to become a black dwarf.

If a white dwarf is close to another star, they can become a nova, which means that an existing star suddenly emits an outburst of light. A nova is only bright for a few days or weeks and then it gradually fades away. The outburst of light occurs when mass is transferred from the normal star to the white dwarf. After the outburst, the white dwarf is calm again but eventually the process is repeated.