White Dwarf Star

Some red giants die quietly. When a small one runs out of fuel, one with about as much mass as the Sun, it cools and begins to contract under gravity. The contraction of the inner parts of the star releases heat which causes the outer parts to expand and gently get blown away to form a so-called planetary nebula. In this way the gas of old stars is released and can be formed into new stars.

The inner part of the star continues to contract until it reaches the size of the Earth. Then its atoms are crushed together so tightly that their electrons begin to overlap. Because two electrons cannot occupy the same space, they repel each other. This is called the exclusion principle. The electrons cannot be compressed further and so create a force called degeneracy pressure. This small squashed cinder of matter is called a white dwarf star, although it does not shine like a normal star.

Note that if the red giant is more than 1.5 times the mass of the Sun it does not become a white dwarf. Instead it becomes a neutron star or a black hole.