A burnt-out star is called a white dwarf. A white dwarf results after a star’s nuclear fuel burns out, which causes it to collapse or implode. While it has a mass similar to that of the sun, its radius is similar to that of the Earth.
Studies by astronomers at the University of California, Los Angeles and University of Kiel, Germany, led to the conclusion that the formation of a white dwarf takes billions of years and progresses through a "red giant" phase. Further degeneration of the red giant leads to the death of the star, transforming it into a white dwarf.