over a relatively short period of time, tens of thousands of years, the expanding shell of material on the outer parts of the planetary nebula disperses into interstellar space and fades away as a visible nebula. The atoms of hydrogen, helium, and a small amount of carbon are in this way added to the interstellar medium, having spent the past few billions of years in the interior of a star.
The carbon core of the dead star, however, continues to go through a few more stages of evolution or degeneration. After ejection of the outer layers of the former red giant star, the remnant core, which is about the size of the Earth, glows white-hot, giving the name white dwarf. The initial surface temperature of the white dwarf is about 50,000 K, but the heat is only heat stored from its formerly active processes of nuclear fusion and gravitational collapse; no new heat is generated in the white dwarf. White dwarfs can be extremely dense, however; for example, sirius B is about a million times denser than Earth. With time this core of a formerly great red giant will continue to cool, fade, and become a virtually invisible black dwarf.
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