Gemini

White dwarfs are essentially dead stars that no longer have nuclear reactions in their core. Thus, they slowly cool off by simply radiating the remaining thermal energy left in the core at the end of their lives. They are also very compact objects with surface gravities on the order of 100,000 times larger than that of Earth. As a result, heavy elements sink rapidly toward the center of the star while light elements tend to float at the surface. Consequently, most white dwarfs are found to have nearly pure hydrogen or pure helium surface compositions. When heavy elements (calcium, magnesium, iron etc.) are detected in the outer layers of a cool white dwarf, their presence must necessarily be attributed to recent accretion episodes from an external source, most likely from small nearby asteroids that survived the earlier evolutionary phases of the white dwarf.