The core of an old star that has lost most of its outer mass is called a white dwarf, also known as a degenerate dwarf.
It is mostly carbon, with a relatively thin shell of burning helium.
The core temperature remains too low to produce nuclear fusion, so the atoms simply continue to contract. The atoms then break down under the compression of gravity until they are little more than nuclei.
A white dwarf once the size of the Sun can squeeze into a space the size of Earth.
Even a teaspoonful of the densely compacted matter of a white dwarf would weigh several tons on Earth.
White dwarfs typically have a solar mass of between 0.5 to 0.7.
Its dim light comes from its burning outer shell.