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There are 3 kinds of Stellar Remnants:
White Dwarfs, Neutron stars (some are also Pulsars) and Black Holes.
They represent a final, end stage of evolution for stars. They are, in a sense, dead stars. However, we DO see them.. though for many, (including most neutron stars and all black holes) to see them means to infer their existence through their interaction with matter in the Universe.
The Sun will evolve to a Planetary Nebula, revealing its central core to become a White Dwarf. White Dwarfs are extremely small and very HOT (initially). Since it is the remnant of fusion processes, it is made up nearly exclusively of either Carbon and or Oxygen, with just a thin atmosphere of Hydrogen and Helium.
Radius of a White Dwarf?
Same as the Earth (1/100th that of the Sun).
Temperature of a White Dwarf?
At first, 150,000 K! But, it will cool as time passes, becoming a BLACK DWARF.
Density of a White Dwarf?
106 g/cc (million times that of water!). This is about 16 tons per cubic inch.