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White dwarfs are the final state of stellar evolution of main-sequence stars with masses from about 0.1 to 10 solar masses. After the hydrogen-fusing period, the star expands to a red giant, and helium fuses to carbon and oxygen in its core. Depending on its mass, no further fusion is possible, and the red giant sheds its outer layers so that the core is exposed. The mass of these remnants is comparable to the sun’s, but their diameter is about the same as that of the Earth. The only energy source of a white dwarf is its residual thermal energy. A thin atmosphere of a few hundred meters in height, composed mainly of hydrogen and/or helium with sometimes traces of heavier elements, covers the core. Surface temperatures range from about 100,000 K to 4000 K. Only hot white dwarfs with surface temperatures above 30,000 K can be observed in soft X-rays or in extreme UV.
