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White Dwarf Stars

A white dwarf is what stars like our Sun become when they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material (creating a planetary nebula), until only the hot core remains, which then settles down to become a very hot (T > 100,000K) young white dwarf. Since a white dwarf has no way to keep itself hot unless it is accreting matter from a nearby star (see Cataclysmic Variables), it cools down over the course of the next billion years or so. Many nearby, young white dwarfs have been detected as sources of soft X-rays (i.e. lower-energy X-rays); recently, soft X-ray and extreme ultraviolet observations have become a powerful tool in the study the composition and structure of the thin atmosphere of these stars.
A typical white dwarf is half as massive as the Sun, yet only slightly bigger than the Earth. This makes white dwarfs one of the densest forms of matter, surpassed only by neutron stars.


To say that white dwarfs are strange is an understatement. An earth-sized white dwarf has a density of 1 x 109 kg/m3. In comparison, the earth itself has an average density of only 5.4 x 103 kg/m3. That means a white dwarf is 200,000 times as dense!

Because a white dwarf is no longer able to create internal pressure, gravity unopposedly crushes it down until even the very electrons that make up a white dwarf's atoms are mashed together. In normal circumstances, identical electrons (those with the same "spin") are not allowed to occupy the same energy level. Since there are only two ways an electron can spin, only two electrons can occupy a single energy level. This is what's know in physics as the Pauli Exclusion Principle. And in a normal gas, this isn't a problem; there aren't enough electrons floating around to completely fill up all the energy levels. But in a white dwarf, all of its electrons are forced close together; soon all the energy levels in its atoms...

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