Composition of a Black Dwarf Star
Black dwarf stars are hypothetical entities. Although scientists have posited their existence, they have not actually been discovered anywhere in the universe. Astronomers believe that the universe is too young for the creation of black dwarf stars. A black dwarf star is the hypothetical end of the life cycle of a white dwarf star. Therefore, it shares certain similarities with a white dwarf.
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White Dwarf Stars
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To understand black dwarf stars and why astronomers hypothesize their existence, it's necessary to understand white dwarf stars. White stars are extremely dense forms of matter. Living stars contain hydrogen that is expended as it is transformed into helium. This causes the star to expand. However, the star also contains gravity that pulls elements inward. These two opposing forces cause pressure to build inside the star. Once the hydrogen has been burned up, gravitational pressure is all that holds the star together. Remaining hydrogen traps heat inside the star, which it uses to fuse the helium into carbon causing the star to again grow. Once this helium energy has been converted, gravity pulls the star inward, but there is not enough heat energy left to transform the carbon. At this stage, the star contracts and then releases the outer layers leaving behind a glowing white dwarf star.
Chemical Composition
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All stars consist of protons, neutrons and electrons, which are found in atoms and present in all chemical compounds. Astronomers refer to this compound as baryonic matter, which is a slightly different definition than normally used by scientists. A black dwarf star would theoretically consist of baryonic matter with the same chemical compounds contained in a white dwarf star. White dwarf stars are typically composed of the elements of oxygen and carbon. However, they can also contain helium, neon and magnesium.
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Electron Degeneracy
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Large stars will often simply collapse into black holes because of the gravitational pressure that exists once all the energy has been transformed. However, smaller stars often resist this process because of electron degeneracy. Essentially, two electronics cannot occupy the same state. This means that some of them will have to transform to a different state, even as a star is collapsing as a result of gravity. In the instance of white dwarfs, electrons are forced into higher energy states, which cause pressure and prevent the star's collapse.
Eventual Existence
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The reason that astronomers can only hypothesize about the eventual existence of black dwarf stars is that the universe is too young to have produced any. White dwarf stars take trillions of years to cool off because the heat trapped inside them is released very slowly through radiation. Theoretically, a white dwarf star should cool off to a point where it no longer contains heat energy and consists only of decomposed matter. At this point, the white dwarf star is considered dead and will become an invisible black dwarf star.
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