White Dwarfs
White dwarfs are among the dimmest stars in the universe. Even so, they have commanded the attention of astronomers ever since the first white dwarf was observed by optical telescopes in the middle of the 19th century. One reason for this interest is that white dwarfs represent an intriguing state of matter; another reason is that most stars, including our Sun, will become white dwarfs when they reach their final, burnt-out collapsed state. In the white dwarf state, all the material contained in the star, minus the amount blown off in the red giant phase, will be packed into a volume one millionth the size of the original star. An object the size of an olive made of this material would have the same mass as an automobile! For a billion or so years after a star collapses to form a white dwarf, it is 'white' hot with surface temperatures of about twenty thousand degrees Celsius.
When they were first discovered, white dwarfs presented a paradox to astronomers. If a white dwarf couldn't produce energy through nuclear fusion, how could it generate the pressure necessary to keep it from collapsing further? It didn't seem possible, yet there they were, glowing dimly and reminding scientists that 'the fault is not in the stars, but in their theories,' to paraphrase Shakespeare.
The paradox was not resolved until the quantum theory of matter was developed in the 1920s. This theory showed that matter in so-called 'degenerate' states of extremely high density could produce a new type of pressure never observed in a terrestrial laboratory. This is because the quantum theory prohibits more than one electron from occupying the same energy state. To think of a white dwarf as a 'burned out' or 'dead' star can be misleading. It is more like a transformation or metamorphosis from one stage to the next. As X-ray observations prove, under the right conditions an old star can be quite lively indeed.
