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White Dwarfs & Planetary Nebulas
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.
 A star experiences
an energy crisis and its core collapses when the star's basic, non-renewable
energy source - hydrogen - is used up. A shell of hydrogen on the edge of the
collapsed core will be compressed and heated. The nuclear fusion
of the hydrogen in the shell will produce a new surge of power that will cause
the outer layers of the star to expand until it has a diameter a hundred times
its present value. This is called the "red giant" phase of a star's existence.
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