Sometimes the names of objects are deeply misleading.
For example, starfish are not actually fish (they are echinoderms) and guinea pigs are not related to pigs in any way (they are rodents). Similarly, planetary nebulas have nothing to do with planets. They were misnamed when scientists looking through small telescopes in the 19th century thought that these objects looked like planets.
Today, astronomers know that a planetary nebula actually represents a phase that stars like our Sun experience after they use up much of their fuel. After cooling and expanding through a “red giant” phase when it begins to expel its outer layers, such a star leaves behind a type of dense and smaller star called a white dwarf. The previously jettisoned shells of gas remain for a relatively short time in cosmic terms — tens of thousands of years — before dissipating into space. Meanwhile they are illuminated and energized by the white dwarf at the center of the system. This will happen to our Sun, but not for another 5 billion years or so.
This image shows the planetary nebula NGC 6302.
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