Planetary nebulae represent the last phase of life of stars of similar solar masses. Once the star reaches the red giant phase it begins to lose its surface layers due to the stellar wind. Meanwhile, the star's core contracts more and more, until it becomes a white dwarf with a very high surface temperature (clearly visible in the center of this image). At this point the ultraviolet radiation of the white dwarf ionizes the gases expelled previously, which consequently starting to emit forming the planetary nebulae we see.
In reality, as shown by NGC 2022, the situation is more complex than that. This planetary nebula, for example, is made up of two spherical shells that expand at two different speeds.
Astronomers are currently still trying to understand the mechanisms that lead to the formation of planetary nebulae with structures so complex and different from each other.
Credit: ESA, Hubble, NASA, R. Wade.
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