A planetary nebula […] is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. […] They are relatively short-lived phenomena, lasting perhaps a few tens of millennia, compared to considerably longer phases of stellar evolution. [1] https://en.wikipedia.org/wiki/Planetary_nebula 9.10.2022[↩]
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Planetary Nebulae
