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Galaxy processors
Planetary nebulae play an important role in the evolution of galaxies. The early universe consisted mostly of hydrogen and helium, but with time as a result of nuclear fusion in stars formed heavier elements. Thus, the substance of planetary nebulae have a high content of carbon, nitrogen and oxygen, as with the expansion and penetration of the interstellar space, it enriches it by these heavy elements are generally called metals by astronomers.
Subsequent generations of stars formed from interstellar matter, will contain a larger initial quantity of heavy elements, although their presence in the composition of stars is small, they significantly affect their evolution. Stars formed shortly after the formation of the universe, contain relatively small amounts of metal - they are the stars of type II. Stars enriched with heavy elements that belong to the stars I type.