Why can planetary nebulae contain elements such as nitrogen carbon and helium?

Planetary nebulae contain elements such as nitrogen, carbon, and helium because they are chemically enriched by nuclear processing that occurs within the central star during its evolution.1

As a star evolves, it undergoes various stages of nuclear fusion in its core. In the later stages of a low-mass star's life, helium is consumed to produce heavier elements like carbon through processes such as the triple alpha process. These elements, along with others like nitrogen, are synthesized within the star and subsequently brought to the surface through convection currents, a phase often referred to as the "dredge-up".23

When the star reaches the end of its evolutionary life, it expels its outer layers into space, forming an expanding shell of luminous gas known as a planetary nebula. Because this ejected material consists of the layers that were previously processed inside the star, the resulting nebula is enriched with the products of that nuclear activity, including helium, carbon, and nitrogen. Consequently, the chemical composition of a planetary nebula serves as a direct reflection of the nuclear history and internal processing of its parent star.421