What is the immediate precursor phase a medium-sized star enters before shedding its outer layers?
Answer
The star swells immensely and becomes a red giant.
The evolution toward a planetary nebula begins after a star of intermediate mass exhausts the hydrogen fuel located in its core. Once core fusion ceases, the star enters a dramatic structural transition. It initiates hydrogen burning in a shell surrounding the depleted core, causing the outer atmospheric layers to expand outward dramatically, resulting in the star becoming a red giant. It is only after this swelling phase that the star sheds these outer layers into space, marking the beginning of the planetary nebula creation process.
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