= Stellar dredge-up
<Stellar dredge-up> is mixing that brings material altered by nuclear reactions from deeper layers into a <star>'s outer envelope and hence its observable surface. As a <convective envelope> changes its depth during <stellar evolution>, it can reach layers with a different composition. Surface <spectroscopy> can then reveal processing that occurred far below the <photosphere>.
The numbered events distinguish evolutionary circumstances rather than a single repeated clock. <First dredge-up> exposes products of prior <hydrogen burning> during giant evolution; <third dredge-up> can carry carbon and neutron-capture products outward after a <thermal pulse of an asymptotic-giant-branch star>. The measured abundance therefore records both prior reactions and subsequent transport. It need not reproduce the composition with which the <star> was born.
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