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.

First dredge-up occurs as a star ascends the red-giant branch. Its deepening convective envelope exposes material processed by hydrogen burning, especially enhanced helium and nitrogen and depleted carbon.

Second dredge-up occurs after core helium exhaustion in sufficiently massive intermediate-mass stars. The convective envelope moves inward, mixes helium- and hydrogen-burning products to the surface, and reduces the hydrogen-exhausted core mass.

Third dredge-up follows a thermal pulse on the asymptotic giant branch, when the convective envelope reaches into the intershell and can carry freshly made carbon and slow-neutron-capture elements to the surface.